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  596405
Fax:  +44 (0)1703  596400
Email:  David.Cromwell@soc.soton.ac.uk
 
 
 

 ABSTRACT
 

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 which 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 and sound speed profiles.

The project began in October 1997 and will run until April 2000. This report comprises an initial assessment of relating ocean colour and other satellite-derived signatures to subsurface hydrography and biology.

KEYWORDS

ALTIMETRY, BIOLOGICAL-PHYSICAL INTERACTIONS, INFRARED RADIOMETRY, MOS, NORTH ATLANTIC, OCEAN COLOUR, OCTS, OMEGA, PLANKTON PATCHINESS, SATELLITE REMOTE SENSING, SEA SURFACE HEIGHT, SEA SURFACE TEMPERATURE, SEAWIFS

CONTENTS

ABSTRACT

KEYWORDS

1. INTRODUCTION

2. Use of altimeter data and hydrography to derive currents

2.1 Azores region: STORM eddies

2.2 OMEGA

3. Surface chlorophyll and its relationship to subsurface hydrography and biology

3.1 OMEGA

3.2 D227 N.E. Atlantic

References

ACRONYMS

APPENDIX. N.E. Atlantic: CRUISE D227 (P²S³⁾
 

1. INTRODUCTION

The overall thrust of this Blue Skies project is to investigate to what extent remotely-sensed data from the ocean surface contain information about what is happening at depth. More specifically, we aim to study the spatial and temporal variability of biological activity ("plankton patchiness") in the upper layers of the ocean, and its relationship to the physical processes occurring there, using a combination of shipborne and satellite measurements from a number of different sensors.

A key issue is the synergistic use of satellite and in situ data. However, the relationship between remotely-sensed signatures of the surface and subsurface features is complex. The first contract report from the current Blue Skies project comprised a data inventory and preliminary assessment of priorities (Cromwell et al., 1998). Ocean colour observations will be used to improve and validate oceanic ecosystem models and particular emphasis will be given to the observation of those phenomena whose surface signature corresponds well to subsurface structures, such as Rossby waves.

This report is a first-look at the relationship between satellite signatures and subsurface hydrography and biology using in situ data from two cruises: one in the Western Mediterranean and the other in the North-East Atlantic. This lays the groundwork for a follow-up report due at the end of October 1999. In a companion report we assess the detectability of Rossby waves from colour data (Cipollini et al., 1999).

2. Use of altimeter data and hydrography to derive currents

2.1 Azores region: STORM eddies

The North-East Atlantic is a particularly interesting region in which to study ocean dynamical phenomena such as propagation of eddies and Rossby waves. The region around the Azores Front/Current is particularly active. For example, as Cipollini et al., (1997a) showed, the presence of the eastward-flowing Azores Current appears to produces a "wave guide" effect which enhances Rossby wave amplitudes, enabling the first three baroclinic modes to be discerned. These features were observed in both the sea surface height and temperature fields.

In this section, we present new results on the zonal propagation of eddies across a coincident altimeter track/hydrographic section in the North-East Atlantic basin near 33^oW (Cromwell et al., 1999).

It is well known that, although it is possible to measure the varying part of the surface geostrophic current with a satellite altimeter, the mean ocean circulation cannot be easily separated from the geoid signal [e.g., Marshall, 1985]. The eventual solution to this problem will be to measure the geoid directly by measuring the Earth's gravitational field. Here, we apply a variation of a powerful method which combines along-track altimetry and hydrography, first presented by Challenor et al. (1996) in their monitoring of variability in absolute surface geostrophic flows in the Antarctic Circumpolar Current through the Drake Passage. In essence, the method works as follows. Since the altimeter can measure the variability of the current, if we can measure the absolute current at the time of one satellite pass, we can infer the absolute current at every pass. In this paper, a conductivity-temperature-depth (CTD) section, with an assumed level of no motion at 2000 dbars, is used to determine a "snapshot" of absolute surface geostrophic flow south of the Azores. We then combine this "snapshot" with altimetrically-derived sea surface slopes to monitor the variability of the absolute surface geostrophic current forwards and backwards in time for as long as the altimeter flies along that ground track. The method applied in the present study has already been used successfully to analyse data from a nearby 3-day repeat ERS-1 track / CTD section south of the Azores, revealing a surprising persistent westward component of the Azores Current (Cromwell et al., 1996).

Pingree and Sinha (1998) proposed that westward-travelling eddies previously observed, for example, by Gould (1985), propagate in a zonal 'corridor' in the eastern basin of the North Atlantic between ~32^o-34^oN at a frequency of ~2 per year. This region is coincident with, or close to, a 'waveband' of enhanced energy in propagating baroclinic Rossby waves observed in both altimeter and sea surface temperature data (Cipollini et al., 1997). By exploiting the repeat nature of along-track altimetry we can investigate the temporal variability in absolute geostrophic flow across a combined near-meridional hydrographic section/altimeter track crossing the zonal STORM corridor. The aim is to infer both the frequency of propagating eddies in the corridor, as well as changes in the southern part of the Azores Current and its southern recirculation. (The northern part of the Azores Current lies north of the CTD section and is not observed here.)
 
 

Figure 1. North East Atlantic Canary Basin and Azores Current region. The ground track patterns of ERS-1/2 in the 35-day repeat phases are indicated. The descending ERS-1 (ERS-2) satellite pass on October 17 (18), 1995 is shown as a heavy line (pass number 448). Bathymetry contours at 2000m and 4000m are plotted. A CTD section was occupied between ~30o -35o N along pass number 448, crossing the 'STORM corridor' just south of the Azores Current.
 
 

Conductivity-Temperature-Depth stations occupied on, or close to, the 35-day repeat ERS-1/2 ground track during RRS Charles Darwin Cruise CD97 in October 1995 (Figure 1) were processed, assuming a level of no motion at 2000 dbars, to yield a profile of absolute surface geostrophic currents. This was then combined with ERS-1/2 altimeter measurements of along-track sea surface slope to generate a time series of absolute surface geostrophic currents from May 17, 1992 - November 28, 1993 (first ERS-1 35-day repeat mission); April 25, 1995 - May 14, 1996 (ERS-1/2 tandem mission); and May 31, 1995 - February 2, 1998 (ERS-2).

By combining the ERS-1/2 altimetry and hydrography, we obtain a time series of absolute surface geostrophic currents at 35-day intervals. These values are contoured and presented as a latitude-time section in Figure 2. [For the period of the tandem ERS-1/2 mission, i.e. May 1995 - May 1996, the ERS-2 profiles have been used].

An event of similar large magnitude and location (~33^oN, 33^oW) to the STORM event of October, 1995 occurred in July, 1992, as observed by ERS-1. The geostrophic velocity profiles of all 'candidate' STORM events are illustrated in figure 3. A striking exception to the 'model' profile exhibited by the STORM event of October 18, 1995 is the feature observed on August 28, 1996 which exhibits a strong and narrow westward flow of ~50 cms^-1 just north of 32^oN.

The results show that STORMs occur with a mean period of 7.5 months, supporting but refining the earlier hypothesis of Pingree and Sinha (1998) that ~2 eddies per year propagate westward in the STORM corridor.

We contend that the zonal Rossby wave corridor or 'waveguide' previously reported by Cipollini et al. (1997) between ~33^o - 34^oN is distinct from, and lies to the north of, the STORM corridor (~31^o - 34^oN), as can be observed on the right of figure 2. In the Rossby wave corridor, the dominant period of Rossby waves is ~7 months, close to that of STORM events. Since these periods are similar, it is likely that the Rossby wave and STORM corridors are dynamically coupled with each other, and with the Azores Current. It remains a challenge to theorists to explain the complex interaction between the Azores Current, eddies which pinch off from current meanders and propagate along the STORM corridor, and the strong presence of westward-travelling Rossby waves just to the north.

We will continue these studies by building up multi-year datasets from the altimeters on board T/P, as well as the altimeter and Along-Track Scanning Radiometer on board ERS-1 and ERS-2, and their successor, Envisat. We will explore the signatures seen in these datasets by continuing to develop data analysis techniques such as the Radon transform extended from 2-D to 3-D (Cipollini et al., 1997b).

Eddy-permitting ocean models at SOC are available to assist in the study of Rossby waves:

(a) DYNAMO-MICOM: a high-resolution (1/3^o) isopycnic model of the North Atlantic;

(b) OCCAM: a high-resolution (1/8^o) level (Bryan-Cox type) model of the global ocean.

Specifically, we intend to compare wave amplitude, propagation velocity and the effect of the Azores Current "wave guide" in the model and satellite observations. These results will be reviewed in the light of recent theoretical advances on Rossby wave propagation - particularly the effect of flat vs. sloping bathymetry (Killworth and Blundell, 1999). We will also examine the possible Rossby wave forcing mechanism revealed in wind stress observations from the ERS scatterometers.

Figure 3. Profiles of absolute surface geostrophic velocity for seven STORM eddy 'candidates' (see Figure 2). The eddy observed in October 1995 was observed during RRS Charles Darwin Cruise CD97. STORM eddies occur around twice a year. Positive current values indicate eastward flow.
 
 

2.2 OMEGA

OMEGA (Observations and Modelling of Eddy scale Geostrophic and Ageostrophic motion) is a European MAST Project (contract no. MAS3-CT95-0001). The project has the following main objectives: to determine the spatial and temporal variability of 3D circulation in an upper ocean front; to evaluate impact of vertical motion on biogeochemical properties (using the "omega equation " to estimate vertical velocity); and to provide the scientific community with a new tool for computing vertical motions from CTD and ADCP data. The second OMEGA cruise (Allen, Guymer et al, 1997) took place in Nov. - Dec. 1996 onboard RRS Discovery, in the region of the Almería-Oran front at the eastern edge of the Alborán sea. The second leg of this cruise (Pugh et al., 1997.) continued observations in the region of the front, with an emphasis on biological measurements. Of the two cruises, Leg 1 is of more immediate relevance to the current study.

Hydrographic observations of the upper 370 metres of the water column were made using SeaSoar, accompanied by ADCP (Allen et al., 1997) and ACCP current measurements, underway physical, chemical and biological analysis of surface water samples, multi-frequency acoustic backscatter measurements, meteorological observations and sea surface radiation measurements. In addition, eight towed deployments of a Longhurst-Hardy Plankton Recorder were made to look at the change in plankton species composition across frontal zones.

A Fine Scale Survey (FSS) pattern was designed, with the aid of near real time AVHRR images, to cross the Almería-Oran front in a series of legs parallel to descending ERS passes. Unfortunately, an error during the survey planning led to some of the legs of the survey not running parallel to the ERS tracks (Figure 4). This fine scale survey pattern was completed three times during Leg 1 of the cruise and then twice during Leg 2. After the completion of the third FSS a brief SeaSoar survey was also made of the head of the Algerian Current. Several additional legs were undertaken, some of which were also along altimeter tracks (see Figure 4).

The first track to be analysed is ERS track 646. Because of the offset between the ERS track and the FSS, the hydrographic data had first to be interpolated to the ERS track. The gridded density profiles from the two closest FSS legs were first aligned at the maximum density gradient in order to interpolate along the front, and so minimise smoothing during the interpolation. The density field was then determined at the midpoints of each pair of along-track altimeter reference locations, using a linear distance-weighted mean of the four closest grid points to the desired interpolation location. The geostrophic velocity relative to 200m was then determined at each altimeter reference point. The ADCP data were interpolated to the altimeter track by aligning the two closest legs at the maximum speed and then using the same weighted mean scheme to give a velocity profile at each altimeter data point. The component of absolute velocity perpendicular to the altimeter track was then calculated and smoothed vertically to give values at each geostrophic velocity position from the SeaSoar.

The barotropic component of the flow was estimated by finding the average difference between the ADCP and SeaSoar from 120-320m for each profile. This was then added to the SeaSoar derived vertical shear, to determine the total across track surface geostrophic current, or reference current.

The sea surface height measured at the closest overpass in time (18 Dec 1997) was used as the reference level for all repeats of the ERS altimeters (ERS-1 and 2). This height profile was removed from all the others to give height residuals relative to this overpass. The surface geostrophic velocity residuals relative to this overpass were then found by taking the linear slope over 3 adjacent points. These residual velocities, and the absolute velocities obtained by adding the reference current, for ERS-2 are shown in Figure 5. The profile for 18 Dec is the reference velocity. The legend indicates that solid lines (alternating between green and red for each profile) are absolute velocities, while dashed lines (alternating between blue and cyan) are relative (i.e. residual) velocities.

The jet along the Almería-Oran front can be seen in the reference velocity profile between 36.2 and 36.4^oN, reaching a maximum velocity of about 1 ms^-1. Although this feature is regularly seen in the time series of profiles, the position and intensity change rapidly, e.g. in both July 1996 and February 1997 the jet appears further North, nearer 36.5^o, whilst at other times the jet is not evident, e.g. March 1996.

The Alboràn Sea typically has a two gyre system, seen in the ATSR image for 2 Dec 1996, near the start of the cruise (Figure 6). Both of these gyres, particularly the Eastern Gyre, are known to collapse periodically. When the eastern gyre collapses, it forms an eastward jet across the eastern Alboràn and would appear much further south on ERS track 646. This may be the situation during March 1996 when the jet is not apparent in the altimetry.

The velocity profiles so far obtained give a quantitative measure of the current across a single ERS track. This view will be expanded to include monitoring the other tracks for which coincident altimetry and in situ data exist (see Figure 4). ATSR SST data yield two-dimensional images of the ocean surface. Since SST and SSH are often correlated, this raises the possibility of using SST data as a 'proxy' for SSH between altimeter tracks. Consequently, a longer time series of sea surface temperature data from ATSR will be examined to place the above altimeter-derived velocity profiles in a two-dimensional context. Changes in purely altimeter-derived velocity profiles could result from either or both of: (a) a change in the orientation of the front with respect to the altimeter track; (b) a real change in the strength of the front. Examining the two-dimensional surface structure by utilising ATSR data will enable us to see what proportion of the changes seen in the altimetry are due to changes in the relative orientation of the front, rather than true changes in front intensity.
 

3 SURFACE CHLOROPHYLL AND ITS RELATIONSHIP TO SUBSURFACE HYDROGRAPHY AND BIOLOGY

3.1 OMEGA

In this section, we will examine the relationship between surface chlorophyll (from in situ measurements and spaceborne ocean colour sensors) and subsurface biology and physics as observed during RRS Discovery cruise no. 224 (OMEGA) leg 1 (Allen, Guymer et al., 1997)
 
3.1.1. In situ measurements

In situ measurements collected during the OMEGA cruise include the following:

1.    underway chlorophyll fluorescence profiles by a fluorimeter on SeaSoar;

2.    underway near-surface (~ 5 m) chlorophyll fluorescence by on-board fluorimeter;

Those fluorimetric measurements were calibrated by means of hourly chlorophyll and phaeopigments from surface samples.

The near-surface chlorophyll displays the characteristic plankton patchiness associated with areas of strong mesoscale activity (Strass, 1992). The near-surface chl plot for the 3rd FSS (21-24 Dec 1996) is in Figure 7. Although on average slightly greater at the front or on its NE side in this particular survey, the concentration shows various scales of patchiness ranging from 10 to 100 Km. This corresponds to the results obtained from the discrete sampling at the surface (see Jimenez-Gomez et al., 1997).
 
 
Figure 7 - Chlorophyll fluorescence at ~ 5 m during the 3rd FSS across the Almeria-Oran front, Eastern Alboran sea (OMEGA cruise leg 1)
 

Chlorophyll patchiness is also evident, along with subsurface maxima, in the chlorophyll fluorescence profiles across the front (Figure 8) taken during the three FSS. A quite peculiar tongue of biological material is clearly visible in the 3^rd FSS. This material is dragged down by the subduction of the surface waters along the sloping isopycnals at the front. Figure 9 shows a plot of the potential temperature on the 27.9 kg/m³ isopycnal taken during the 3^rd FSS, in which the subduction of warmer MSW (Mediterranean Surface Water) can be easily spotted. The MSW approaches the front on the NE side, and is found at depth to the south as a result of the subduction process. This is confirmed also by a computation of the 3-D circulation at the front from the primitive equation of motion and the quasi-geostrophic omega equation (Allen et al., 1998), which shows vertical velocities downwards of several tens of m/day.
 
 
Figure 8 - Fluorescence signal along transect e for the three OMEGA/leg1 FSS (as measured by the fluorometer on SeaSoar - the black arrows mark the position of the front at the surface as seen in the 27.5 kg/m3 isopycnal (Guymer et al., 1997, Snaith et al., 1997)
 

Figure 9 - Potential temperature on the 27.9 kg/m3 isopycnal during the 3rd OMEGA FSS
 
 
3.1.2 Satellite imagery

Unfortunately, SeaWiFS was still unavailable at the time of the campaigns. To compare ocean colour data with in situ OMEGA data we have to rely on the Ocean Colour and Temperature Scanner (OCTS), on board the Japanese satellite ADEOS, and on the Modular Optoelectronic Scanner MOS, built by DLR and on board the Indian satellite IRS-P3.

During the observational campaign weather conditions were quite unfavourable for ocean colour. However, some partially cloud-free OCTS images have been acquired by NASDA but level2 LAC (Local Area Coverage) scenes of the area have not been released as yet (contact are undergoing with NASDA to get the relevant data). The only L2 LAC scene of the Algerian Current available so far (not shown here), taken on 1 April 1997, does render the meandering of the current and the corresponding biological activity with very good resolution and accuracy, so it is hoped that L2 LAC images should infer valuable information on the phytoplankton distribution in the area. It is however possible to analyse the level3 weekly data already released by NASDA. Figure 10 shows the 4 weekly composites available during late November and December 1996. Despite the low spatial resolution (L3 data are gridded on a 0.09 x 0.09 degree grid, which correspond to a resolution of about 8 x 10 Km at ~35°N) and the fact that data are averaged over one week, it is still possible to see the increased biological variability (especially in the week starting on 24 November) and the patchiness at the front.
 

Figure10 - OCTS L3 weekly chlorophyll concentration composites during late November and December 1996
 

As regards MOS on IRS-P3, it was in stellar pointing mode in November 1996 and at the beginning of December, and went back to Earth observation mode on 15 Dec 1996. Two images are available on the area of the Algerian Current east of the AO front: 22 Dec (reasonably cloud-free) and 27 Dec (with scattered clouds, not shown here). These images are affected by some radiometric problems, the most important being the striping due to the different responses of the single elements in the detector array. This type of noise will be dealt with in the near future by a calibration algorithm which is being developed at the University of Pisa in cooperation with DLR Berlin (Corsini et al, 1998). It is hoped that the de-striping algorithm should be applied to the images of the Algerian current in the near future. Moreover, due to the low sensitivity of the instrument in the near infrared bands, the atmospheric correction has proved to be somewhat troublesome. An atmospheric correction algorithm (Sturm, 1998) has been developed by B. Sturm at JRC Ispra and incorporated in a level2 processing scheme as a result of a joint SOC/JRC cooperation. Preliminary result of the application of the scheme to MOS imagery of the Black Sea have been very encouraging and led to the development of a set of bio-optical algorithms for MOS (Barale et al., 1998), which will soon be applied to the images of the Algerian current. The MOS image of 22 Dec 1996 (Figure 12), taken around 0° in front of the Algerian Coast, shows a significant filament-like intrusion (possibly of Levantine Intermediate Water) enclosed by two areas of Modified Atlantic Water to the south and Mediterranean Surface Water to the North; this feature is also clearly visible in Figure 11, which shows an AVHRR SST image of the same day (Guymer et al., 1997).
 
 

Once all the OCTS LAC data have been acquired and the MOS processing is completed, satellite-derived chlorophyll data will be calibrated and integrated with the in situ data.
 
 
3.2 Preliminary results for cruise D227 (P²S³)

In this section, we will discuss the preliminary results on the relationship between surface and subsurface biology and physics as observed during RRS Discovery cruise no. 227 (hereafter referred to as D227 or P²S³; ). The discussion will focus on the two large-scale surveys carried out during the cruise, surveys B and F, as these are the ones for which the data have been considered in more detail so far.

Background

D227 was a cruise designed to study the phenomenon of plankton patchiness using a combination of ship and satellite observations (see for more details). It took place in the eastern North Atlantic from 15^th April to 16^th May 1997 (Julian days 105 to 136). The satellite data of relevance to this discussion are the ocean colour measurements from the Ocean Colour and Temperature Sensor (OCTS) on the Japanese satellite ADEOS, and sea surface temperature (SST) measurements from AVHRR on the NOAA satellites and ATSR-2 on ESA's ERS-2. Due to the weather conditions during the cruise, few cloud-free satellite images (colour or SST) were obtained. For this reason the discussion here will focus more on the in situ observations.

Survey B was a combination of SeaSoar legs and CTD stations (to 2000m), while survey F was simply a SeaSoar survey. The results presented here will be based on the SeaSoar observations only.

A further background point to note is that during the cruise there was a major storm, around day 125, which caused the mixed layer to deepen (approximately double in depth). Prior to the storm, the ocean was gaining heat from the atmosphere, while after the storm there was little net heat gain or loss (although there were diurnal variations in heating / cooling; see Figure 3 of Srokosz et al. (1997) ). Survey B was carried out prior to the storm and survey F after the storm.

SeaSoar measurements

The SeaSoar was equipped with conductivity and temperature sensors, a PAR (photosynthetically active radiation) sensor, a fluorimeter and an optical plankton counter (OPC). These provided data on salinity, (potential) temperature, chlorophyll (an indicator of phytoplankton abundance), light and zooplankton, respectively. The details of the calibration and processing of the data are given by Morrison et al. (in preparation) and will not be discussed here. Suffice to note that the measurements that will be presented below are based on the 4km (horizontal) by 8m (vertical) gridded version of the data.

The only non-standard measurement is that provided by the OPC, so we will discuss the data from that sensor briefly. The OPC measures zooplankton in what is termed the equivalent spherical diameter (ESD) range 250 microns to 1.4cm (). Here the data are aggregated into four size classes 250-500 microns, 500-1000 microns, 1000-2000 microns, and >2000 microns. The largest numbers of zooplankton measured are in the two smallest size classes, but on conversion to bio-volume or biomass more "weight" is given to the larger zooplankton. The conversion to bio-volume (or displaced volume DV; cm³ m^-3) is achieved by calculating the volume of each animal from its ESD and knowing the flow rate of water through the OPC (see Morrison et al., in prep.). The conversion to biomass (mg C m^-3) was made using the equation relating biomass to displaced volume. Further work is necessary on the problem of conversion of OPC measurements to biomass and verification of the results obtained, but this will not be considered here. The OPC data are qualitatively representative of the zooplankton present, even if the quantitative results can be questioned. This will be adequate for the discussion that follows.

Surveys B and F - horizontal structure

Survey B consists of 180km legs spaced 30km apart, alternating SeaSoar and CTD measurements, which means that some SeaSoar legs are 60km apart (see Figure A.3 of ). Survey F repeats part of survey B, with legs of 150km spaced at 30km (see Figure A.7 of ). We have constructed horizontal maps of the salinity (S), potential temperature (T), chlorophyll (chl; units of mg chl m^-3), and three size classes of zooplankton (250-500 microns, 500-1000 microns, 1000-2000 microns; units of mg C m^-3). These have been produced using a distance-weighted interpolation scheme to link data on adjacent SeaSoar legs (5km along-track, 40 or 80km across-track for 30 or 60km spaced legs, respectively). Varying the interpolation scheme does not significantly alter the results obtained, except when too short or too long length scales are used. In the former case data on adjacent tracks cannot be linked, in the latter all variations are smoothed out. Clearly the along-track resolution (at 4km) is much better than the across-track (at 30 or 60km).

In producing the horizontal maps no account has been taken of the possible non-synopticity of the data. From Acoustic Doppler Current Profiler (ADCP) current measurements made during the cruise (), we know that strong flows (up to 50cm s^-1) were present in the area. Thus advective effects will have an influence on the observations obtained. Survey B, which mixed SeaSoar and CTD observations and took approximately 10 days to complete (), is less synoptic than survey F, which used only SeaSoar, and took approximately 3 days (). Further work will be necessary to understand the effect of the non-synopticity of the surveys on the preliminary results presented here.

The results shown in Figures 1-8 are for survey B and in 9-16 for survey F, with the cruise track overlaid. Figures 1-6 & 9-14 give T, S, chl, C_250-500, C_500-1000, C_1000-2000, at a depth of 20m for the two surveys. Figures 7 & 8 and 15 & 16 show T and S at a depth of 200m for the two surveys. No biological variables are shown at the deeper level as the biological activity there is negligible. Phytoplankton, needing light to grow, are in the near surface waters, and the smaller zooplankton stay near their food source (larger zooplankton, >1cm, were seen to exhibit diurnal vertical migration from the ADCP backscatter data - not shown). The two depths were chosen so that one was within the mixed layer (20m) and the other well below the mixed layer (200m). The 20m metre depth data are at sufficient depth to avoid surface quenching problems, a consequence of high light levels during daylight hours, in the fluorimeter estimates of chlorophyll (see ). Due to a variety of data and interpolation problems some of the Figures (4-6, 9-11, 15 & 16) exhibit some gaps. These are not sufficiently numerous to impair an initial discussion of the observations.

What can be deduced from the observations in these figures regarding the relationship between surface and sub-surface biological and physical structures for this data set? First, if the 20m temperature T can be taken as representative of the SST (assuming the mixed layer is well mixed and there is no significant skin effect), then comparisons of Figures 1 and 7 show that the surface and sub-surface T structures are related. Comparison of Figures 1 and 2 suggest that for survey B the horizontal structure of T and S in the mixed layer are also closely related. For survey F (Figures 9, 10 & 15), after the storm, the links between the 20 and 200m T structures is less clear, as is that between T and S at 20m.

Consider next the biological variables, chl, C_250-500, C_500-1000, C_1000-2000, for survey B. Figures 1-3 show that the high chl values to the south and east of the survey area are linked to the physical structures there. The zooplankton data (Figure 4-6) do not show as clear a relationship, though there is some indication of low values associated with the eddy-like feature at 18^oW, 47.5^oN (Figures 1 & 2). There is also some indication of higher C_250-500 values linked to higher chl (>1mg chl m^-3) values to the south and east. It should be noted that the highest zooplankton biomass values are associated with the smallest size class, and peak at ~15mg C m^-3.

In contrast to survey B, the chl in survey F (Figure 11) is more uniform across the area at ~0.6mg chl m^-3. Its distribution seems to be largely de-coupled from the physical structure (Figure 9 & 10). The latter appears to be true of the zooplankton biomass as well (Figures 12-14). However, note that there has been a significant increase in the biomass over the north-west part of the area in the two largest size classes. The increase for C_500-1000 is from <10 to up to ~90mg C m^-3, and for C_1000-2000 from <10 to up to ~180mg C m^-3. There is no obvious relationship between the zooplankton biomass distribution and the phytoplankton chlorophyll. The uniformity of the chl distribution for survey F might be attributable to the mixing effects of the storm and subsequent grazing controlled growth. This remains to be investigated. The large increase in zooplankton biomass in the two larger size classes is difficult to explain given typical zooplankton growth rates. Advective effects may be important, but again this remains to be investigated.

Surveys B and F - vertical structure

Having looked at the horizontal structure on the 20 and 200m levels, we now consider some examples of the associated vertical structure. The vertical SeaSoar profiles from all the D227 surveys may be found in Morrison et al. (in prep.), and a detailed study of these remains to be carried out. In appendix 2 we give a complete set of plots of the vertical profiles obtained from SeaSoar for surveys B and F. Here, in Figures 17-20, we consider two example sections B11 and F03 (see Figures A.3 and A.7 of for details of survey legs and positions). The choice of these vertical sections is somewhat arbitrary, but they are illustrative of the data overall. Work is in progress to examine the complete data set.

The vertical section plots show, over a depth of approximately 300m, potential temperature T, salinity S, potential density (s₀ in units of kg m^-3), light (PAR in units of log₁₀(W m^-2)), chlorophyll, and zooplankton biomass (aggregated into the size class 250-2000 microns, C_250-2000).

We begin with leg B11, the results for which are shown in Figures 17 (T, S, s₀)and 18 (PAR, chl, C250-2000). B11 is 180km in length and considerable meso-scale structure can be seen in the temperature, salinity and density fields (Figure 17). It can also be seen that the mixed layer at this stage of the cruise was about 40-50m deep. From the light levels (Figure 18) it can be seen that during this leg a night-day transition occurred, and there is some evidence of vertical migration in the zooplankton data at the beginning of the day. Clear evidence of vertical migration was found in the ADCP backscatter data for the leg (not shown). However, the ADCP "sees" much larger (>1cm) zooplankton than the OPC, which are known to exhibit strong vertical migration. From the chl data there is some suggestion of surface quenching in the measurements during the day. No attempt has been made to correct the data for this effect. The levels of zooplankton are relatively low, while those of chlorophyll relatively high (up to ~1.7mg chl m^-3). The chl distribution is horizontally patchy, and that patchiness appears to be related to the underlying physical structure to some degree. The vertical structure of T and chl are such that surface measurements (say from a satellite) would appear to be representative of the values to a depth of ~40m (allowing for the possibility of surface quenching in the daytime chl measurements, which would not affect satellite measurements of ocean colour).

Turning to leg F11, which is 150km long, and for which the results are given in Figure 19 and 20. The light levels (Figure 20) show that this leg was during the day. The major difference from B11 is that the effect of the intervening storm has deepened the mixed layer to approximately double (80-100m) its earlier depth (compare Figures 19 and 17). There is still evidence of meso-scale structure in the physics (Figure 19). For the biology (Figure 20), the chl levels have been halved (maximum values of ~0.8mg chl m^-3), while zooplankton biomass has increased greatly (up to ~180mg C m^-3). The decrease (halving) of the chl concentration can be linked to the increase (doubling) of the mixed layer depth. As noted in the previous section, the increase in zooplankton biomass is more difficult to explain, given the growth rates of zooplankton. Clearly visible is the patchiness in the chl and, particularly, the zooplankton distributions. Again this is linked to the physical structure, but now there is also evidence of biologically induced patchiness as well (which could not really be seen in B11). Thus at around 47^oN there is a "gap" in the zooplankton, where the phytoplankton appear to be growing (chl levels are higher). Whereas around 47.6^oN there is a patch of high zooplankton, possibly grazing on a patch of phytoplankton (indicated by slightly higher chl levels). A further point to note about the zooplankton is that their distribution is not uniform in the vertical within the mixed layer (the phytoplankton appear to be more uniformly distributed vertically). This may be due to the zooplankton swimming and swarming, whereas the phytoplankton tend to be passively advected by the turbulence in the mixed layer. As for survey B, these observations suggest that surface measurements of T and chl might be taken as representative of the conditions in the mixed layer, given the fairly uniform vertical structure of T and chl.

From this initial look at the vertical structure on one leg each in surveys B and F, we might conclude that surface measurement of T and chl may be representative of the values in the mixed layer. Given the dominance of T over S in determining the near surface density structure for this area, information on T will also give an indication of the density. There is no obvious link between the chl observations and those of the zooplankton, which suggests that either models or in situ data would be required to understand the biological distributions, in addition to any ocean colour observations from space.

Satellite data

Due to cloud cover during the cruise period few AVHRR or OCTS images were obtained of the area, and most of those that were obtained had a high percentage of cloud cover. Figure 21 shows the best AVHRR SST data available for the cruise. This is a composite of two images acquired on days 114 and 117 (afternoon passes) during survey B, the track of which is superimposed on the composite. Comparing the composite with the 20m potential temperatures from SeaSoar in Figure 1, shows that there are similarities, such as the existence of an eddy in the centre of the survey area and the intrusion of warmer water to the south east of the area. In fact the 200m potential temperatures (Figure 9) also show some correspondence with the SST data, but features (such as the eddy) are displaced relative to their surface expression. Due to the non-synopticity of the SeaSoar survey it is likely that some of the features in the AVHRR composite have been "smeared out" somewhat, but the similarity is still striking. This suggests that, in this case, the SST data are representative of the physical structure at depth.

For survey F the AVHRR SST images were basically cloud covered, with the occasional small gap that allowed the surface to be seen. By combining these with ATSR-2 data it may be possible to piece together a SST composite for comparison with survey F. This possibility remains to be investigated.

Moving on to consider the OCTS data (both chl and SST), we have had difficulty reading the LAC (Local Area Coverage) data that had been provided to us (as for the OMEGA area, we are currently liaising with NASDA to get LAC data for the D227 area in standard HDF format). Therefore, here we will only briefly consider the version 3 OCTS monthly composite GAC (4km spatial resolution) data that we do have to hand. Figures 22-25 show the April and May 1997 OCTS SST and chl data. Comparing the April OCTS SST (Figure 22) with the AVHRR composite (Figure 21) suggests that the thermal structures are not dissimilar (allowing for the different time scales and spatial resolution). Similarly, comparing the April OCTS chl (Figure 24) with the ship observed chl from survey B (Figure 3) shows overall agreement (qualitatively and quantitatively), particularly the enhanced chl to the south east of the survey region.

On comparing results from survey F (Figures 9 and 11) with the May OCTS SST and chl (Figures 23 and 25) the observations (ship and satellite) differ. A partial explanation for the difference is that the first half of May (during the cruise period) was cloudy, so the monthly composite probably more representative of the situation after the cruise ended. Clearly looking at weekly GAC or even better daily LAC data will allow us to carry out improved comparisons. This too remains to be done.

Initial conclusions

On the basis of this initial look at the ship and satellite data associated with the P²S³ cruise the following conclusions can be drawn:

a) the OCTS April surface chl values and those for at 20m for survey B seem in reasonable agreement. This is consistent with the OCTS being able to "see" an optical depth into the ocean. For chlorophyll concentrations of ~0.1 - 1mg chl m^-3 the optical depth varies from ~25 - 10m (Smith (1981)), which fits with the observations here. The differences observed, between survey F and the OCTS May values, are probably due to the fact that that this monthly composite is not representative of the cruise period.

b) the vertical structure observed during the cruise suggests that the surface T and chl values are representative of the mixed layer values. This is probably due to wind-induced mixing keeping the layer well-mixed during the later part of the cruise. During the earlier part of the cruise the upper ocean was being heated and re-stratifying. Despite this, the mixed layer appeared to remain well-mixed as it was shallowing.

c) for survey B, there is a clear correspondence between the spatial pattern of SST from AVHRR, and the 20m and 200m potential temperatures observed using SeaSoar. After the storm the surface to subsurface links are less clear in the survey F data. The detailed impact of the storm on the surface to subsurface links in both the physical and biological structure will be the subject of further investigations.
 
 

3.3 Final Comment

This initial report has presented preliminary conclusions on biophysical interactions in two cruise areas: Western Mediterranean and the North-East Atlantic. It is intended that the successor report will present further work, including possible insight gained by comparing the two regions/datasets.
 
 
Acknowledgements

We are grateful to Sam Lavender of RSDAS, PML for providing the OCTS GAC data used in this section.
 
 
Figures for Section 3.2

1. Survey B 20m potential temperature.

2. Survey B 20m salinity.

3. Survey B 20m chlorophyll (mg chl m^-3).

4. Survey B 20m zooplankton 250-500 microns (mg C m^-3).

5. Survey B 20m zooplankton 500-1000 microns (mg C m^-3).

6. Survey B 20m zooplankton 1000-2000 microns (mg C m^-3).

7. Survey B 200m potential temperature.

8. Survey B 200m salinity.

9. Survey F 20m potential temperature.

10. Survey F 20m salinity.

11. Survey F 20m chlorophyll (mg chl m^-3).

12. Survey F 20m zooplankton 250-500 microns (mg C m^-3).

13. Survey F 20m zooplankton 500-1000 microns (mg C m^-3).

14. Survey F 20m zooplankton 1000-2000 microns (mg C m^-3).

15. Survey F 200m potential temperature.

16. Survey F 200m salinity.

17. Vertical section for leg B11; from top to bottom - potential temperature, salinity and potential density (kg m^-3).

18. Vertical section for leg B11; from top to bottom - light (log₁₀ W m^-2), chlorophyll (mg chl m^-3), zooplankton biomass 250-2000 microns (mg C m^-3).

19. Vertical section for leg F03; from top to bottom - potential temperature, salinity and potential density (kg m^-3).

20. Vertical section for leg F03; from top to bottom - light (log₁₀ W m^-2), chlorophyll (mg chl m^-3), zooplankton biomass 250-2000 microns (mg C m^-3).

21. AVHRR SST composite for days 114 and 117, with ship track for survey B superimposed.

22. OCTS monthly GAC SST composite for April 1997.

23. OCTS monthly GAC SST composite for May 1997.

24. OCTS monthly GAC chlorophyll composite for April 1997.

25. OCTS monthly GAC chlorophyll composite for May 1997.
 
 
References

ACCP Acoustic Correlation Current Profiler

ADCP Acoustic Doppler Current Profiler

ADEOS ADvanced Earth Observation System

AIMS Analysis, Interpretation, Modelling and Synthesis (phase of WOCE)

ATSR Along-Track Scanning Radiometer (instrument on board ERS-1/2)

AVHRR Advanced Very High Resolution Radiometer

CTD Conductivity-Temperature-Depth

DYNAMO Dynamics of North Atlantic Models

EOS Earth Observation System (NASA satellite series)

ERS European Remote Sensing satellite

ESA European Space Agency

ESD Equivalent Spherical Diameter

GAC Global Area Coverage

GFO GEOSAT Follow-On

GIM Global Isopycnic Model

IRS Indian Remote Sensing satellite

LAC Local Area Coverage

LHPR Longhurst-Hardy Plankton Recorder

MERIS Medium Resolution Imaging Spectrometer (on board Envisat)

MICOM Miami Isopycnic Code Ocean Model

MODIS Moderate Resolution Imaging Spectrometer (on board EOS)

MOS Modular Optoelectronic Scanner (ocean colour sensor on board IRS)

NASA National Aeronautics and Space Administration

OCCAM Ocean Circulation and Climate Advanced Model

OCTS Ocean Colour and Temperature Scanner (on board ADEOS)

OPC Optical Plankton Counter

PAR Photosynthetically Active Radiation

P²S³ Phytoplankton Patchiness Studies by Ship and Satellite

SBWR ShipBorne Wave Recorder

SeaWiFS Sea-viewing Wide Field-of-view Sensor (US ocean colour satellite)

SOC Southampton Oceanography Centre

SSH Sea Surface Height

SST Sea Surface Temperature

T/P TOPEX/POSEIDON

TSG Thermosalinograph

WOCE World Ocean Circulation Experiment
 
 

APPENDIX: N.E. ATLANTIC CRUISE  D227 (P²S³)

Vertical sections for surveys B and F

The figures in this appendix show vertical sections (0-350 dbar) of SeaSoar data between 46^oN and 48.5^oN, for each leg of the large-scale surveys (B and F). Each figure comprises eight plots (a) through (h) (four per page a-d, e-h), as described below. The number of each leg is indicated at the top of the page, and the position of the leg in the survey is indicated by the small diagram at the top right of each page.

(a) Potential temperature referenced to 0 dbar, in units of degrees Celsius (^oC).

(b) Salinity calculated according to the Practical Salinity Scale 1978 (PSS78), which has no units, and cross-calibrated with the underway thermosalinograph and discrete surface samples.

(c) Potential density (s₀) referenced to 0 dbar, in units of kg/m³, calculated from the temperature and salinity measurements.

(d) Irradiance, in units of log₁₀W/m², calculated from measurements by the PAR light meter.

Section B00 (a)-(d)
Section B03 (a)-(d)
Section B07 (a)-(d)
Section B11 (a)-(d)
Section B13 (a)-(d)
Section F01 (a)-(d)
Section F03 (a)-(d)
Section F05 (a)-(d)
Section F07 (a)-(d)
Section F09 (a)-(d)
Section F11 (a)-(d)
Section F12 (a)-(d)

(e) Chlorophyll a, in units of micrograms/l (identical to mg/m³), calculated from the fluorimeter measurements and cross-calibrated with an underway fluorimeter and discrete surface samples.

(f) Displaced volume for zooplankton of equivalent spherical diameter (ESD) in the range 250-500 microns, in units of cm³/m³.

(g) Displaced volume for zooplankton of ESD in the range 500-1000 microns in units of cm³/m³.

(h) Displaced volume for zooplankton of ESD in the range 1000-2000 microns, in units of cm³/m³.

Note that an approximate estimate of the zooplankton biomass (in mg C m^-3) may be obtained from the displaced volume by multiplying the values by 40.

Section B00 (e)-(h)
Section B03 (e)-(h)
Section B07 (e)-(h)
Section B11 (e)-(h)
Section B13 (e)-(h)
Section F01 (e)-(h)
Section F03 (e)-(h)
Section F05 (e)-(h)
Section F07 (e)-(h)
Section F09 (e)-(h)
Section F11 (e)-(h)
Section F12 (e)-(h)