James Rennell Division for Ocean
Circulation and Climate
Southampton Oceanography Centre
University of Southampton
Waterfront Campus
European Way
Southampton
Hants SO14 3ZH UK
Tel: +44 (0)1703 596404
Fax: +44 (0)1703 596400
Email: Paolo.Cipollini@soc.soton.ac.uk
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CIPOLLINI, P, CHALLENOR, P G, CROMWELL, D, GUYMER, T H & RAFFAGLIO, S |
PUBLICATION
DATE 1999 |
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| ABSTRACT
This report comprises an initial assessment on the detectability of Rossby waves in ocean colour data from the OCTS and SeaWiFS radiometers. By producing longitude/time plots of the merged OCTS and SeaWiFS datasets we observe at some latitudes westward propagating signals whose characteristics are consistent with those expected for baroclinic Rossby waves. The propagating signals are evident both in the OCTS data and in the SeaWiFS data, even though they may be superimposed on the much stronger annual phytoplankton cycle. Their propagating speed depends on latitude as expected for Rossby waves, with speed increasing equatorward. A preliminary comparison with altimeter data shows that in some cases the waves propagate at the same speed, while in other occurrences the relationship from what we see in altimetry and what we see in ocean colour is more complex and needs investigating further. One possible explanation is that biology could be more sensitive to higher order baroclinic modes. We believe that i) from the data presented it is possible to conclude
that effects due to Rossby waves are detectable in ocean colour, and ii)
this novel result is well worth extending and quantifying; this will be
the subject of a future report.
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Southampton Oceanography Centre
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This is a contract report on work funded under a Joint Grant Scheme project on the study of biophysical interactions using multi-sensor satellite data and in situ measurements. The main objective is to improve our understanding of those biophysical interactions that are responsible for plankton growth and distribution, using multi-sensor satellite data, in situ measurements and oceanic ecosystem modelling. A basic understanding of these biophysical interactions is an essential stepping stone on the way to building models of ambient noise. The project began in October 1997 and will run until April 2000.
This report comprises an initial assessment on the detectability of Rossby waves in ocean colour data from the OCTS and SeaWiFS radiometers. By producing longitude/time plots of the merged OCTS and SeaWiFS datasets we observe at some latitudes westward propagating signals whose characteristics are consistent with those expected for baroclinic Rossby Waves. The propagating signals are evident both in the OCTS data and in the SeaWiFS data, even though they may be superimposed on the much stronger annual phytoplankton cycle. Their propagating speed depends on latitude as expected for Rossby waves, with speed increasing equatorward. A preliminary comparison with altimeter data shows that in some cases the waves propagate at the same speed, while in other occurrences the relationship from what we see in altimetry and what we see in ocean colour is more complex and needs investigating further. One possible explanation is that biology could be more sensitive to higher order baroclinic modes.
We believe that i) from the data
presented it is possible to conclude that effects due to Rossby waves are
detectable in ocean colour, and ii) this novel result is well worth extending
and quantifying; this will be the subject of a future report.
altimetry; biological-physical interactions; ocean colour; OCTS; phytoplankton; propagation modes; propagation speed; Rossby waves, satellite remote sensing; sea surface height; SeaWiFS.
INTRODUCTION - ROSSBY WAVES IN THE FRAMEWORK OF THE BLUE SKIES PROJECT
APPENDIX - ATLAS OF LONGITUDE/TIME PLOTS OF OCTS AND SEAWIFS COMBINED DATA
INTRODUCTION - ROSSBY WAVES IN THE FRAMEWORK OF THE BLUE SKIES PROJECT
One of the specific objectives that was listed in the initial report for the present Blue Skies project (Cromwell et al., 1998) was "to assess the detectability of Rossby waves from colour data and compare with results from infrared and altimetry, and to investigate further aspects of Rossby wave physics". These waves are of extreme importance in the oceans for a number of reasons (Gill, 1982, Jacobs et al. 1994, Chelton and Schlax, 1996). A contribution to a better Rossby wave theory and to a better characterisation of their properties and effects will lead to a better knowledge of the density field which is one of the inherent purposes of the present project. The earlier Blue Skies project (Cromwell et al., 1997) and some results published in the scientific literature (Cipollini et al., 1997a, 1997b) had demonstrated that by utilising both Sea Surface Temperature (SST) and Sea Surface Height (SSH) datasets, a more complete picture of the propagation properties of these waves could be obtained. As a natural consequence of this, it was proposed to investigate whether the new ocean colour datasets could provide additional information not present in SST or SSH and thereby improve our knowledge of the subsurface structure and our ability to monitor it remotely.
It must be stressed that at the time
of writing of the proposal and of the initial report the above objective
was still purely tentative - no one had observed before a direct effect
of Rossby waves on biology, at least at mid-latitudes. In the present work
we present the first results of our investigation on the detectability
of Rossby waves in ocean colour data, from which it is clear that their
detection is possible at least in some cases. Such original findings encourage
a further study of the relationship that the signature of Rossby waves
in ocean colour has with the corresponding signatures in altimetry and
infrared data.
A simplified schematic of a Rossby wave is presented in figure 1. Although their signature in the SSH field (which is detectable and often measurable by a spaceborne altimeter) is only a few centimetres, they have a signature of several tens of metres in the thermocline. The density profile is variously affected by the different modes of propagation of the waves so the overall picture can be more complex than that shown in the figure. Rossby waves also affect, albeit indirectly, the temperature as well as the local heat budget. This is why they can be detected in SST (Cipollini et al, 1997a, 1997b, Hill et al, 1999), depending on the extent to which surface and subsurface fields are coupled. In short, if the SST can be regarded as being correlated with the surface density, and if this surface density is in turn correlated with the density at some depth, then perturbations in the density field could also be visible as perturbations in the SST field.
The above considerations lead to some intriguing questions about a possible relationship between Rossby waves and ocean colour, which is affected by phytoplankton:
For our purposes we have taken advantage of the only two global coverage instruments in the new generation of ocean colour sensors launched so far. These are:
For OCTS we used Global Area Coverage (GAC) level 3 Binned Map data (monthly chlorophyll-a composites) which are currently available from NASDA-EORC at ftp://ftp2.eorc.nasda.go.jp/pub/ADEOS/OCTS/GAC3BM. The data have been processed with version 3 chlorophyll algorithm (OCTS Team, 1998) and cover the period November 1996 to June 1997 (8 months in total). As of January 1999, NASDA are reprocessing the data with a new version of the algorithms (version 4) but version 4 composites have been released only for 4 out of 8 months so in order to work with a uniform dataset we relied on version 3 products. However, we believe that for the kind of observations with which we are concerned (possible effect of Rossby waves on biology, thus large-scale phenomena in open ocean waters) the accuracy of version 3 chlorophyll is more than acceptable.
For SeaWiFS we used Global Area Coverage (GAC) level 3 Binned Map data (monthly chlorophyll-a composites) from NASA-GSFC DAAC which are available to registered users at http://daac.gsfc.nasa.gov/CAMPAIGN_DOCS/BRS_SRVR/seawifsbrs_main.html. The data have been recently reprocessed by the GSFC team with version 2 chlorophyll algorithm. The updated version 2 chlorophyll algorithm yields very similar results, in the range up to 1.5 mg/m3, to the version 1 one described in O'Reilly et al., 1998, while in more eutrophic waters the updated algorithm generates lower concentration than the original equation. This could have affected our results, especially in bloom conditions at high latitudes, so it was decided to wait until the whole range of version 2 monthly composites had been released. They are now available and cover 11 months in total, from October 1997 to August 1998, with more months to be added soon.
OCTS and SeaWiFS data are distributed in a similar format (HDF - Hierarchical Data Format). We have imported data from HDF files into MATLAB, where they have been subjected to further binning. The original data are on a 0.0879° x 0.0879° grid, which is far too detailed for the purposes of this work. So the data have been rebinned onto a 0.5° x 0.5° grid, still more than enough to detect large-scale propagating signals. This additional binning reduces the noise and the effect of potential remnant cloud contamination on the data. Finally OCTS and SeaWiFS datasets have been merged into a single dataset covering 22 months from November 1996 to August 1998, with a three-month gap in summer 1997
It is well known that, since Rossby
waves propagate mainly zonally, they can be observed in longitude-time
plots of satellite data. Propagating waves appear as diagonal features
in the plots. Thus, we built longitude-time plots for every 0.5° step
in latitude from 42°S to 42°N. The whole set of plots is presented
in the appendix at the end of this report.
In some of the longitude/time plots we do see signals propagating westward. They are not as ubiquitous as those observed in altimeter data (Chelton and Schlax, 1996) and infrared data (Hill et al., 1999); nevertheless they appear at various latitudes in all the three main oceanic basins.
Their speed, which can be measured either by eye directly on the plots or with more objective methods such as the Radon Transform (Chelton and Schlax, 1996, Cipollini et al., 1999), depends on latitude; the closer to the equator, the faster they propagate, which is what we expect for Rossby waves. Also, it is worth noting that we do not see anything similar propagating eastward. For these reasons we believe that the features observed are the effect of Rossby wave propagation on ocean colour. A few examples will help to support this statement.
The Indian Ocean at 34.25°S is
one of the places where the propagating signals in the longitude-time plots
are more evident (figure 2). It is possible to observe that the alignments
in the OCTS data (first 8 months) and those in the SeaWiFS data (last 11
months) have approximately the same slope; indeed the latter lie on the
continuation of the former. In the same ocean at 14.75°S (figure 3),
the propagating signals travel significantly faster, as demonstrated by
their slope.
The effect of the annual phytoplankton
cycle are quite visible in the North Atlantic at 31.25°N, where the
propagating signals are still visible but appear to be modulated by the
much stronger annual bloom (figure 4).
A comparison of the results obtained
from ocean colour with those obtained from altimetry goes beyond the scope
of this report, and will be the subject of a following one. For the moment,
it is worth noting that in a few cases there is a significant correlation
between the two datasets. Figure 5 shows one of these cases, around 22°S
in the Pacific. The propagating signals in ocean colour and altimetry (from
TOPEX/POSEIDON) have similar slope (that is, similar speed) and some of
the alignments almost coincide in the two datasets. More often, the speed
of propagation observed in ocean colour differs from that observed in altimetry.
One example is shown in figure 6, for ~15°S in the Pacific. The alignments
spotted in the TOPEX/POSEIDON longitude-time plot propagate significantly
faster than those in the combined OCTS/SeaWiFS dataset do. One possible
explanation of this could be that in the two datasets we see different
baroclinic modes of propagation (higher order modes in ocean colour), as
already observed in altimetry versus SST by Cipollini et al., 1997.
This report provides evidence that it is possible to observe Rossby waves in ocean colour, thereby suggesting strongly that Rossby waves have an effect on biology in the oceans. In fact, the characteristics of the propagating signals that we observe in the ocean colour field match or are very similar to those of Rossby waves. Moreover, the speed of these signals depends on latitude and increases equatorwards. In some cases they seem to correspond to the Rossby waves we see in the altimetry field. In other cases they go slower, which could suggest higher modes of propagation.
This opens many intriguing questions, of which the following are only a subset:
Chelton, D.B., and M.G. Schlax, 1996, "Global observation of oceanic Rossby waves", Science, vol. 272, pp. 234-238.
Cipollini, P., D. Cromwell, M. S. Jones, G. D. Quartly, P. G. Challenor, 1997a, "Concurrent altimeter and infrared observations of Rossby wave propagation near 34° N in the Northeast Atlantic", Geophysical Research Letters, vol. 24 , no. 8 , pp. 889-892.
Cipollini, P., D. Cromwell, M. S. Jones, G. D. Quartly and P. G. Challenor, 1997b, "The potential of ERS for the detection of Rossby waves in the Northeast Atlantic", ESA SP-414 Proc. of 3rd ERS Symposium - Space at the Service of our Environment, Florence (Italy), 17-21 March 1997, vol. 3, pp. 1473-1478, European Space Agency.
Cipollini, P., D. Cromwell, G. D. Quartly, 1999, "Observations of Rossby wave propagation in the Northeast Atlantic with TOPEX/POSEIDON altimetry", Advances in Space Research, in press.
Cromwell, D., P. G. Challenor, M. S. Jones and T. H. Guymer, 1997, Relationship between remotely-sensed signatures of the ocean and subsurface structure: end of contract report.. Southampton Oceanography Centre Internal Document no. 15, 29 pp. (Unpublished manuscript).
Cromwell, D., P. G. Challenor, P. Cipollini, T. H. Guymer, M. A. Srokosz, 1998, The study of biophysical interactions using multi-sensor satellite data and in situ measurements: inventory and preliminary assessment of priorities, Southampton Oceanography Centre Internal Document no. 31, 23 pp. (Unpublished manuscript).
Gill, A. E., 1982, Atmosphere-Ocean Dynamics, Academic Press, San Diego, 662 pp.
Hill, K. L., I. S. Robinson, P. Cipollini, 1999, "Observations of Rossby wave characteristics from satellite-derived global sea surface temperature", in preparation
Jacobs, G. A., H. E. Hurlburt, J. C. Kindle, E. J. Metzger, J. L. Mitchell, W. J. Teague, and A. J. Wallcraft, 1994, "Decade-scale trans-Pacific propagation and warming effects of an El Niño anomaly", Nature, vol. 370, pp. 360-363.
OCTS Team, A cal/val report on the OCTS version 3 products, Proceedings of the 3rd ADEOS Symposium/Workshop, Sendai (Japan), 26-30 January 1998, pp. 39-56.
O'Reilly, J. E., Maritorena, S.,
Mitchell, G., Siegel, D. A., Carder, K. L., Garver, S. A., Kahru, M., McClain,
C., 1998, "Ocean color chlorophyll algorithms for SeaWiFS", Journal
of Geophysical Research, vol. 103, no C11, pp. 24937-24953.
ADEOS ADvanced Earth Observation System
DAAC Distributed Active Archive Center at GSFC
EORC Earth Observation Research Centre (of NASDA)
GAC Global Area Coverage
GSFC Goddard Space Flight Centre (of NASA)
HDF Hierarchical Data Format
NASA National Aeronautics and Space Administration
NASDA NAtional Space Development Agency (of Japan)
OCTS Ocean Colour and Temperature Scanner (on board ADEOS)
SeaWiFS Sea-viewing Wide Field-of-view Sensor (US ocean colour satellite)
SSH Sea Surface Height
SST Sea Surface Temperature
T/P TOPEX/POSEIDON
APPENDIX
- ATLAS OF LONGITUDE/TIME PLOTS OF OCTS AND SEAWIFS COMBINED DATA