Earth and Ocean Sciences, Duke University, Durham, North Carolina, USA
A widely held view is that the Deep Western Boundary Current (DWBC) is primarily responsible for the export of recently-ventilated watermasses from the northern North Atlantic to the rest of the global ocean. New subsurface float observations and simulated “e-floats”, however, have revealed that the DWBC is in fact not a continuous pipeline for the export of Labrador Sea Water from the subpolar to subtropical latitudes. Only a small percentage (<10%) of observed and simulated floats released in the DWBC in the subpolar region continuously followed the path of the DWBC to the subtropics. Together, these observational and modeling results challenge the concept of the DWBC as a “conveyor belt”, supporting instead a more diffusive process including interior pathways and eddies. Results from the RAFOS float observational program, its accompanying modeling study and an analysis of historical hydrographic data will be discussed.
Christopher P. Atkinson, Harry L. Bryden, Torsten Kanzow
National Oceanography Centre, Southampton
The Atlantic Meridional Overturning Circulation (AMOC) is a fundamental component of the Earth’s climate system, transporting heat from the tropics to higher latitudes. Modelling studies have shown that a weakening of the AMOC in response to global warming could have a profound impact on European and North American climate. Since 2004, continuous measurements of the AMOC have been made along 26.5°N using the transatlantic Rapid Climate Change array of moored instruments combined with time series of Gulf Stream transport and satellite derived surface-layer Ekman transport. In the first year of observation, average overturning was 18.7 ± 5.6 Sv, with a range of 4.0 to 34.9 Sv. With measurements planned until 2014, understanding the intra-annual and interannual variability in the AMOC signal is key to identifying longer-term trends and variability. Here, Ekman transport variability at 26°N is analysed using the NCEP-NCAR climatology plus QuikSCAT scatterometery where available. The timescales and patterns of Ekman transport variability at 26°N are discussed.
William Austin1, Charlotte Bryant2, Joy Sangarayer3
1) University of St Andrews2) NERC Radtiocarbon Laboratory, East Kilbride3) University of Bristol
Records of changing atmospheric radiocarbon concentration from the last deglaciation suggest that an anomaly during the Younger Dryas (YD) cold phase is the largest of the last 15,000 y. However, the cause of such variations are debated, and are either attributed to the production rate of 14C due to changes in solar activity or the Earth’s magnetic field and/or changes in the carbon cycle. The latter is strongly influenced by carbon exchange between the atmosphere and other reservoirs, such as the deep ocean. In particular, reorganisation of the North Atlantic’s overturning circulation, widely associated with intervals of abrupt climate change such as the YD, may have a significant effect on ocean-atmosphere carbon exchange. During the last deglaciation, as the northern hemisphere climate warmed, meltwater from the North American icesheet introduced a large volume of freshwater to the surface North Atlantic. It is widely held that such freshwater perturbations severely reduced the strength of the ocean conveyor and, in this case, triggered the YD cold phase. Here we reconstruct apparent surface water C-14 ages (reservoir ages (Rt)) in the Atlantic Ocean north of 50°N through the YD interval. Within 300 calendar years of the start of the YD cold phase, Rt increased dramatically, reaching values of 1000 y. After 12,300 y BP, Rt gradually decreased, approaching modern North Atlantic surface ocean values of 400 y by the end of the YD. The 14C concentration of the surface North Atlantic changed in direct opposition to atmospheric radiocarbon concentration throughout the YD, suggesting that extensive sea-ice cover limited air-sea exchange and that a direct link exists between the strength of Atlantic overturning circulation and the 14C ventilation rate of the deep ocean on sub-centennial timescales. We review the evidence from an Earth system model to show that spatially disparate marine 14C datasets from the Atlantic and Pacific Oceans can provide a constraint to the causal mechanisms of past Δ14Catm variation. We confirm that the increase in Δ14Catm at the start of the YD was principally caused by ocean circulation change. Furthermore, changes in tropical Atlantic reservoir age mean that the Δ14Catm increase is rather less than that inferred, questioning the accuracy of radiocarbon calibrations during the YD. The results of the study by Sangarayer et al. (in press) constitute the first model-data reconciliation of the YD radiocarbon record. By quantifying changes in surface reservoir age, our approach provides a means for improving the robustness of the radiocarbon calibration curve. REFERENCE Joy S. Singarayer, David A Richards, Andy Ridgwell, Paul J. Valdes,William E. N. Austin, J. Warren Beck (in press) An oceanic origin for the increase of atmospheric radiocarbon during the Younger Dryas, Geophysical Research Letters.
Sheldon Bacon, N. Penny Holliday, Peter Saunders
The project "Cape Farewell and Eirik Ridge: Interannual to Millennial Thermohaline Circulation Variability" (CFER), funded by the UK Natural Environment Research Council's Rapid Climate Change Directed Research Programme, contained an element of modern physical oceanographic measurements in the vicinity of southern Greenland, and in this presentation, we give an overview of results to date from this project's field programme. In particular we show how aspects of the three-dimensional circulation in the area are illuminated by the first hydrographic survey, conducted on RRS Discovery in late summer 2005, and we describe their significance. We also describe results from the current meter array, deployed in 2005, then recovered and redeployed in 2006, established to provide the first long-term measurements of the Deep Western Boundary Current in this area. We will conclude with a brief description of plans for further measurements around southern Greenland.
Johanna Baehr
MIT
We test the incorporation of local temperature/salinity observations from the RAPID mooring array, as well as the cable estimates of volume transport in the Florida Current (FC), in the ECCO-GODAE estimation system for their impact on the meridional overturning circulation (MOC) and the meridional heat transport in the Atlantic. An experimental one year setup covering the first deployment period of the RAPID array from March 2004 - March 2005 is used to test different strategies. Using both monthly means of the FC data and monthly means of the RAPID temperature and salinity measurements at the eastern and western boundary of the basin as an observational constraint in a one year experiment results in a twofold change in the MOC at 26N and the adjacent latitudes: (i) an increased MOC of about 1 Sv for the northward branch of the MOC above 1000 m, and (ii) an increased MOC of about 1 Sv for the southward branch of the MOC between about 2000 m and 3000 m. The meridional heat transport increases by about 0.05 PW between 26N and about 40N. These results suggest that the North Atlantic MOC is sensitive to changes in both the FC transport and the zonal density gradient across 26N, presupposing the continuous observation of both to facilitate continuous MOC monitoring in the Atlantic.
Andy Baker1, Valerie Trouet2, Christoph Spoetl3, Ian Fairchild1, Rob Wilson4, Jan Epser2
1) School of Geography, Earth and Environmental Sciences, The University of Birmingham, Birmingham, B15 2TT, UK2) Swiss Federal Research Institute WSL, Zuercherstrasse 111, CH-8903 Birmensdorf, Switzerland3) Institute of Geology and Paleontology, Innsbruck University, 6020 Innsbruck, Austria4) School of Geography & Geosciences, University of St Andrews, North Street, St Andrews, Scotland
Stalagmite SU-96-7 from NW Scotland grew continuously for ~1000 years up to 1996 AD, the year of its sampling. It contains a continuous annual lamina chronology, which was previously shown to correlate with North Atlantic climate. We have combined the low frequency components of this proxy and a tree ring record of drought severity from Morocco: these two millennial length proxy records are located in the opposing nodes of the NAO dipole and reconstruct multi-decadal atmospheric phases back for almost the full millennium. We find a persistent positive NAO phase (1050 – 1350 AD) that suggests that present precipitation anomalies lie within the realm of natural variability over the last millennium. Additionally, we have undertaken annual to biannual resolution analysis of d13C and d18O on the stalagmite. The presence of a precisely constrained annual lamina chronology allows us to undertake instrumental calibration of the isotope records for the last 100 years. We observed no statistically significant correlations between annual, seasonal or monthly climate parameters and d13C and d18O, confirming the lack of high frequency climate signal in this stalagmite. However, we also investigated the relationship between stalagmite isotopes and smoothed annual, seasonal and monthly climate parameters. For d13C, this revealed a statistically significant relationship between d13C and February temperature, with the strongest correlation with the smoothing of the preceding 24 yrs of February T (r = -0.80, with weaker correlations at both longer and shorter smoothing; r=-0.80 is statistically significant despite the reduced degrees of freedom). We hypothesise that this correlation with February temperature is due to the freezing of the peat that overlies the cave in cold winters; the subsequent melting of this water will recharge the underlying limestone with d13C that is relatively isotopically heavy compared to autumn recharge waters. These two recharge waters are then mixed in the aquifer with ground water derived from previous years recharge, leading to a smoothed d13C signature in the stalagmite. Such a hypothesis also explains the low frequency signal contained in the stalagmite annual lamina width proxy. d13C in SU-96-7 is therefore used to provide a record of low frequency variability in winter temperature for the last millennium. Our results demonstrate the importance of both instrumental calibration and a multi-proxy approach to speleothem research.
Jonathan Bamber1, Janneke Ettema2, Michiel van den Broeke2, Erik van Meijgaard3, Rupert Gladstone1, Jenny Griggs1
1) University Bristol2) Utrecht University3) Royal Netherlands Meteorological Institute
The results presented here are part of the RAPID-project “Mass balance and freshwater contribution of the Greenland ice sheet: a combined modeling and observational approach”. The aim of the project is to produce a benchmark data set of freshwater fluxes from the Greenland ice sheet from 1957-2005 using an atmospheric limited area model to estimate runoff and satellite observations for the solid-ice (iceberg) fluxes. We use the Regional Atmospheric Climate Model version 2.1 (RACMO2.1) of the Royal Netherlands Meteorological Institute (KNMI). ECMWF Re-Analysis (ERA-40) fields force the model at the lateral boundaries, while the interior of the domain is allowed to evolve freely. To date, we have runoff estimates for the period 1989-2005 at a resolution of 11 km, sufficient to resolve the ablation zone in some detail. We compare, here, our surface mass balance time series with other regional climate modelling studies and in-situ observations. Substantial difference are found between estimates, particularly in the south-east of Greenland where accumulation gradients are large. Since 1990 the surface mass balance has become more negative but only by ~5 Gt/yr. The surface mass balance, however, represents less than half the total mass loss from the ice sheet, the remainder taking place by processes controlled by ice dynamics. In the last decade, the dynamics have shown substantial variability. Although it is not possible, at present, to model this variability, we use observational data on calving front position and ice motion to estimate the envelope of variability in the solid ice flux entering the North Atlantic and Arctic Oceans.
Jonathan Bamber1, Marion Bougamont1, Dan Lunt1, Bob Marsh2, Jonathan Gregory3, Jeff Ridley4, Tony Payne1, Paul Valdes1, Robin Smith3
1) University Bristol2) National Oceanography Centre3) University Reading4) Met Office, Hadley Centre
The aim of this project was to investigate how future changes in the mass balance of the Greenland ice sheet and Arctic sea ice might affect the behaviour of the AMOC, using a suite of coupled climate models. Here, we present results of several hosing experiments, placed in the context of the probable range for the freshwater flux (FWF) from Greenland. One set of experiments was designed to investigate the sensitivity of the AMOC to the amplitude and duration of freshwater forcing using the Earth Model of Intermediate Complexity: GENIE-1. We also report on the sensitivity of GENIE-1 and GENIE-2 (which contains a dynamic atmosphere) to “standard” CMIP hosing experiments with fluxes of 0.1 and 1.0 Sv and an ensemble of hosing experiments with a low resolution version of HadCM3, called FAMOUS. We also discuss the plausible range of FWF from the Greenland ice sheet over the next 200 years to place the fluxes used in the hosing experiments into context. The input from Greenland is highly localised and also seasonally modulated and both these factors may be important for mixing and transport. It has been suggested that Greenland runoff may be largely entrained in narrow boundary currents and advected away from the key areas of overturning. We investigate this using two approaches: a global ocean model, NEMO, running at 0.25º, which, although not adequate to fully resolve narrow currents, does capture the general characteristics of the East Greenland Current. The second approach uses a high resolution (0.08º) pan-Arctic ocean model (Los Alamos POP model), with passive tracers in collaboration with US colleagues and is currently in progress. Preliminary results from these analyses and some outstanding questions arising from them are presented.
Molly Baringer1, William Johns2, Christopher Meinen1, Deborah Shoosmith3, Harry Bryden4
1) NOAA/AOML2) University of Miami3) British Antarctic Survey4) University of Southampton
The structure of the Florida Current variability is examined using more than 100 sections of the Florida Current temperature and velocity taken between June 1982 and July 1984 during NOAA’s Sub(T)ropical Atlantic Climate Studies(STACS) program. The goal of such analyses is to determine what vertical structure of the Florida Current can be estimated from simple metrics like the total volume transport. Through an examination of the statistically coherent fluctuations in the transport, both as a section as averaged vertical profiles across the section we have identified the major modes of variability of the current and have used this to estimate the vertical structure of the flow for use with the RAPID/MOCHA array in the ocean interior. Together, these vertical profiles of transport provide the estimate of the strength of the MOC along 27N. This poster describes in detail how the Florida Current portion of the MOC is obtained. Using the historical data, we first grid the velocity and temperature fields onto a 2 km by 10 m uniform grid. The velocity data is de-tided using the tidal constituents derived by Mayer and Larsen (1986). Empirical orthogonal functions of these two-dimension sections show a predominance of the first mode structure that represents strong fluctuations along the coast, suggestive of a coastally trapped wave. Integrating this two dimensional velocity field to generate a total transport profile as a function of time and depth, shows a similar predominance of the first mode of variability. The leading mode contains about 87% of the variance. The first mode is well correlated at zero lag with the total transport anomaly and the total transport can be linearly fit as a function of the principal component associated with this mode. We show that this fit can be used to generate a vertical transport anomaly time series that can be used with the Rapid/Mocha transport anomalies in the ocean interior.
Grant Bigg, Richard Levine
Department of Geography, University of Sheffield
We introduce explicit icebergs from a dynamic and thermodynamic iceberg model into an intermediate complexity climate model, with icebergs seeded from all known ice-sheets. Last Glacial Maximum (LGM) simulations produce a model state that is consistent with a steady state climate. Adding icebergs to the PD climate model has only a minimal impact but in the LGM the Atlantic overturning strength is reduced by a third. We test the sensitivity of the coupled model at the LGM to additional Heinrich event-scale fluxes of icebergs from three possible sources of meltwater: Hudson Strait, the Gulf of St Lawrence, and the Norwegian Channel Ice Stream (NCIS) or North Sea Ice Sheet. The sensitivity of the ocean is similar for all three locations, with the main difference due to the varying times it takes for the iceberg meltwater to reach the main convection region in the northeast Atlantic, with more variability produced for the NCIS events and a distinctly different, more northerly, salinity anomaly. Heinrich events modeled as freshwater floods produce similar results, except for the NCIS events. Our results suggest that 0-3-0.4 Sv of additional freshwater flux, either as icebergs or freshwater, is required to shut down the North Atlantic meridional overturning, a larger freshwater flux than sometimes suggested because of the localised nature of the release of the freshwater.
Adam T. Blaker, Bablu Sinha
NOCS
The FORTE (Fast Ocean Rapid Troposphere Experiment) climate model comprises of a 2 x 2 degree MOMA (Modular Ocean Model Array) ocean coupled to IGCM3 (Intermediate General Circulation Model), a T42 spectral atmosphere (approximately 2.8 x 2.8 degree). Both components have 15 layers. Results from two climate forcing scenarios are presented: a 0.1 Sv hosing applied uniformly over the North Atlantic between 50N and 70N; and a 1% transient increasing CO2 experiment which achieves 4 x CO2 after 140 years. The FORTE model responds strongly to the hosing experiment, with a complete shutdown of the Atlantic Meridional Overturning Circulation (AMOC) within 100 years. The response to the transient CO2 forcing is weaker, with a maximum reduction of the AMOC of 4 Sv after 140 years. Further details of the nature of the climate change in each experiment are presented.
David Brayshaw1, Brian Hoskins2, Mike Blackburn1
1) Walker Institute for Climate Research, Department of Meteorlogy, University of Reading2) Department of Meteorlogy, University of Reading
The North Atlantic storm track exerts a strong influence on the weather of Europe, driving surface westerly winds and supporting a relatively mild and wet climate over western Europe. The storm track itself is sensitive to changes in the strength of the Atlantic ocean's meridional overturning circulation (MOC) with previous studies suggesting that a forced shut down of the MOC (under constant pre-industrial conditions) generally produces a stronger storm track in the eastern North Atlantic. The overall mechanisms controlling the storm track are complex, however, and have not yet been investigated using GCM-level models in a systematic manner. This presentation therefore provides a "step back" from the detail, describing a series of GCM experiments that investigate the how the shape, strength and character of the storm track is controlled by the interaction of the large scale flow with mountains, land and the ocean surface on geophysically relevant spatial scales. The importance of the sea surface temperature gradient in the North Atlantic is discussed along with its relevance to a potential weakening of the MOC. This provides a framework for interpreting storm track changes in both future and past climate simulations and understanding the atmospheric response to slow variations in sea surface temperature.
Josephine Brown1, Mat Collins2, Sandy Tudhope3
1) School of Geography and Environmental Science, Monash University, Melbourne, Australia.2) Hadley Centre for Climate Prediction and Research, Met Office, Exeter, UK.3) School of Geosciences, University of Edinburgh, Edinburgh, UK.
Evidence from proxy records including fossil corals suggests that the tropics may have undergone substantial shifts in mean climate and interannual variability over the Holocene, which in turn may have contributed to significant regional or global climate change. Coral evidence for a greatly reduced El Niño-Southern Oscillation (ENSO) amplitude in the early to mid-Holocene provides a target for model simulations to test the ability of climate models to reproduce such changes in ENSO behaviour. Corals from the north coast of Papua New Guinea, in the western Pacific warm pool, record variability in precipitation and sea surface temperature associated with ENSO. Interannual variability in fossil corals from the early and mid-Holocene is reduced by up to 60% compared with modern corals from the same site. The HadCM3 coupled ocean-atmosphere model is used to simulate mid-Holocene and pre-industrial climate. An ensemble of model versions with perturbed physical parameters are used to investigate the uncertainty in model sensitivity associated with model parameterisations. The model simulates a reduction in mid-Holocene ENSO amplitude of up to 15%, although the perturbed physics simulations show little response due to the use of seasonally-varying ocean surface heat flux adjustments. This study has highlighted the importance of using a consistent measure of ENSO amplitude to carry out quantitative comparisons between proxy records and model output. The relative magnitude of changes in local ENSO signals recorded by coral and basin-scale ENSO simulated by the model cannot be assumed. Instead, the relationship between local and large-scale ENSO signals is demonstrated for instrumental records and a set of model simulations which include a range of ENSO amplitudes. Remaining disagreement between model and proxy estimates of changes in mid-Holocene ENSO amplitude requires additional proxy records to determine whether the model is insufficiently sensitive to palaeoclimate forcing.
Maria Paz Chidichimo1, Torsten Kanzow2, Stuart A. Cunningham2, Jochem Marotzke 1
1) Max Planck Institute for Meteorology, Bundesstrasse 53, 20146 Hamburg, Germany.2) National Oceanography Centre, Empress Dock, Southampton, SO143ZH, UK.
The RAPID-MOC array makes use of moored time series measurements of vertical density profiles at the western and eastern boundaries at 26.5°N of the Atlantic to estimate the transatlantic, absolute zonally integrated meridional geostrophic transport. Here we study the contribution of eastern boundary density to the Meridional Overturning Circulation (MOC), based on mooring data between April 2004 and October 2006. Among the mechanisms that may change densities at the eastern boundary (and thus the strength of the MOC) are Kelvin waves propagating poleward and wind-driven upwelling. It is generally expected that the density variability near the eastern boundary of the North Atlantic is smaller than near the western boundary. However, neither the amplitude nor the frequency distribution of eastern boundary densities contribution to MOC variability have been studied systematically. To highlight the eastern boundary variability, the MOC is calculated assuming that density at the western boundary is time-invariant and only the eastern boundary density varies over time. At the eastern boundary there are two methods of sampling density profiles: by two 5000 m long (full water column) moorings located at the base of the African continental slope, and with an array of small moorings distributed between the African shelf and the base of the continental slope. The eastern boundary contribution to the basin-scale meridional transports inferred from the inshore (small moorings) and offshore (tall moorings) data sets are investigated for potential redundancy. There are considerable differences between the two data sets in terms of amplitude, vertical structure and frequency distribution of the resulting mid ocean geostrophic transport fluctuations.The vertical correlation scale of the density anomalies that account for the major changes in the transports is much larger at the offshore site (throughout the entire water column) with maximum density changes at ~1000 m. Density anomalies in the inshore data set display larger density anomalies than offshore but these are mostly confined to depths above 1000 m. Transports inferred from the inshore data show pronounced variance in the high frequency limit, with dominant periods of 3 and 13 days. Near boundary processes appear to play an important role in setting the time scale. Transports inferred from the offshore data exhibit maximum variability in the low frequency limit with dominant periods of 5 months. The two transports signals are uncorrelated. Mechanisms which are unrelated to the MOC (such as eddies) may mask MOC related density signals in the offshore data set on the time scales under consideration. The full water column mooring is too far offshore to detect potential boundary waves and / or upwelling signal. Therefore we conclude that the inshore data set should be used to compute the eastern boundary density contribution to the MOC. Contribution of eastern boundary density variability to MOC variability is ± 2.1 Sv. The overall MOC variability is ± 5.6 Sv (Cunningham et al., 2007).
Stuart A. Cunningham1, Torsten Kanzow1, Darren Rayner1, Harry L. Bryden1, Molly O. Baringer2, William E. Johns3, Jochem Marotzke4, Joel J.-H. Hirschi1, Lisa M. Beal3
1) National Oceanography Centre, Southampton2) NOAA-Atlantic Oceanographic and Meteorological Laboratory3) University of Miami, Rosenstiel School of Marine and Atmospheric Sciences4) Max-Planck Institute for Meteorology, Hamburg
The first objective of the RAPID programme is to establish a pre-operational prototype system to continuously observe the strength and structure of the Atlantic meridional overturning circulation (MOC). Observing the Atlantic MOC is the fundamental observational requirement of a programme to assess the role of the Atlantic thermohaline circulation (THC) in climate. The Rapid-MOC array at 26.5N incorporates observations from pre-existing and new systems to estimate the complete MOC signal. Components include: 1. Gulf Stream transport through Florida Strait by cable and repeat direct velocity measurements; 2. Ekman transports by satellite scatterometer; 3. deep western boundary currents by direct velocity measurements; 4. the basin wide interior baroclinic circulation from moorings measuring vertical profiles of density at the boundaries and on either side of the Mid-Atlantic Ridge and; 5. barotropic fluctuations using bottom pressure recorders. The array became operational in late March 2004 and is expected to continue until at least 2014. This paper aims at (i) giving a general overview over the different measurement components, (ii) demonstrating that the array works as proposed, and (iii) showing to modelers and other potential users the standard data products we derive from the observations. We show that the zonally integrated meridional flow tends to conserve mass, with the fluctuations of the different transport components largely compensating. We take this as experimental confirmation that the Rapid array is measuring the MOC. We then present a 30 month long time series of the MOC and of heat transport associated with it. We also present a first example of a model-observation synthesis application.
Lars Czeschel, David P. Marshall, Helen L. Johnson
University of Oxford
Under anthropogenic greenhouse forcing the Atlantic meridional overturning circulation is expected to weaken over the next century due to high-latitude warming and freshening. However, the ongoing global warming as seen in the upper North Atlantic ocean has not been monotonic. Departures from a steady warming on decadal timescales is well known. It is widely accepted that the meridional overturning circulation (MOC) plays an important role in driving decadal nera-surface temperature variations. Here, using the MIT adjoint model in an 1 degree global configuration, we show that anomalous forcing at high-latitudes triggers a chain of processes which results in an oscillation of the Atlantic meridional overturning and sea surface temperatures on decadal time-scales. This leads to the counter-intuitive result that a positive Atlantic overturning anomaly can emerge as a result of, although some years after, anomalous warming at high latitudes. Our study suggests that the variability of the MOC is sensitive to changes in the buoyancy fluxes and SST in the North Atlantic of at least the last four decades. This long-term memory makes it challenging to detect secular change in the MOC in ongoing observations.
Elizabeth J. Farmer, Julian E. Andrews, Mark R. Chapman
University of East Anglia, School of Environmental Sciences
Recent studies on the climate variability of the present interglacial have highlighted the presence of clear fluctuations, albeit on a lesser scale than those seen during the last glacial. These fluctuations are generally thought to be related to freshwater forcings influencing the nature of the North Atlantic meridional overturning circulation. It has been suggested that the well documented and widely recorded 8.2 ka BP event is the result of such forcing, caused by the catastrophic final drainage of the proglacial meltwater lakes Agassiz and Ojibway during the collapse of the Laurentide Ice Sheet. High resolution records of Mg/Ca ratios from planktonic foraminifera (Globigerina bulloides and Globorotalia inflata) have been produced for the Holocene sequence of the North Atlantic core MD99-2251, complementing existing multi-proxy data from the same core (Ellison et al. 2006). The G. bulloides Mg/Ca record extends to Termination 1 at ~ 100 year resolution, with a higher resolution (~ 20 year) section through the 8.2 ka BP cooling event. Mg/Ca derived temperatures reveal marked fluctuations throughout the Holocene. G. bulloides Mg/Ca temperatures range from ~8 - 12.5 ºC and show a slow warming into the interglacial, with a brief warm period encompassing the 8.2 ka BP event. This is followed by a mid-Holocene cooling before temperatures make a stepped increase ~ 4 ka BP. This late Holocene period is shown to have the highest temperatures of the whole interglacial but also greater variability. In order to further study this late Holocene warming, Mg/Ca ratios from the deeper dwelling planktonic foraminifera G. inflata were analysed for the last 5 ka section of the core. Mg/Ca ratio studies indicate greater Holocene variability than previously seen in other proxies from the same core. Comparisons between these different proxies and Mg/Ca records from a range of foraminiferal species highlight the nature of changes in surface hydrography during periods of rapid climate change, as well as potential ecological issues related to depth habitat and seasonality.
Silvia L. Garzoli1, Molly O. Baringer1, Shenfu Dong2, Christopher Meinen1
1) NOAA/AOML/PHOD2) Un. of Miami/CIMAS
This presentation will describe the current and planned observations of the Atlantic Meridional Overturning Circulation (AMOC) in the South Atlantic undertaken by the Atlantic Oceanographic and Meteorological Laboratory of NOAA. The cornerstone of AOML’s observational and analysis program in this region is centered around the high-density line expendable bathythermograph (XBT) line from Cape Town to Buenos Aires that has been occupied since July 2002 with 20 successful cruises to date. Using this data, a methodology has been developed to obtain heat transport from the XBT temperature profiles in which salinity is estimated from Argo profiles and CTD casts for each XBT temperature observation using statistical relationships developed for the region. The results from the analysis indicate a mean meridional heat transport of 0.54 PW (PW ~1015 W) with a standard deviation of ±0.11 PW. The geostrophic component of the heat flux has a marked annual cycle following the variability of the Brazil-Malvinas Confluence Front, and the geostrophic annual cycle is 180° out of phase with the annual cycle observed in the Ekman fluxes. As a result, the total heat flux shows significant interannual variability with only a small annual cycle. One of the largest uncertainties in the measured heat transport is the lack of direct measurements of the barotropic component of the flow that is largest to the west of 47°W. This is particularly important because at the western boundary the Malvinas Current and the North Atlantic Deep Water flow both in the same direction, creating a strong barotropic flow whose magnitude and variability are poorly described. To improve the observations at the western boundary of the AX18 line AOML will deploy an array of instruments consisting in three inverted echo sounders equipped with bottom pressure sensors (PIES) and one CPIES (a PIES also equipped with a near-bottom current meter). In addition to this new observational program, AOML is working to describe and characterize of the AMOC variability in the South Atlantic using both available observations and a non data-assimilative numerical model simulation of the AMOC. AOML is focusing on two model runs using the Hybrid Coordinates Ocean Model (HYCOM), with the aim of defining the importance of variations in inter-ocean and inter-basin exchange and the connectivity of the AMOC. It is expected that results will allow us to evaluate possible observing system components needed to characterize and monitor the AMOC in the South Atlantic.
Jonathan Gregory1, Remi Tailleux2, Grant Bigg3, Adam Blaker4, David Cameron5, Neil Edwards6, Alex Megann4, Pardaens2, Beena Sarojini1, Len Shaffrey1, Bablu Sinha4
1) Walker Institute, Department of Meteorology, University of Reading, and Met Office Hadley Centre2) Met Office Hadley Centre3) Department of Geography, University of Sheffield4) National Oceanography Centre, Southampton5) Centre for Ecology and Hydrology, Edinburgh6) Earth and Environmental Sciences, The Open University
The main tools that are used for making projections of anthropogenic climate change in the coming century are coupled atmosphere-ocean general circulation models (AOGCMs). According to the recent assessment of IPCC (2007) based on current AOGCM simulations, it is very likely that the meridional overturning circulation of the Atlantic Ocean will slow down during the 21st century. The multi-model average reduction by 2100 is 25% (range from zero to about 50%) for SRES emission scenario A1B. The wide range reflects an important uncertainty in model projections. The RAPID UK THC coupled model intercomparison project is investigating this systematic modelling uncertainty using a range of models of various resolution and complexity (CHIME, FAMOUS, FORTE, FRUGAL, GENIE, HadCM3, HadGEM1 and HiGEM). Integrations have been carried out (a) for a steady-state climate, (b) with CO2 increasing at 1% per year, and (c) in which a freshwater flux of 0.1 or 1.0 Sv is applied to the north Atlantic. Initial results will be presented on the response of the overturning circulation to these forcings. We also compare the unforced high-frequency variability with the RAPID array data.
Ian R. Hall1, Karin P. Boessenkool1, Harry Elderfield2
1) School of Earth and Ocean Sciences, Cardiff University, Cardiff, U.K.2) Department of Earth Sciences, University of Cambridge,
Variations in surface ocean conditions of the subpolar North Atlantic and near-bottom flow speed of Iceland-Scotland Overflow Water (ISOW) are documented in a 230-year-long deep-sea sediment record (RAPID-21-12B, 2630 m water depth) where the interaction of ISOW with the underlying sea-floor topography results in sediment focusing and enhanced sedimentation rates. 210Pb derived sedimentation rates are 2.3 mm/year. The subdecadal time scale proxy flow speed record of the last 230 years (Boessenkool et al., GRL 10.1029/2007GL030285, 2007) is presented along with paired oxygen isotope and Mg/Ca based sea surface temperature (SST) estimates from the surface-dwelling planktonic foraminifera Globigerina bulloides in co-registered samples from this sediment core. There is a large amount of scatter in the Mg/Ca-based SST, which probably reflects the large seasonal amplitude of SSTs in the region, and the nearby position of the subpolar front. Interannual changes in the timing of the occurrence or calcification of G. bulloides could also play a role. A lag-correlation is observed between records of deep flow speed of ISOW and of the stable oxygen isotopic composition of the surface ocean with strongest correlation when sortable silt mean leads seawater Oxygen-18O by 15-20 years. This offset most likely reflects size-selective biological mixing of the sediment record. Nonetheless, these records reveal a decadal-scale coupling between surface and deep ocean variability over the past 230 years, with implications for the Atlantic meridional overturning circulation and climate.
Paul R. Halloran1, Elena Colmenero-Hidalgo2, Ian R. Hall3, Rosalind E. Rickaby1
1) Oxford University, Department of Earth Sciences2) Departamento de Geología, Facultad de Ciencias, Universidad de 3) Cardiff University, School of Earth, Ocean and Planetary Sciences
Ocean acidification in response to rising atmospheric CO2 partial pressures is widely expected to reduce calcification by marine organisms. From the mid-Mesozoic, coccolithophores have been major calcium carbonate producers in the world’s oceans, today accounting for about a third of the total marine CaCO3 production. Here we present field evidence from the high latitude North Atlantic core RAPID 21-12-B of a 40% increase in average coccolith mass since ~1960. Furthermore we present data showing the complexity of the ecosystem response, and consider the implication for the ballasting and the calcification-CO2 feedbacks.
Ed Hawkins, Rowan Sutton
Walker Institute, University of Reading
Future decadal climate forecasts are likely to rely on ensembles initialised using small perturbations to ocean and atmosphere conditions. In order to design efficient ensembles there is a need to identify those perturbations that grow most rapidly. Such perturbations may also be useful to identify where new ocean observations could improve forecast skill. We have employed two different methods to estimate such optimal perturbations for decadal forecasts of the Atlantic Ocean in the HadCM3 GCM. Firstly, we use linear inverse modelling (LIM) to find the initial condition anomalies which grow most rapidly under a particular norm of interest. Significant non-normal amplification is found in the GCM, and the physical processes identified. We also demonstrate multi-decadal predictability of the overturning strength, and of basin-wide temperature and salinity fields. Secondly, we are using an ensemble based technique which, unlike the LIM approach, enables optimal perturbations to be estimated for specific initial conditions, e.g. a high or low overturning strength. The latest results will be discussed.
We explore the predictability of rapid changes in the Atlantic thermohaline circulation (THC) using a coupled global climate model (HadCM3). Rapid changes in the temperature and salinity of surface water in the Nordic Seas, and the flow of dense water through Denmark Strait, are found to be precursors to rapid changes in the THC, with a lead time of around 10 years. The mechanism proposed to explain this predictability involves variations in convection in the Nordic Seas which create density anomalies which propagate through Denmark Strait and along the deep western boundary current, affecting the overturning. These rapid changes in the THC have significant, and widespread, climate impacts which are potentially predictable a few years ahead. The presence of such predictability motivates the monitoring of water properties in the Nordic Seas and Denmark Strait.
Faced by the realities of a changing climate, decision makers in a wide variety of organisations are increasingly seeking quantitative predictions of regional and local climate. An important issue for these decision makers, and for organisations that fund climate research, is what is the potential for climate science to deliver improvements in such predictions? In particular, what is the potential to reduce the current high levels of uncertainty? Uncertainty in climate predictions arises from several distinct sources, which we separate and quantify using data from a suite of climate models. Focussing on regional predictions of surface air temperature change, we estimate the reduction in total prediction uncertainty that can potentially be delivered by climate science activities. Our method provides a possible basis for assessing the potential value of new investments in climate science, which could be compared with the potential economic savings associated with a quantified reduction in prediction uncertainty. We demonstrate that such investments could deliver significant improvements in predictions, especially for predictions of the next few decades.
Wilco Hazeleger
Global Climate Division, KNMI, The Netherlands
The Atlantic meridional overturning circulation (MOC) transfers a substantial amount of heat northward where it is released to the atmosphere. It is no surprise that when the MOC reduces strongly, the reduced ocean heat transport has a large impact on the global energy transport. Coupled climate models show that the atmospheric energy transports enhance, leaving the top of the atmosphere radiation remarkably constant. Bjerknes hypothesized this response already in his 1964 paper, hence the term ŒBjerknes compensation‚ has been used to describe the effect of compensating oceanic and atmospheric energy transport variations. In this presentation, ensemble simulations with the ECHAM5/OM model from the ESSENCE project will be used to study the details Bjerknes compensation, separating the impact of transient eddies, zonal mean flow, and different components to the moist static energy budget. The shift in the tropical zonal mean circulation and changes in transient eddies in the midlatitudes act to enhance atmospheric energy transport. Despite these large impacts on the global energy transports and Atlantic sea surface temperatures, the impact of a MOC collapse on 2-meter temperatures in Western Europe is surprisingly small. The small imprint appears to be the result of strong local cloud feed backs enhancing the oceanic cooling, but heating the continental regions. The result is a sharp gradient along the coastline of Western Europe and a relatively small impact on Europe.
Joël J.-M. Hirschi, Peter D. Killworth, Jeffrey R. Blundell, David Cromwell
A numerical model is used to test if the sea surface height (SSH) can be used as an indicator for the variability of Atlantic meridional oceanic mass transports. Our results suggest that if the transports over the western boundary current region and those in the eastern part of the basin are considered separately, high correlations are found between zonal SSH differences and the meridional transports in the top 1000m. A much weaker correlation is found for the basin-wide transport which corresponds to the surface branch of the meridional overturning circulation (MOC). For the eastern and western branches of the meridional transport combining the SSH signal with the baroclinic structure obtained from Rossby wave theory allows us to calculate a quantitative estimate of the transport variability in the top 1000m. The results of the method are less convincing for the variability of the MOC. The reason for this is that the variability for the eastern and western transports is large compared to the MOC. Therefore, even small relative errors in the variability estimates of the eastern and western branches can be large compared to the MOC variability obtained by adding the eastern and western branches. Nevertheless, being able to infer transport anomalies from SSH signals in the eastern and western parts of the Atlantic might prove useful in interpreting observations from the RAPID mooring array at 26N which show a large subannual variability that is mainly due to changes at the western boundary. Transports inferred from the SSH could help to identify the origin of this variability and whether transport anomalies propagate into the western boundary region from the basin interior or from other latitudes.
Jonathan Holmes, ISOMAP-UK Project Members
University College London
Oxygen-isotope ratios of the water molecule within precipitation provide an excellent means of tracing climate and climate-related processes. Stable isotopes of water are preserved in a range of natural archives both directly, for example in ice sheets and fluid inclusions, and indirectly in minerals and organic material within lake sediments, peat and speleothems. In certain cases, such palaeo-isotope records can be used to reconstruct the former oxygen-isotope composition of precipitation and so provide an insight into past climate. Past variations in isotope ratios from are often used to reconstruct past temperature, under the assumption that there is a close correlation between precipitation isotope composition and condensation temperature. Although temperature does indeed control precipitation isotope composition, especially in the mid- and high-latitudes, other factors are important. These factors include vapour source, airmass trajectory and rainout history. The conversion of isotopes values into estimates of temperatures not only introduces uncertainty, it also effectively excludes a wealth of other potentially valuable palaeoclimatic information. The aim of the RAPID ISOMAP-UK project was to compare estimates of past precipitation isotope composition from geological records with model simulations for late glacial and Holocene abrupt climatic events. Here, we focus on the early Holocene 8200-year event in Europe and the eastern seaboard of North America. We have incorporated isotope diagnostics into the atmospheric, oceanic and land surface components of the Hadley centre’s General Circulation Model, HadCM3. Present day model simulations for both oxygen isotopes and deuterium excess compare well with results from other isotope-enabled models and with observations from the IAEA GNIP (Global Network of Isotopes in Precipitation) database. We then Modelled the response of temperature and the oxygen-isotope composition of precipitation to a freshwater (-30 ‰ per mil) flux of 5 Sv for one year over the North Atlantic. This is approximately the amount of freshwater associated with the outburst from Lake Agassiz at the time of the 8200-year event. The ten-year average of the difference between the experiment with freshwater added and a control experiment shows marked reductions in the oxygen-isotope composition of precipitation. The ~1 per mil reduction in the isotopic composition during the 8200-year event along the western margin of Europe is in good agreement with geological evidence from lake sediments and speleothems that has been generated in this study and also obtained from published sources. However, the model simulations underestimate the amplitude of the perturbation over northeastern North America, which peat-bog cellulose isotope records suggest was as much as -4 per mil. The model shows that a marked temperature reduction accompanied the negative excursion in precipitation isotopes over Europe (especially northern Europe and Fennoscandia), Greenland and northeastern North America. However, the correlation between changes in temperature and precipitation isotopes is weak, confirming that factors other than temperature are likely to have controlled the isotope composition of precipitation during the 8200-year event. ISOMAP-UK is a UK contribution to the IGBP-PAGES ISOMAP (ISOtope calibration and MAPping study) initiative.
Chris W. Hughes1, Rory J. Bingham1, Miguel Angel Morales Maqueda1, Torsten Kanzow2
1) POL2) NOC, Southampton
While many forms of measurement of ocean dynamics produce results which are representative of only eddy dynamics in very local regions, ocean bottom pressure measurements are, for good dynamical reasons, representative of large-scale dynamics. This makes bottom pressure measurements an ideal means of monitoring the MOC. We give the dynamical reasoning behind this argument, and show that is supported by model diagnostics, demonstrating a reconstruction of more than 90% of variance in the MOC based on western boundary bottom pressures alone. We also show that it is supported by in-situ measurements from the RAPID WAVE and 26N arrays. We then go on to show how such measurements can be practically realised over multi-year timescales, given the current technical limitations of bottom pressure recorders, by using a combination of near-bottom density and current measurements. Sampling errors are discussed based on a variety of scenarios.
Vladimir Ivchenko1, Neil Wells1, Dmitry Aleynik2
1) National Oceanography Centre, Southampton, U.K.2) University of Plymouth
The Argo temperature and salinity profiles were used as our data for the calculations of anomaly of heat content (AHC) and anomaly of salinity content (ASC). For the analysis we used the data between January 1999 and December 2006 for the area between 10 deg. N and 70 deg. N. The temperature and salinity anomalies fields were calculated with an objective analysis scheme, based on Gandin [1965] and Bretherton et al. [1976].The AHC and ASC demonstrate positive trends in the upper 2000m of the North Atlantic over the last 8 years. The decisive contribution to the trend comes from the northern part of the basin (i.e. between 50 deg. N and 70 deg. N), whilst there is no obvious trend for both AHC and ASC in the southern and central basin. The strongest interannual variability occurs in the central North Atlantic. We should also add a caveat about the salinity data for the southernmost and northernmost sub regions between the end of 1999 and the beginning of 2000 where the paucity of data is an issue. The averaged values of the AHC and ASC for the whole period of time for the upper 2000m are negative for AHC and positive of ASC. The negative values of the AHC are unexpected, because in a number of studies a warming of the ocean was observed. The most plausible explanation lies in the climatology, which is based on a combination of data sets from different instruments (e.g. CTD, XBT), and can result in a possible bias (Gouretski et al., 2007). Gouretski et al. [2007] have shown that the XBT data has a positive bias in the temperature field when compared to CTD data. For the basin-averaged values the Argo data provides a rather stable base for the estimation of the AHC, and to some extent for ASC. Removing 50% and even 75% of the data leads to AHC and ASC values not too different to those values from the total data set. In most parts of the upper 2000m the time averaged AHC is statistically significant, as well as the time dependent series of the depth averaged AHC. There are clear signs for a negative AHC in the upper 1000m in most parts of the southern and mid-North Atlantic. There is also evidence for a positive sign of the AHC in the northern part of the North Atlantic. The number of salinity data in the first three years (i.e. 1999-2001) does not allow a decisive estimation of the ASC in the Northern Atlantic, especially in the southern and northern parts. The time-averaged ASC as a function of depth does not provide statistically significant values for estimation in most parts of the North Atlantic. However, at some places and some depths significant values are obtained, for example in the upper 200-300m in the south western North Atlantic, and in the layer between 900-2000m of the western North Atlantic between 50 deg. N and 60 deg. N. For this latter belt in the eastern North Atlantic a significant positive ASC is evident in the upper 900m.
Vladimir Ivchenko1, Sergey Danilov2, Dmitry Sidorenko2, Jens Schroeter2, Manfred Wenzel2, Dmitry Aleynik3
1) National Oceanography Centre Southampton2) Alfred Wegener Institute for Polar and Marine Research, Bremerhaven3) The University of Plymouth
Steric height (SH) variability computed from Argo profiling buoys data for the North Atlantic for period 1999-2006 is analysed and compared to the variability computed from the (TOPEX-Poseidon-Jason) satellite altimetry data. It is demonstrated that although the contribution from halosteric contraction is smaller than that from the thermal expansion it is not negligible in wide areas in the North Atlantic and cannot be discarded (the regression of trends in full steric and thermosteric heights is 0.73). It is found that SH variability is not really sensitive to increasing the reference level from 1000 to1500m. The range of 2000m is seemingly sufficiently deep to catch the trends of SH even on decadal time scales almost everywhere in the North Atlantic with exception, perhaps, of the Deep Western Boundary Current. Increasing the period of observations could make the contributions from deep layers more pronounced. It may also lead to the increase in the relative importance of the halosteric contribution. The comparison of SH and altimetric height variability shows qualitative agreement of both for the amplitude of the annual harmonics and trend, but reveals significant local differences which cannot be explained by bottom pressure variability. Seasonal variability of a few Sverdrups in major currents in the Northern Atlantic can easily be explained by a simple mechanism of nonlinear dependence of density on background temperature.
William Johns1, Harry Bryden2, Molly Baringer3, Lisa Beal1, Stuart Cunningham2, Torsten Kanzow2, Joel Hirschi2, Jochem Marotzke4, Zulema Garraffo1, Chris Meinen3, and Ruth Curry5
1) Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL USA2) National Oceanography Centre, Southampton, England3) NOAA Atlantic Oceanographic and Meteorological Laboratory, Miami, FL USA4) Max Planck Institute for Meteorology, Hamburg, Germany5) Woods Hole Oceanographic Institution, Woods Hole, MA, USA
Continuous estimates of the oceanic meridional heat transport in the Atlantic are derived from the RAPID-MOC observing system deployed along 26.5°N, for the period from April 2004 to October 2006. The basin-wide meridional heat transport (MHT) is derived by combining temperature transports (relative to a common reference) of the following continuously measured flow components: (1) the Florida Current in the Straits of Florida from calibrated submarine electromagnetic cable, (2) the western boundary region offshore of Abaco, Bahamas measured directly by current meter moorings, (3) the Ekman layer derived from satellite (QuickScat) wind stresses and Reynolds and Smith SST, and (4) the interior ocean measured by “endpoint” dynamic height moorings in combination with a seasonal hydrographic climatology across the ocean interior. The interior eddy heat transport arising from spatial covariance of the velocity and temperature fields is estimated independently from repeat XBT and CTD sections. The results for 2.5 years of available data show a mean MHT of 1.36 PW with standard deviation 0.41 PW for weekly-averaged estimates, on which time scale a basin-wide mass balance can be reasonably assumed. This mean estimate is higher by about 0.1 PW than most available MHT estimates from hydrographic sections and inverse model results at this latitude in the Atlantic. The continuous estimates range from a minimum of 0.2 to a maximum of 2.5 PW, indicating a very large MHT variability on intraseasonal time scales. Ekman heat transport variability accounts for 57% of the total MHT variance, and upon its removal the MHT variability arising from the geostrophic circulation drops to ±0.27 PW. The data suggest a seasonal cycle of the MHT with amplitude ~0.4 PW, with the maximum occurring in summer (July-September). As expected, the MHT variability is highly correlated with the strength of the meridional overturning circulation (also derived from the array), with an r2 of 0.93. Based on the autocorrelation statistics of the MHT time series, and accounting for measurement uncertainties, the overall uncertainty of the annually averaged MHT is ±0.11 PW, or less than 10% of the mean value. The MHT estimate derived from the RAPID-MOC array should therefore provide one of the best constrained benchmarks for indirect estimates from satellites or surface flux climatologies as well as numerical models.
Simon A. Josey, Jeremy P. Grist, Robert Marsh, Bablu Sinha
Various processes by which variability in air-sea heat and freshwater fluxes at interannual to interdecadal timescales affect the mid-high latitude North Atlantic Ocean are discussed through a combination of model and observation based results obtained under our RAPID funded project. First, the impact of extreme Nordic Seas heat loss on Denmark Strait (DS) dense water transport is examined in a.) control runs of the Hadley Centre HadGEM1 and HadCM3 coupled climate models, and b.) perturbation experiments with the fast coupled model FORTE which allow heat flux effects to be isolated from wind stress. All three models show an approximately linear increase in southward DS transport of cold dense water with increasing Nordic Seas winter heat loss in the range -80 to -250 Wm-2. In addition, a common response time is found with the strongest decrease in DS temperature occurring within 8-12 months of the heat loss signal. Second, an extension of the surface-forced overturning stream function approach of Marsh (2000) is used to estimate the maximum value of the meridional overturning circulation (MOC) at 48 oN. The method provides good agreement with model MOC variability when a past averaging window of 10 years is employed. This method is then applied with NCEP/NCAR reanalysis surface flux fields to reconstruct MOC strength over 1953-2007. Finally, observational datasets and atmospheric reanalyses are used to link freshening of the eastern subpolar gyre over the last 40 years to increases in the surface freshwater flux from the atmosphere to the ocean.
Torsten Kanzow1, Rory Bingham2, Harry Bryden1, Chris Hughes2, Stuart Cunningham1
1) NOC, Southampton, UK2) Proudman Oceanographic Laboratory, Liverpool
Ocean bottom pressure (OBP) is a measure of the mass of the entire water column and the overlying atmosphere. Continuous observations of spatial differences in OBP allow for the calculation of time-variable near bottom spatially-integrated geostrophic flow. Since April 2004 OBP measurements have been carried out in the subtropical North Atlantic (along 26 N) - at the base of the western margin, at the base of both flanks the Mid-Atlantic Ridge (MAR) and at the eastern margin - under the auspices of the RAPID-MOC programme. Simultaneous OBP measurements from the tropical Northwest Atlantic (MOVE project) and from the western margin of the mid-latitude North-Atlantic (RAPID-Wave project) are also available. The current study focuses on OBP variability at periods shorter than 10 days (high frequency) and seasonal time scales. In the high frequency limit, OBP displays coherent (in phase) fluctuations over the entire zonal extent of the Atlantic (~ 6000 km). These fluctuations are most likely to be linked to the global atmospheric Rossby-Haurwitz pressure pattern, as our correlation analysis of global daily fields of sea level pressure (SLP) implies. We find positive correlations between observed OBP and SLP over large parts of the Atlantic and negative correlations with the West-Pacific. Although fluctuating in phase, the amplitudes of OBP are significantly larger at the western than at the eastern margin (± 1.4 mbar vs. ± 0.9 mbar), implying that there is meridional flow (showing fluctuations of ± 8 Sv) associated with it. A mode analysis reveals that time variable pressure gradients (or geostrophic transports) are significantly larger across the western basin of the Atlantic than across the MAR or the eastern basin. The de-correlation scale of the transports clearly exceeds 1000 km, and could be the signature of Rossby waves. Thus, parts of the variability in OBP appears to be a rather static response to atmospheric pressure, which does not involve changes in sea surface height (SSH). Another part of the response is dynamical, which - given the large transport coherence scales – should result in large scale high-frequency SSH fluctuations. We also observe pronounced OBP variability on seasonal time scales at different sites in the subtropical Atlantic. Parts of the signal appear to be related to temporal changes in regional wind stress curl. The de-correlation scale for these signals clearly exceeds 1000 km. Thus, like in the high-frequency limit, seasonal changes in the zonal pressure gradient across the MAR appear to be weak, which has an important bearing on the observation of the meridional overturning circulation. Given the large coherence scales, OBP derived from remote sensing (GRACE) appears to detect these variations. However, in the region of the deep western boundary current, where local variations dominate, GRACE fails to recover the variability observed with the in-situ measurements.
Torsten Kanzow1, Stuart A. Cunningham1, Darren Rayner1, Molly O. Baringer2, William E. Johns3, Joel J.-M. Hirschi1, Lisa M. Beal3, Christopher Meinen2, Harry L. Bryden1
1) National Oceanography Centre, Southampton2) NOAA-Atlantic Oceanographic and Meteorological Laboratory3) University of Miami, Rosenstiel School of Marine and Atmospheric Sciences4) Max Planck Institute for Meteorology, Hamburg
The vigour of Atlantic Meridional Overturning Circulation (MOC) is thought to be vulnerable to global warming, but a lack of understanding of its shorter-time variability means that changes inferred from sparse observations on the decadal timescale of recent climate change are uncertain. From continuous measurements of the MOC (from 29th March 2004 to 31st March 2005) using the purposefully designed Rapid array of moored instruments deployed along 26.5 N and cable measurements of Florida Current transport the year-long average overturning is 18.7 ± 5.6 Sv (range 4.0 to 34.9 Sv). In this talk we will present the first analysis of a further 18 months of observations from the Rapid array (1st April 2005 to October 2006). We will compare transports and variability from 2004 to 2006 and examine the contributions to MOC variability from Gulf Stream, Ekman and interior baroclinic and barotropic transport fluctuations. The 30 month long time series displays pronounced seasonal variations with an amplitide of roughly 4 Sv. While the seasonal variations are dominated by geostrophic upper mid-ocean and Gulf Stream transports, the sub-seasonal variation are mostly due to changes in Ekman transports. Finally, a feasibility study to use sea surface height measurements as an indicator of changes in MOC transports will be presented.
Thomas Kleinen1, Tim Osborn2, Keith Briffa2
1) Potsdam Institute for Climate Impact Research, Potsdam, Germany2) Climatic Research Unit, University of East Anglia, Norwich, UK
We are investigating the climate of the Little Ice Age. While cooling as a simple, direct response to the reduced insolation due to solar minima, as well as high concentrations of volcanic aerosols, may explain part of the changes in climate during the Little Ice Age, it is possible that dynamic changes in the climate system also play an important role. Dynamical changes in the atmosphere or ocean may themselves be part of the response to solar and volcanic forcing, but this is currently unclear because many climate models simulate only a relative weak dynamical response to changes in external forcings. In order to explore the potential causes of the Little Ice Age climate anomaly, we are conducting sensitivity experiments with the GCM HadCM3 to investigate the consequences of three candidate mechanisms for North Atlantic climate. The candidate mechanisms are: (i) a reduction in North Atlantic MOC, (ii) changes in North Atlantic atmospheric circulation to negative NAO states, and (iii) reduced radiative forcing due to solar minima and volcanic aerosols. We will present results from experiments implementing these mechanisms. The climate change resulting from these model experiments will be compared to the existing palaeoclimatic data on Little Ice Age climate in order to determine, if possible, which of these mechanisms were likely to have been important for the climate during the period we call the Little Ice Age. We will highlight the spatial patterns of climate change resulting from the candidate mechanisms, and investigate whether available data is sufficient to prove or disprove any of the hypotheses.
Greta B. Kristjansdottir, Harry Elderfield, Nick McCave, Luke Skinner
The Godwin Laboratory for Palaeoclimate Research, Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
Material and data were collected at 41 sites in the subpolar North Atlantic Ocean between Scotland and Newfoundland during the RRS Charles Darwin CD159 cruise. Sites were selected to reflect the major inputs of water that becomes the North Atlantic Deep water (NADW); the Iceland-Scotland overflow water (ISOW), the Denmark Strait Overflow water (DSOW) and the Labrador Sea water (LSW). A total of 29 box cores, 19 piston cores, 6 kasten cores, 9 short gravity cores and 20 CTD casts as well as 28 surface water samples were collected during the cruise. In this project we focus on the sediment core-top samples and water samples collected from the CTD casts. The cores were sampled in 1 or 0.5 cm intervals and we used the top 1 or 2 cm, depending on availability of foraminifera in the samples. Sediment samples were disaggregated on an end-over-end wheel, wet sieved at >63 um, and dry sieved to 63-150 and >150 um. Accelerator Mass Spectrometer (AMS) radiocarbon dating was done for each core top based on between 900-1600 monospecific planktonic foraminifera (Globigerina bulloides or Neogloboquadrina pachyderma (sinistral)). All dates were of modern or late Holocene age except site RAPID-08-5B (9806+/-38 uncorrected 14C BP) and site RAPID-14-10B (11543+/-40 uncorrected 14C BP). The >150 um fraction was split until approximately 300 foraminifera remained and counted for number of lithic grains, benthic foraminifera, planktonic foraminifera and foraminifera fragments. In all but the shallowest sample (Greenland rise, 761m water depth) benthic foraminifera constituted less than 2% of the total >150 um fraction of the sample. A variety of benthic foraminifera species were picked from the >150 um fraction for geochemical analyses (stable isotopes and trace elements) with the aim of improving our understanding of benthic Mg/Ca-temperature calibrations, an important tool of paleoceanographers.
John Lowe1, Simon Blockley2, Alison Macleod1, Jonathan Merrit3, Adrian Palmer1, Sean Pyne-O'Donnell1, James Rose1, Michael Walker4
1) Royal Holloway, University of London2) Research Laboratory for Archaeology, Oxford University3) British Geological Survey, Edinburgh4) Archaeology & Anthropology, University of Wales, Lampeter
The mission of this RAPID-sponsored project was to develop more precise procedures for synchronising palaeoclimate records for the last glacial-interglacial transition (the LGIT, 16,000 to 8,000 years ago), a period that witnessed a series of abrupt climatic changes. Two main approaches have been employed: volcanic ash stratigraphy and varve chronology. Used in combination, they provide independent tests of age models derived using other methods while better constraining them. Volcanic ash layers are increasing in importance as a correlation tool due to the discovery of widespread non-visible, distal ash layers in a range of sedimentary contexts. We will present a synopsis of the significant contributions that this RAPID team has made to the development of the ash chronology of the LGIT in the North Atlantic/NW Europe region. Key developments have been: • discovery of new distal ashes, not previously reported in the tephra record • discovery of fine ash layers in marine sediments located in the western North Atlantic • extension of the footprint of one ash bed, which originated from a volcano in Iceland, to as far east as the Ural Mountains and as far south as Switzerland • improved geochemical procedures for determining which ash layers possess diagnostic chemical signatures (tephra ‘fingerprinting’). The potential of these data for synchronising palaeoclimatic records and their application beyond RAPID will be briefly explained. Varves are seasonally-generated sediment layers with alternating ‘winter’ and ‘summer’ units, distinguishable on sedimentary or other properties. Identification of varved deposits enables events to be dated with an annual precision, rivalling tree-ring and Greenland ice-core records in temporal resolution. The RAPID team has been examining varved records of Younger Dryas age from sites in Scotland that accumulated in lakes created when glacier ice blocked valley exits. The aims of the study are to determine the potential of these sequences for (a) dating past climatic events with at least a decadal precision, and (b) permitting high-resolution comparisons between environmental reconstructions from Britain and the Greenland ice-core records. The results provide more precise age estimates for the time of maximal ice advance in the Scottish Highlands as well as for subsequent rapid melting of the glaciers, both developments being responses to markedly different climatic impacts. The varve records also reveal short-term (decadal) oscillations in character and development, which appear to closely match decadal variations in the annual ice layers of Greenland. This in turn suggests that both the Greenland and Scottish ice masses were responding synchronously to the same external forcing factor. The presentation will close with a brief overview of the climatic implications that can be drawn from the data furnished (thus far) by this project
M. Susan Lozier
In 2007, the U.S. Joint Subcommittee on Ocean Science and Technology identified as a near-term priority in the Ocean Research Priorities Plan the “improved understanding of the mechanisms behind fluctuations of the MOC, which will lead to new capabilities for monitoring and making predictions of the MOC changes.” In response to this priority, U.S. scientists drafted a five-year implementation strategy for a new inter-agency program that, together with activities from the U.S. Climate Change Science Program and international partnerships, will develop components of an AMOC monitoring system and AMOC prediction capability. The recommended activities and foci are designed to take advantage of rapidly advancing observing, modeling, and assimilation capabilities, as well as to leverage substantial international investment. Current and planned U.S. AMOC activities and priorities will be discussed. Part 2: In Pursuit of the Conveyor Belt and its Variability A widely held view is that the Deep Western Boundary Current (DWBC) is primarily responsible for the export of recently-ventilated watermasses from the northern North Atlantic to the rest of the global ocean. New subsurface float observations and simulated “e-floats”, however, have revealed that the DWBC is in fact not a continuous pipeline for the export of Labrador Sea Water from the subpolar to subtropical latitudes. Only a small percentage (<10%) of observed and simulated floats released in the DWBC in the subpolar region continuously followed the path of the DWBC to the subtropics. Together, these observational and modeling results challenge the concept of the DWBC as a “conveyor belt”, supporting instead a more diffusive process including interior pathways and eddies. Results from the RAFOS float observational program, its accompanying modeling study and an analysis of historical hydrographic data will be discussed.
M.A. Morales Maqueda1, I. Walkington2, A.J. Willmott1
1) Proudman Oceanographic Laboratory, 6 Brownlow Street, Liverpool, L3 5DA2) Department of Engineering, The University of Liverpool
Sea ice growth in wind-driven, coastal polynyas is believed to be an important mechanism for the formation of dense, intermediate and deep waters in the Arctic Ocean and subarctic seas. Coastal polynyas are typically only a few kilometers wide and so are difficult to resolve in current sea ice-ocean models. For this reason, the dynamics of these polynyas have been often investigated with relatively simple flux models that use the ice continuity, or mass balance, equation to calculate polynya evolution [Willmott et al., 2007]. Here we discuss the strengths and weaknesses of the flux model approach and extend it to include conservation of momentum as well as mass. We show that mass and momentum conserving models behave in ways sensibly different from traditional flux polynya models. In particular, while flux models admit polynya solutions that open to a steady state in times scales of hours to days, models that conserve both ice mass and ice momentum tend to create polynyas that open indefinitely. REFERENCES Willmott, A. J., Holland, D. M. and Morales Maqueda, M. A., 2007. Polynya Modeling. In Polynyas: Windows into Polar Oceans. Eds. W. O. Smith and D. Barber. Elsevier Oceanography Series (David Halpern, series Ed.).
Robin McCandliss, Kevin Marsh, Mark Hebden, Julie Collins, Zoe Aston
RAPID Data Centre
Overview of progress on archiving observations and model output from RAPID projects.
Robin McCandliss1, Julie Collins1, Mark Hebden1, Zoe Aston1, Kevin Marsh2
1) British Oceanographic Data Centre2) British Atmospheric Data Centre
The RAPID Data Centre (RDC) provides data support to all projects in the RAPID Directed Mode programme. It is made up of the British Oceanographic Data Centre (BODC) and the British Atmospheric Data Centre (BADC), the NERC Designated Data Centres for Marine and Atmospheric sciences, respectively. The BODC is responsible for the archival of observational data, and the BADC holds numerical model output. The RDC provides a long term archive for RAPID datasets. It also gives comprehensive advice on all aspects of data management, and promotes data sharing and collaboration between the various RAPID projects. The RDC supports the RAPID community by providing third-party datasets, on-line tools for data analysis, operating RAPID mailing lists, and a dedicated user helpdesk.
Lorna M. McLean, Brian A. King
The scales over which ocean properties vary play an important part in the assimilation of ocean data. In this study Argo data have been used to develop a method of estimating the correlation scales of salinity on a potential temperature surface. The correlation scales of salinity anomalies relative to a reference field from WOA05 have been examined. For the development of the method, three test regions in the Pacific Ocean were chosen and scales are estimated on the 6° theta surface (approximately 500 m in depth). For pairs of data in a region, the difference in salinity anomaly is found. 7 years of Argo data are used but pairs are only included when the observations fall within a 10-day window. The salinity differences are then divided into 50 km bins according to the distance between the data points. The median difference in salinity is then calculated for each bin. A curve is fitted to the data varying exponentially from the near field to a far field limit based on a function devised by Boehme and Send [2005, Deep Sea Research II, 52, 651-664]. By varying the scale parameter in the equation and finding the best fit to the data (the lowest rms error) the correlation scale is found. This method is used to examine the variability of correlation scales with region and on varying potential temperature surfaces. A comparison with the variability of these scales on depth levels is also made.
Jerry F. McManus
1) Dept. Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA2) Dept. Earth and Environmental Sciences, Columbia University,
Past ocean circulation changes may be expressed in water mass tracers and dynamical proxies. We have examined a combination of the 231Pa/230Th and sediment grain size indicators with nutrient proxies to reconstruct the evolution of the North Atlantic meridional overturning circulation since the last ice age. Deep-sea core locations in the western basin monitor the export of northern-sourced waters, especially along the deep western boundary. Cores from a range of water depths allow reconstruction of both intermediate and deep waters. During the last glacial maximum, northern source waters shoaled relative to the modern circulation and may not have overflowed from the Nordic seas, but they appear to have been well ventilated and part of a vigorous overturning cell. Intermediate depth sediments south of Iceland display a similar grain size spectrum as the modern, and sedimentary 231Pa/230Th at shallower sites was generally reduced, in contrast to higher ratios at deep sites. During the deglaciation, large-amplitude variability characterized the climate as well as the meridional overturning circulation at all depths. The most prominent excursion to high 231Pa/230Th indicating reduced export occurred at deep sites along the western boundary. The circulation rebounded at all depths at the onset of the Bolling-Allerod, and declined again during the Younger Dryas. A major increase in the strength of the deep circulation accompanied the onset of the Holocene. Only smaller oscillations occurred once the modern configuration of overflows from the Greenland and Norwegian Seas was established, although an early Holocene reduction was significant.
Douglas J. McNeall1, Peter G. Challenor2
1) Met Office Hadley Centre, UK2) National Oceanography Centre, Southampton, UK
We present a calibrated uncertainty analysis of the strength of the MOC over the 21st century; such an analysis is a prequisite for estimating the probability of MOC collapse. We combine observational data from the RAPID array, with a 92 member ensemble of the Earth system model of intermediate complexity, GENIE-1. The calibration of GENIE-1 is based on a rejection sampling approach, accepting an input configuration if the output from the model is close to the observational data. We use a Gaussian process emulator, trained on the model ensemble, to ensure that we have enough samples from the input space for the calibration. Calibrated model inputs are then used to predict the MOC in the future. We combine the emulator with principal components analysis to reconstruct the probability density function of the timeseries of the MOC through the 21st century.
Alex Megann, Adrian New, Adam Blaker
The CHIME coupled climate model comprises the atmosphere and ice models used in the Hadley Centre's HadCM3, coupled to the Hybrid-Coordinate Ocean Model (HYCOM), which uses layers of constant potential density in the interior and constant-depth layers near the surface. CHIME is designed to be identical to the Hadley Centre's HadCM3 model except for its ocean component, allowing us to investigate the sensitivity of the climate to the choice of vertical coordinate in the ocean model. A 200 year control experiment using CHIME is well underway, and transient experiments with 0.1Sv freshwater hosing and with CO2 increasing at 1% per year have also completed several decades. The effect of these perturbations on the Atlantic overturning circulation and on the meridional steric height gradient will be presented.
Christopher S. Meinen1, Molly O. Baringer1, Rigoberto F. Garcia2
1) NOAA/Atlantic Oceanographic and Meterological Laboratory2) Univ. Miami/Cooperative Institute for Marine and Atmospheric Studies
As the Florida Current passes through the northern Straits of Florida it carries both the bulk of the Subtropical Gyre western boundary current flow and the majority of the warm upper limb of the Meridional Overturning Circulation. More than forty years of Florida Current transport estimates are used to illustrate the difficulties in extracting annual and longer time scale variability in the presence of a large amount of higher frequency energy. It is shown that for the Florida Current the annual cycle represents less than 10% of the total variance in the long-term record, while interannual (13-42 month) variability represents only 13% of the total variance and periods longer than 42 months (out to a maximum observable period of about 12 years) represents less than 10% of the total. Given the observed variability of the Florida Current, in order to get a monthly mean value that is accurate to within 0.5 Sv at the one standard error level more than 20 daily observations are needed. To obtain an estimate of the annual cycle that is accurate to within 20% of its own standard deviation at the one standard error level at least 24 years of daily data is needed. More than 40 observations spread throughout a year are required to obtain an annual mean that is accurate to within 0.5 Sv. Despite these daunting data requirements, there is sufficient data now to evaluate both the annual cycle of the Florida Current transport with a high degree of accuracy and to begin to determine the longer period variability of the Florida Current. Comparison of the 40+ year record of the Florida Current to the North Atlantic Oscillation index and to the wind stress curl over the basin interior illustrates that a recently described Sverdrup-based mechanism explains a significant fraction of the long-period variability, although the forcing for the mechanism does not appear to be consistent over the full record.
Katie Miller1, Mark Chapman1, Julian Andrews1, Nalân Koç2
1) School of Environmental Sciences, University of East Anglia (UEA), Norwich, NR4 7TJ, UK.2) Norwegian Polar Institute N-9296 Tromsø, Norway
While it is widely accepted that interglacial climates are more stable than glacial climates, palaeoceanographic studies of the subpolar North Atlantic have indicated significant climate variability during the current interglacial period. A number of periods of lower temperatures and increased iceberg discharge have been identified, most notably the ‘8.2 kyr cooling event’, throughout the Holocene. This study looks at Holocene variability as recorded in marine sediment cores MD99-2251 and MD99-2252 from the Gardar Drift in the subpolar North Atlantic. Core MD99-2251 extends throughout the Holocene, while Core MD99-2252 is a record of the last 7kyr, based on a calibrated 14C time scale. We have generated high resolution multi-proxy data using quantitative diatom assemblages as an indicator of sea surface conditions and analyses of terrigenous material in the sediments as an indication of drift ice flux over the coring site. Sea surfaces temperatures generated from diatom assemblages for sediment core MD99-2251 indicate an initial warming at the beginning of the Holocene followed by a broad cooling from around 9kyr to 7kyr with an increase in the dominance of the subarctic flora associated with this cooling. Variations in the amount of ice rafted material in core MD99-2251 reveal distinct but low level multidecadal and centennial scale inputs but no strong enhancement in terrigenous influx associated with the 8.2 kyr cooling. A subsequent warming and the establishment of a typical temperate Atlantic water flora is recorded in both sediment cores at ~7kyr, with relatively muted variability through the remainder of the Holocene.
Aazani Mujahid, Torsten Kanzow, Harry Bryden
The subannual variability in the mid-ocean thermocline recirculation of the Atlantic meridional overturning circulation as measured by the Rapid array 26°N has a standard deviation of 3.1 Sv as reported by Cunningham et al. (2007). Such variability is notably smaller than estimates of 6 Sv by Ganachaud (2005) and of 16 Sv by Wunsch (2008) based on models of eddy variability. Here we discuss the reasons for the smaller than expected variability in the upper layer flow by examining the nature of compensation mechanisms for the mid ocean flow and by comparing sea surface height variability based on satellite altimetry with dynamic height variability observed by the Rapid array.
Eleanor O'Rourke1, Chris Hughes1, Ric Williams2, Vassil Roussenov2
1) Proudman Oceanographic Laboratory2) Department of Earth & Ocean Sciences, University of Liverpool
Johnson & Marshall (2002) idealised model study found that the North Atlantic responded very rapidly to changes in deep water formation. This result obviously has important implications for the rapid climate change problem. However, their work neglected all topographic influences on the surface dynamics. Therefore an idealised model study using the MIT general circulation model (MITgcm) has been used to investigate the effect of a change in deep water formation on an ocean with topography. An initial control run with vertical sidewalls found adjustment taking place on rapid timescales. Further experiments were then completed using a selection of topographic profiles and compared to both the control run and a detailed analysis of the Johnson & Marshall (2002) work.
Adrian Palmer, Alison MacLeod, John Lowe, Jim Rose
Department of Geography, Royal Holloway University of London, Egham Hill, Egham, Surrey, TW20 0EX, UK
The Greenland ice-core records demonstrate that pronounced, decadal-scale climate shifts occurred during the last glacial cycle. To conduct high-resolution regional correlations, palaeoclimatic records obtained from marine and terrestrial sedimentary environments need to be reconstructed with comparable temporal resolution. The only sediment records that will normally enable this degree of age precision are annually-laminated (varved) deposits. Varved sediments which span the Last Termination have been reported from sites in continental Europe, Scandinavia and Greenland, but no detailed investigations of assumed varved deposits have been reported from sites in the British Isles. Here we report on progress with the construction of a varve chronology for the Last Termination in the British Isles. Initial investigations have focussed on varved deposits that accumulated in two areas of Scotland where ice-dammed lakes were formed by the advance of glacier ice during the Younger Dryas Stadial (GS-1 in the Greenland stratotype sequence). The first concerns a series of lakes in the Glen Roy-Glen Spean area, and the second, Glacial Lake Blane, was blocked by ice advancing beyond the southern shores of Loch Lomond. We also report on attempts to employ tephrochronology as an independent tool for correlating the Scottish varved sequences with other records in Greenland, NW Europe and the North Atlantic. Identification, measurement and detailed analysis of the Scottish varved deposits are based on thin section micromorphology and sediment geochemistry (XRF). The summer layers vary in number and thickness of sub-laminations (sediment flux events), while both winter and summer layers show considerable structural variation. We contend that detailed examination of the internal structure of summer layers provides a more reliable index of variations in local summer climatic conditions than measurement of varve thickness. The latter is more likely to reflect changes in sediment supply and surge events not directly related to climate forcing. Comparison of summer layer data with the δ18O variations in the GRIP ice-core record (ss08sea and ss09) for the latter part of GS-1 shows a visual cross-match between both records. The comparison suggests that: (i) 515 varves accumulated between 12,119 and 11,506 GRIP years BP; (ii) GS-1 ice cap on mainland Scotland reached its maximum extent ca. 840 years after the onset of GS-1 in Greenland (11,797 GRIP years BP); (iii) Greenland ice and glaciolacustrine sedimentation in Scotland appear to have responded to a common forcing factor - perhaps decadal migrations of the North Atlantic Polar Front or variations in solar radiation output. Current work aims to replicate and test for consistency between varved sequences, and to independently constrain their ages and detailed correlations with other records using tephrostratigraphy. Future work will focus on comparison to the GICC05 timescale.
C.F. Postlethwaite, M.A. Morales, G.R. Tattersall, J. Holt, Willmott
Proudman Oceanographic Laboratory, 6 Brownlow Street, Liverpool, L3 5DA
Dense water formation occurs in ice covered seas when brine is rejected from newly forming ice. Dense water formation can be modulated by tides in several ways. Tidal mixing can hinder ice growth or even cause melting, as oceanic heat is transported upwards. A layer of fresh melt water can inhibit convection. Additionally the ebb and flow of the tide can cause sea-ice to pile up (in areas of convergence) or separate (in areas of divergence). Thicker, piled up ice thermally insulates the ocean from the atmosphere and thus further dense water formation becomes less likely. Conversely, areas of open water exposed as the tides pull the ice cover apart, start to produce dense water as brine is rejected from newly formed sea-ice. We present results from a dynamic/thermodynamic sea-ice model coupled to a baroclinic coastal ocean model of the Barents and Kara Seas. By introducing tides into the model, sea-ice volume is found to increase. Interestingly, this ice is distributed over a smaller surface area, likely due to the mechanical redistribution of both existing and newly formed sea-ice. Introducing tides into the model does not alter the annual salt flux to the ocean significantly. However, the distribution of increased brine rejection due to tides is spatially inhomogeneous and some coastal regions show significantly increased salinity throughout the water column. Further work will determine the significance of these results and indicate whether future Global Climate Models should include tide/sea ice interactions to make more accurate predictions.
Sean D.F. Pyne-O'Donnell1, Simon P.E. Blockley2, Adrian Palmer1, Alison MacLeod1, J. John Lowe 1, J. Rose 1, Mike J.C. Walker 3, Jon Merrit 4
1) Centre for Quaternary Research, Geography Department, Royal Holloway University of London, UK2) Research Laboratory for Archaeology and the History of Art, Oxford University, UK3) Department of Archaeology and Anthropology, University of Wales, Lampeter, UK4) British Geological Survey, Murchison House, West Mains Road, Edinburgh, UK
A number of Icelandic ash isochrons of Last Termination and early Holocene age occur throughout the North Atlantic region, both in visible (macrotephra) and distal fine ash (‘micro-‘/’crypto-tephra’; <100 μm) form. Some can be traced between terrestrial, marine and ice-core sequences and hence offer considerable potential for testing precise correlations between diverse sedimentary records (Davies et al. 2002). Two well-established ashes, the Saksunarvatn Ash and Vedde Ash, are of particular importance in this respect due to their widespread geographic distribution and stratigraphic positions. They have been detected in the Greenland ice-cores, giving ages of 10,347 (mce 89) GICC05 yr b2k and 12,171 (mce 114 ) GICC05 yr b2k respectively (mce is the ‘mean counting error’ of the GICC05 timescale; Lowe et al., 2008). Additional ash isochrons have been detected in recent years which enhance the tephrostratigraphical framework and potential for the North Atlantic region (Turney et al., 1997; Björck & Wastegård, 1999; Wastegård, 2002; Davies et al, 2003; Pyne-O’Donnell, 2007). However, it is also becoming more apparent that this framework is not yet as fully established or clearly defined as previously thought, but includes a number of chemically equivalent or equivocal ashes, several of which are closely spaced in time (Davies et al., 2004; Pyne-O’Donnell et al, in press). Here we report on attempts to trace these ash isochrons into North Atlantic shelf and deep marine sequences, outline some of the problems involved, and assess the potential for overcoming them. Resolving the difficulties will lead to clarification of the roles played by ice-rafting and air-fall transport in the dissemination of the various tephras, to a fuller understanding of the distorting effects of bioturbation on ash layers in marine sequences, and ultimately to more precise correlations between marine, terrestrial and ice-core records. This research forms part of the NERC ‘RAPID Climate Change’ thematic programme, project no. NE/C509158/1 Björck, J., Wastegård, S. (1999). Climate oscillations and tephrochronology in eastern middle Sweden during the last glacial-interglacial transition. Journal of Quaternary Science 14, 399-410. Davies, S. M., Branch, N. P., Lowe, J. J., Turney, C. S. M. (2002). Towards a European tephrochronological framework for Termination 1 and the Early Holocene. Philosophical Transactions of the Royal Society of London A 360, 767-802. Davies, S. M., Wastegård, S., Wolfarth, B. (2003). Extending the limits of the Borrobol Tephra to Scandinavia and detection of new early Holocene tephras. Quaternary Research 59, 345-352. Davies, S. M., Wohlfarth, B., Wastegård, S., Andersson, M., Blockley, S. P. E., Possnert, G. (2004). Were there two Borrobol Tephras during the early Lateglacial period: implications for tephrochronology? Quaternary Science Reviews 23, 581-589. Lowe, J. J., Rasmussen, S. O., Björck, S., Hoek, W. Z., Steffensen, J. P., Walker, M. J. C., Yu, Z., INTIMATE group. (2008).Synchronisation of palaeoenvironmental events in the North Atlantic region during the Last Termination: a revised protocol recommended by the INTIMATE group. Quaternary Science Reviews 27, 6-17. Pyne-O’Donnell, S. D. F., (2007). Three new distal tephras in sediments spanning the Last Glacial-Interglacial Transition in Scotland. Journal of Quaternary Science 22, 559-570. Pyne-O’Donnell, S. D. F., Blockley, S. P. E., Turney, C. S. M., Lowe, J. J. (in press). Distal volcanic ash layers in the Lateglacial Interstadial (GI-1): problems of stratigraphic discrimination. Quaternary Science Reviews 27, 72-84. Turney, C. S. M., Harkness, D. D., Lowe, J. J. (1997). The use of microtephra horizons to correlate Late-glacial lake sediment successions in Scotland. Journal of Quaternary Science 12, 525-531. Wastegård, S. (2002). Early to middle Holocene silicic tephra horizons from the Katla volcanic system, Iceland: new results from the Faroe Islands. Journal of Quaternary Science 8, 723-730.
Detlef Quadfasel
Universität Hamburg, Zentrum für Meeres- und Klimaforschung, Institut für Meereskunde,
Direct observations and model simulations of the exchanges across the Greenland-Scotland Ridge reveal remarkably stable fluxes over the past decades, as changes in space and time of the water mass formation in the Arctic Mediterranean appear to be buffered and filtered out in the Nordic Seas. We also report on process studies seeking to quantify the entrainment of ambient waters into the sinking overflow plumes south of the ridge. Both processes, overflows and entrainment, contribute about one third each to the volume transport in deep limb of the Atlantic Meridional Overturning Circulation.
Darren Rayner1, Stuart A. Cunningham1, Torsten Kanzow1, Harry L Bryden1, Joel J.-M. Hirschi1, Jochem Marotzke2, William E. Johns3, Lisa M. Beal3, Molly O. Baringer4
1) National Oceanography Centre, Southampton2) Max-Planck Institute for Meteorology, Hamburg3) University of Miami, Rosenstiel School of Marine and Atmospheric Sciences4) NOAA-Atlantic Oceanographic and Meteorological Laboratory
Objective 1 of the RAPID programme is to establish a pre-operational prototype system to continuously observe the strength and structure of the basin wide Atlantic circulation. The meridional circulation is a balance of the northward flux of the Florida Current plus Ekman transport by the southward thermocline and North Atlantic Deep Water flux. The zonal integral of this circulation defines the meridional overturning circulation (MOC). While parts of the MOC are wind-driven, the basin-scale Atlantic MOC is largely buoyancy-forced. Hence, observing the Atlantic MOC is the fundamental observational requirement of a programme aiming to assess the role of the Atlantic thermohaline circulation (THC) in rapid climate change. The northward flow of warm water in the Florida Current is measured using telephone cables between the US and Bahamas; and the compensating southward circulation of cool thermocline and cold deep water are observed using the RAPID-MOC transatlantic array of moorings. The moorings measure top-to-bottom density profiles and bottom pressure near America and Africa and on either side of the mid-Atlantic Ridge. The transatlantic mooring array was first deployed in spring 2004, and has now been operating continuously for 3 years. The array consists of 3 sub-arrays; the Eastern Boundary; the Mid Atlantic Ridge; and the Western Boundary. This poster presents the current status of the array including changes implemented in the Autumn 2007 Eastern Boundary service cruise and the Spring 2008 Western Boundary service cruise. The Western Boundary sub-array is a joint operation between the National Oceanography Centre (Southampton), the Rosenstiel School of Marine and Atmospheric Science (Miami); and the Atlantic Oceanographic and Meteorological Laboratory (Miami).
Rosalind E. M. Rickaby1, David J. Harding1, Ian Hall2
1) Department of Earth Sciences, Oxford University2) School of Earth, Ocean and Planetary Sciences, Cardiff University
Ocean ventilation and temperature are key to controlling the meridional distribution of heat, and to the partitioning of carbon between the atmosphere and the ocean. Changes in the pattern and heat transport of the ocean ventilation system, have been implicated in the abrupt climatic swings of the last deglaciation. In particular, subsurface warming has been proposed as a candidate to account for the synchronous iceberg discharge from various sources around the North Atlantic during the cold phases of millenial scale D-O events during the last glacial period and during Heinrich 1, which heralds the beginning of the deglacial transition. Here we will contrast new high-resolution deglacial planktonic and benthic foraminiferal histories of Mg/Ca, Cd/Ca and δ13C from intermediate depths in the high latitude North Atlantic with other records from the global mid-depths to address the role played by ocean ventilation in abrupt climatic swings. Despite the potential that foraminiferal chemistry may not capture a pure signal of temperature under the extreme cold conditions of the high latitudes, our preliminary reconstruction of ocean temperature indicate that both the Heinrich event and the Younger Dryas are associated with periods of subsurface warming. This warming could be associated with a lack in cooling by convection due to a weakening of the northern overturning cell, which leaves the intermediate depths to accumulate heat from downwards diffusion at the low latitudes. Because some of this heat is transported poleward by horizontal mixing, a temperature inversion in the North Atlantic will grow which ultimately weakens upper ocean stratification and promotes increased ocean mixing. Further evidence for such a phenomenon is to be found in our intermediate depth records of benthic δ13C which demonstrate a deglacial minimum δ13C in parallel with surface waters and δ13C atmospheric CO2 from the Taylor’s Dome. Globally, intermediate depth δ13C signatures converge to the same value at the start of the deglaciation. These data are consistent with enhanced vertical mixing as a result of the temperature inversion, and perhaps release of carbon and nutrients from the poorly mixed LGM deep reservoir during the last glacial, due to reventilation at the deglaciation. The sequestered carbon is mixed via the intermediate waters to the surface and atmosphere.
Jon Robson1, Rowan Sutton1, Doug Smith2
1) Dept of Meteorology, University of Reading2) Hadley Centre, UK Met Office
A rapid warming of the North Atlantic subpolar gyre region is observed in oceanic analyses in the mid to late 1990s. Previous modelling studies, (which investigate the effect of the North Atlantic Oscillation (NAO) on the Atlantic Ocean circulation) suggest that the warming is probably a lagged response to the persistent positive NAO forcings seen throughout the previous decade. The literature suggests that the Atlantic Meridional Overturning Circulation would spin-up in response to the persistent positive NAO forcing, causing an increase in the northward heat transport, and hence a warming. If the rapid warming of the North Atlantic is a lagged response to the NAO then this event could potentially be predictable. This predictability is being investigated by using the UK Met Office's Decadal Prediction System (DePreSys, Smith et al, 2007) which aims to predict both the signal of anthropogenic climate change and internal variability, by initializing forecasts with the observed state of the climate system. An extensive hindcasts dataset is being analysed to understand DePreSys' capacity to predict this rapid warming and to understand the mechanisms at work. The initial results are promising with some of the hindcasts showing highly skilled predictions of the rapid warming, but others are too eager to warm early. Idealized experiments are planned to test hypotheses concerning the cause of the warming and to understand the successes and failures of the DePreSys hindcasts. This paper will present a detailed analysis of the warming and the performance of the DePreSys hindcasts for this event.
Vassil Roussenov1, Richard Williams1, Chris Hughes2, Rory Bingham2
1) Department of Earth and Ocean Sciences, University of Liverpool2) Proudman Oceanography Laboratory
The relationship between changes in sea-surface height, bottom pressure and overturning is explored using isopycnal model experiments for the North Atlantic. Changes in high latitude forcing are communicated rapidly over the basin through boundary wave propagation along the continental slope, involving a hybrid mixture of Kelvin and topographic Rossby waves, as well as spreading more slowly through advection along the western boundary. This adjustment process is revealed in the variability of sea-surface height and bottom pressure, where correlation patterns for sea-surface height from the model are in broad accord with altimetric diagnostics. This wave communication process occurs in model experiments with fine and coarse resolution with the propagation timescale only altering in detail with the realism of the topography. The adjustment in bottom pressure is directly linked to a change in overturning, since west-east contrasts in bottom pressure are associated with a zonal integral in the meridional geostrophic flow. Correlation patterns reveal that temporal changes in overturning are primarily connected to the vertical contrast in bottom pressure, across the shelf and continental slope, along the western boundary.
Gregory C Smith, Keith Haines
ESSC, University of Reading
Our main aim is to produce a high-resolution reconstruction of the global ocean over the last 50 years suitable for the study of ocean climate signals and in particular the Atlantic meridional overturning circulation (AMOC). The relative abundance of collocated temperature and salinity observations provided by Argo are used to develop an assimilation scheme whereby salinity is assimilated on isothermal surfaces. This allows us to exploit the larger spatial and temporal decorrelations of these quantities, compared with assimilation on geopotential surfaces, allowing flow dependent assimilation and recovery of water mass information. The ¼ degree NEMO ice-ocean model developed by DRAKKAR is used to perform a series of experiments over the Argo period, assimilating on depth and temperature surfaces. These experiments are evaluated in terms of the ability of the different methods to capture observed water mass properties, and the effect of assimilating Argo data on the AMOC. A comparison will also be made with estimates of the AMOC from the RAPID Array at 26N. Results from a 50 year reanalysis at 1 degree resolution and a 21 year reanalysis at ¼ degree resolution will also be shown.
Remi Tailleux
Department of Meteorology, University of Reading
An important question about which oceanographers have been wondering for the past century is how to quantify the amount of mechanical energy that can be produced by heating and cooling. The stakes are important, for they ultimately bear on understanding the driving mechanisms of the so-called ocean meridional overturning circulation. From a theoretical viewpoint, one of the main riddles in addressing this question is that the integrated values of the surface heating and cooling do not appear explicitly in the mechanical energy balance, unlike the work of the wind stress for instance. In absence of a clear theoretical framework to tell them how one should then estimate the work of buoyancy fluxes, the so-called Sandstrom's theorem has remained until now the main theoretical basis for arguing that the work of surface heating and cooling should be a negligible contributor to the mechanical energy balance in the oceans, and therefore that they cannot drive the meridional overturning circulation. In this work, I will argue that the main reason why understanding how to compute the work of surface heating and cooling has remained elusive so far is because fluid dynamicists have overlooked until now an important non-viscous dissipation term in the mechanical energy balance. Such a dissipation term is provided by turbulent mixing, which is well known to contribute for about 20 percent of the total irreversible kinetic energy decay. So far, however, it has usually been assumed that this fraction of the kinetic energy ultimately ends up in mean gravitational potential energy. We show, however, that this conflicts with the second law of thermodynamics, as well as with other pieces of evidence, and that in fact, turbulence mixing degrades kinetic energy in dead internal energy, exactly like viscous dissipation. This result is important, because if we subtract the non-viscous dissipation term from the work of expansion/contraction, i.e., the only term through which buoyancy fluxes can act, then a new term emerges that we argue is naturally interpreted as the work of surface heating and cooling. Preliminary results suggest that this term is about O(0.2 TW) in the oceans, and therefore a significant contributor to the mechanical energy balance. Moreover, it is also shown that this production is mostly achieved by cooling, which supports the idea that the overturning circulation is driven by deep water formation, as was commonly assumed about a decade ago. In the present view, turbulent mixing is best regarded as a dissipation mechanism, and not as a driver of the overturning circulation, in contrast with recent claims to the contrary.
Elizabeth Thomas, Eric Wolff, Rob Mulvaney
British Antarctic Survey
We present high-resolution chemistry and isotope data from two large and abrupt climate oscillations, observed in the Greenland ice core record. The first is the most prominent cold event to have occurred during the Holocene, the cold event 8,200 years ago (the 8.2 kyr event) and the second is one of the strongest and longest glacial oscillations, Dansgaard-Oeschger event 8 (DO-8). The 8.2 kyr event is investigated using a composite of four ice core records, to reveal a 160.5 year period during which decadal-mean isotopic values were below average. However, the changes in chemical concentrations are small, suggesting only minor changes in atmospheric circulation accompanied this event. The warming at the onset of DO-8, investigated using the North Greenland Ice Core Project (NGRIP) ice core, is first observed as a rapid decrease in chemical deposition to Greenland, indicating a large and abrupt shift in oceanic and atmospheric circulation. The change in the chemical deposition is followed over a decade later by an increase in temperature of approximately 13 °C, from extreme cold stadial conditions to warm interstadial conditions.
David J.R. Thornalley, Harry Elderfield, I. Nick McCave
The Godwin Laboratory for Palaeoclimate Research, Department of Earth Sciences, University of Cambridge, UK.
The Atlantic meridional overturning circulation (AMOC) transports warm and salty surface waters to high latitudes, where they cool, sink, and return southwards at depth. Through its attendant meridional heat transport, the AMOC helps maintain a warm NW European climate, and acts as a control on global climate. Past climate fluctuations during the Holocene (0-11.7 ka) have been linked with changes in North Atlantic Ocean circulation. However, the behavior of surface flowing salty water that helps drive overturning is not well-known during past climate change. Here we investigate how the temperature and salinity of a major surface inflow to a region of deepwater formation varied throughout the Holocene. The inflow has undergone millennial variations in temperature and salinity (~3.5 oC and ~1.5 psu) controlled by subpolar gyre dynamics. These variations correlate with worldwide glacier advances and cold events. The inflow becomes more saline during enhanced freshwater flux to the subpolar North Atlantic. Under global warming scenarios, models predict a weakening of AMOC in response to enhanced Arctic freshwater fluxes, although the inflow can compensate on decadal timescales by becoming more saline. We provide evidence for the operation of this negative feedback during past intervals of climate change. This behavior suggests that the AMOC may be more stable than previously expected during future global warming.
Craig Wallace
RAPID Knowledge Transfer Co-ordinator
Overview of RAPID knowledge exchange activities since the last Annual Meeting.
Neil Wells1, Rachel Hadfield2, Simon Josey3
1) University of Southampton2) BMT3) National Oceanography Centre
Since 1999, Argo floats have collected almost 50,000 temperature profiles in the North Atlantic Ocean. The accuracy with which this dataset can be used to estimate the upper ocean temperature and heat storage in the North Atlantic has been investigated. A hydrographic section across 36 N was used to assess uncertainty in the temperature field. The RMS difference in the Argo based temperature field relative to the section measurements is about 0.6 deg. C. In comparison, the difference of the section with respect to the World Ocean Atlas (WOA) is 0.8 deg.C. For the upper 100 m, the improvement with Argo is more dramatic, the RMS difference being 0.56 deg.C, compared to 1.13 deg.C with the WOA. The Ocean Circulation and Climate Advanced Model (OCCAM) was used to determine the Argo sampling error in mixed layer heat storage estimates. Using OCCAM subsampled to typical Argo sampling density, it is found that outside of the western boundary, the mixed layer monthly heat storage in the subtropical North Atlantic has a sampling error of 10-20 Wm-2 when averaged over a 10°x10° area. This error reduces to less than 10 Wm-2 when seasonal heat storage is considered. Further results will be presented that demonstrate closure of the heat budget to within 10 Wm-2 in the central and eastern subtropical ocean.
Paul Williams1, Eric Guilyardi1, Rowan Sutton1, Jonathan Gregory1, Gurvan Madec2
1) NCAS Climate, Reading University, UK2) LOCEAN/IPSL, Paris, France
An intensification of the hydrological cycle is a likely consequence of global warming. But changes in the hydrological cycle could affect sea-surface temperature by modifying diffusive ocean heat transports. We investigate this mechanism by studying a coupled general circulation model sensitivity experiment in which the hydrological cycle is artificially amplified. We find that the amplified hydrological cycle depresses sea-surface temperature by enhancing ocean heat uptake in low latitudes. We estimate that a 10 per cent increase in the hydrological cycle will contribute a basin-scale sea-surface temperature decrease of around 0.1 K away from high latitudes, with larger decreases locally. We conclude that an intensified hydrological cycle is likely to con-tribute a weak negative feedback to anthropogenic climate change.
Tim Woollings, Brian Hoskins, Mike Blackburn
Dept. of Meteorology, University of Reading
Current global climate models are low resolution and there is concern that they may be missing key aspects of the dynamics as a result. In particular, it is well known that the sea surface temperature (SST) field contains much fine-scale detail which is not represented in global models. This is especially true in the vicinity of the Gulf Stream, where strong SST gradients are thought to be important for the genesis of the mid-latitude cyclones which make up the North Atlantic storm track. Here we investigate this issue by using a high resolution atmospheric Regional Climate Model (RCM) over the Atlantic. We take advantage of a new high resolution SST dataset spanning 15 years to show that the storm track is sensitive to both spatial and temporal SST resolution. Increasing spatial SST resolution acts to shift the storm track slightly off the east coast of North America and over the Gulf Stream SST gradient. Increasing the temporal resolution strengthens the storm track along its core region and into Northern Europe. These changes are subtle but significant. Work is continuing to investigate the atmospheric response to realistic variations in the high resolution Gulf Stream SSTs.