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Tracking the PRIME eddy using satellite altimetry
Deep-Sea Research II 48 (2001) 725}737
Tracking the PRIME eddy using satellite altimetry Ian P. Wade*, Karen J. Heywood School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK Received 11 September 1997; received in revised form 6 May 1998; accepted 5 August 1998
Abstract The PRIME cruise to the North Atlantic during June/July 1996 surveyed and sampled an extremely vigorous and deep-reaching eddy with a signi"cant barotropic component. Although it exhibited anticyclonic #ow and featured a warm core at depth, it had been capped at some point during its lifetime, so appeared as a cold feature in the upper 500 m. Satellite-derived sea-surface temperatures (SST) showed it to have moved little during the few weeks prior to the cruise. In this paper we discuss the origin of the PRIME eddy including where and when it is likely to have formed. Consistently large amounts of cloud cover restrict the use of SST imagery to track such features. Altimetry provides a better method to trace this eddy back in time and space since microwave radiation is not signi"cantly a!ected by cloud cover. Sea-level anomaly (SLA) data from the TOPEX/POSEIDON and European Remote Sensing (ERS) satellites were used. Results show that the eddy remained almost stationary in the Iceland Basin since "rst being detected in late 1995 and that it almost certainly formed locally, probably as a result of an instability in the current #ow around the northwest of the Hatton Bank. Comparisons between satellite SLAs and hydrographic estimates of sea-surface elevation con"rm that the eddy had a substantial barotropic #ow. Both the altimeter data and the sea-surface height derived from the acoustic Doppler current pro"ler agree that the PRIME eddy had a sea-surface elevation of about 20 cm and that its diameter was about 120 km. 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction As part of the UK Plankton Reactivity In the Marine Environment (PRIME) research programme, a cruise on RRS Discovery (D221) was conducted in the Northeast Atlantic during June and July 1996. The cruise was divided into two legs. The major aim of leg 1 was to perform a Lagrangian time series of sampling for biology and chemistry in the Iceland Basin. It was decided that an eddy would make a suitable site as the water contained within the eddy would remain * Corresponding author. Now at: SEADATA Division, Fugro GEOS Ltd., Southampton Oceanography Centre, Empress Dock, Southampton SO14 3ZH, UK. E-mail address: [email protected] (I.P. Wade). 0967-0645/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 0 0 ) 0 0 0 9 4 - 1
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Fig. 1. Bathymetry of the study area showing isobaths at 1000 m intervals with eddy location (E) and relevant TOPEX/POSEIDON tracks (labelled) and ERS tracks (unlabelled). Ascending ERS tracks (southwest to northeast) are 448, 906, 362 and 820 (respectively west to east). Descending ERS tracks (northwest to southeast) are 717, 173, 631 and 87 (respectively west to east). H"Hatton Bank, R"Rockall Bank. (Inset) AVHRR sea-surface temperature image of the PRIME eddy taken on 6 July 1996 during the early stages of leg 2 of the cruise. Enlargement of area marked E 59}603N, 22}203W. White areas represent cloud cover. The centre of the eddy has an SST of approximately 10.33C compared to an ambient SST of approximately 11.53C. (Images supplied by Steve Groom, RSDAS, PML; from data supplied by the Satellite Receiving station, Dundee.).
coherent for longer periods of time relative to the surrounding waters. In order to locate a suitable feature, infra-red images of sea-surface temperature (SST) obtained using the advanced very high resolution radiometer (AVHRR) from the National Ocean and Atmosphere Administration (NOAA) satellites were examined for the six weeks preceding the cruise. They showed a suitable feature within the Iceland Basin with a cold surface core for several weeks prior to the cruise. An example of SST image from the time of the cruise (03:53 GMT on 6 June 1996) is shown in Fig. 1 (inset). A decrease in temperature of approximately 13C between the centre of the eddy and the surrounding water matched that observed during the SeaSoar surveys of the feature (Martin et al., 1998). The formation of the PRIME eddy is a topic of interest, not only because of its particularly strong and deep-reaching circulation, but also because the location and timing of its formation will determine the biological and chemical characteristics of the water contained within the eddy. This will include its nutrient concentration and the planktonic species within it. Knowing the age of the eddy should enable more accurate modelling of the productivity within this semi-enclosed system.
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The PRIME eddy was chosen as it was the most coherent of three possible cold-core eddies located sporadically using AVHRR SST images. Unfortunately cloud cover is frequent in the Northeast Atlantic, so tracking this eddy through time using SST data proved unsatisfactory. Tracking the eddy by other satellite-based methods appears to be the answer to the problem of extensive cloud cover. Eddies have marked e!ects on the sea-surface height, appearing as either depressions (cyclonic rings) or elevations (anticyclonic rings) relative to the surrounding waters. Satellite altimetry, therefore, o!ers an alternative tracking method that is una!ected by cloud cover. Satellites capable of measuring sea-surface height to within a few cm over the entire globe are routinely used. In this paper we use TOPEX/ POSEIDON and ERS altimeter data to track the PRIME eddy back in time and space.
2. Physical oceanography of the PRIME Eddy Cold-core rings are cyclonic and appear as depressions in the sea surface. However, despite having a cooler SST, the PRIME eddy rotated anticyclonically at the surface (shown in the shipborne acoustic Doppler current pro"ler (ADCP) data during the initial SeaSoar surveys of the region; Fig. 4c of Martin et al., 1998). Analysis of the deeper hydrographic structure explained this apparent anomaly. A zonal section comprising seven full-depth CTDs was undertaken from 59.13N, 19.43W to 59.13N, 21.73W across the eddy at the end of leg 1 (27}29 June). The section was approximately 130 km in length, with a mean station spacing of approximately 21 km and sea bed depths between 2800 and 2900 m. The density anomaly (p ) section through the eddy (Fig. 2) F showed a reversal in the slope of the isopycnals above and below a large warm-core pycnostad centred around 600 m. The expected cyclonic contribution to the #ow in the surface 600 m was countered by a stronger anticyclonic #ow due to the isopycnal slopes below this depth. Because of its strong anticyclonic circulation, we expected the eddy to manifest itself in satellite altimeter data as a doming of the sea surface. Winter mixing depths in this region are typically in the order of 500} 600 m. Because there is decreased strati"cation through the upper few hundred meters of the eddy after formation, such mixing may preferentially cool the surface water within the eddy by enhanced entrainment at the base of the mixed layer, thus creating, and enhancing, a surface cold signature (Arhan et al., 1994). The physical oceanographic characteristics of the PRIME eddy are discussed further by Martin et al. (1998), who show that the temperature and salinity properties of the water inside the eddy were not inconsistent with local formation, possibly as a result of an instability in the #ow to the northwest of the Hatton Bank. Knowing the location at which the eddy was shed should assist in determining the processes by which it formed, and its relationship to the larger scale circulation.
3. Methods 3.1. Estimation of sea-level anomalies (SLAs) Satellite altimeter data from the TOPEX/POSEIDON (henceforth T/P), ERS-1 and ERS-2 missions were analysed. These missions use microwave altimetry to make precise observations of
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Fig. 2. Density anomalies (p ) across the eddy centre at the end of leg 1 of the PRIME cruise. An extensive pycnostad is F observed centred at 600 m with a doming upwards of isopycnals above this depth. Flow above 600 m is driven anticyclonically by the deeper #ow.
sea level, with an accuracy of 2}3 cm . The T/P satellite mission is jointly run by the United States, National Aeronautics and Space Administration (NASA) and the French Space Agency, Centre National d'Etudes Spatiales (CNES). The T/P satellite has been on a 10-day Exact Repeat Mission (i.e. the satellite over#ies the same location every 10 days) from its launch in August 1992 until present (Fu et al., 1994). The ERS-1 and ERS-2 missions are run by the European Space Agency (ESA). ERS-1 was launched in July 1991 and deployed into its second series of 35-day repeat orbits (Phase G) in March 1995 to coincide with the launch of ERS-2 (April 1995), which also operates with a 35-day repeat on the same ground tracks but o!set by 1 day. The ERS 35-day repeat orbits o!er superior spatial resolution over the T/P 10-day repeat orbit. Data used in these analyses have been generated by the Collecte Localisation Satellites (CLS) Space Oceanography Division and are distributed on CD-ROMs by Archivage, Validation et InterpreH tation des Donnees des Satellites OceH anographiques (AVISO). Altimeter measurements require a number of geophysical corrections. The data have been corrected, by CLS, for instrumental errors, environmental perturbations, tidal in#uences and inverse barometer e!ects to give a corrected sea-surface height (CORSSH). Full details of these corrections are available (AVISO, 1996). CORSSH is dominated by the geoid, which is not known accurately on scales less than basin-scale. Therefore, sea-level anomaly (SLA) "les are generated from several CORSSH "les. The T/P anomaly is taken as the deviation from a 3-year mean of cycles 11}121 (January 1993 to December 1995). The ERS anomaly is taken from a mean derived from ERS-1 phases C and G
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("rst and second 35-day repeat orbits, respectively) using T/P data to correct for orbit errors (AVISO, 1997). This yields an ERS 3-year mean compatible with the T/P 3-year mean. Because SLA data are calculated relative to a 3-year mean, quasi-constant slopes in sea level, associated with permanent currents, are e!ectively removed and temporary, mobile features, such as eddies, remain. This type of data is therefore well suited to the study of eddies. The PRIME eddy was located at approximately 59.53N, 21.03W (Fig. 1). The inset to this "gure shows the PRIME eddy clearly identi"ed during the cruise by AVHRR SST. Six T/P tracks (241, 165 and 89 ascending, 146, 70 and 248 descending) and eight ERS tracks (448, 906, 362 and 820 ascending and 717, 173, 631 and 87 descending) that pass closest to the eddy are shown. T/P 10-day repeat track spacing at the latitude of the PRIME eddy is approximately 100 km. ERS 35-day repeat track spacing is approximately 40 km. Processed T/P SLA data were analysed between cycle 2 (October 1992) and cycle 149 (September 1996, the latest processed data available). These cycles cover the time of the June}July 1996 PRIME cruise (cycles 138}141). ERS-1 and ERS-2 data from March 1995 to June 1996 were used to complement the T/P data. 3.2. Sea-level variability For a number of altimeter passes along a certain track, i"1 to n, the sea-level anomaly is de"ned as *HG"SLA"HG!HM , where H is the sea-surface height, and the sea-level variability (i.e. the rms of the SLA) as p "( L (SLA)/n. 1* G Sea-level variabilities were calculated from T/P altimeter data for the years 1993}1996. Because the 1996 data only extended to September of that year (cycle 149), sea-level variability for the years 1993}1995 was calculated only using data from January to September. This results in directly comparable variabilities between the years that are una!ected by any seasonal changes (White and Heywood, 1995).
4. Results 4.1. Comparisons of sea-surface height between altimetry and hydrography The currents at 200 m associated with the eddy were large, of order 50 cm s\. This was likely to be associated with a signi"cant doming of the sea surface, which should be detectable by satellite altimeters. The dynamic height relative to 2500 m (the baroclinic contribution to the sea-surface elevation) is plotted in Fig. 3. Since the reference level was arbitrary, the dynamic heights were adjusted so that the height at the edge of the eddy (station 1) was zero. The CTD section passed through the centre of the eddy (station 5), where the dynamic height was about 8 cm. This assumed a zero velocity at 2500 m. Fig. 3 also shows the T/P SLA along track 70 (see Fig. 1 for the location of the track in relation to the eddy) for cycle 141 at the end of the PRIME cruise in July 1996. Although the satellite track did
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Fig. 3. Sea-surface height from the TOPEX/POSEIDON altimeter along track 70 (cycle 141, end of PRIME cruise) compared with dynamic height (referenced to 2500 m) and ADCP current-derived height along the zonal CTD section. The abscissa is the distance (km) to allow comparisons of the di!erent sampling strategies that crossed the eddy from di!erent orientations. It has a purely arbitrary starting point.
Fig. 4. TOPEX/POSEIDON sea-level anomalies (10-day "elds) from cycles 140 and 141 (leg 2) with corresponding track spacing. Solid lines represent anomalies greater than 10 cm, contour interval is 2 cm.
not cross the eddy zonally, a comparison with the zonal CTD section was justi"ed if we assumed the eddy to be radially symmetrical. The altimetry showed a positive anomaly of almost 18 cm (dashed line), considerably larger than the dynamic height relative to 2500 m (circles). This was not surprising since Martin et al. (1998) showed that the eddy had a substantial barotropic component and also, as is always present in eddying or meandering #ow, a small ageostrophic cyclostrophic (or rotational) component. In anticyclonic #ow the cyclostrophic component augments the horizontal pressure gradient. Using the ADCP to provide a reference level for the geostrophic shear showed that the currents at 2500 m were about 20 cm s\, far from zero (Martin et al., 1998). Our comparison of the satellite altimetry with the hydrographic estimate con"rms that the eddy has a large component of #ow that is not baroclinic. Although we believe that altimeter track 70 passed close to the eddy centre at the time of the cruise, any deviation from it would result in an underestimate of the sea-level anomaly associated with the eddy.
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The di!erence in sea-surface height between two stations is directly proportional to the surface geostrophic #ow (< ) between them g tan(i) g*H " , < " f¸ f where g is the acceleration due to gravity, f is the Coriolis parameter, i is the angle between the sea surface and a level surface, *H is the height di!erence and L the horizontal distance between the stations. Using this relationship and the absolute current measurements taken with the shipborne ADCP, we derived the sea-surface height di!erence between station pairs and therefore a height estimate across the eddy including the non-baroclinic terms. These ADCP-derived sea-surface heights (crosses) are shown integrated from station 1 (Fig. 3). The agreement between these ADCP-derived sea-surface heights and the altimetric sea-level anomaly was encouraging. Both datasets showed the eddy to be approximately 120 km in diameter and to exhibit a doming of the sea surface of almost 20 cm. Assuming a comparatively small cyclostrophic component of #ow, this suggested that the T/P altimeter track 70 did indeed pass close to eddy centre. It is interesting to note that all three cold-surface eddies considered for sampling, from the initial AVHRR images, had positive SLAs in the altimetry data sets. The other features are not discussed in detail here due to a lack of hydrographic data. However, the repeated cold-core/positive SLA combination does suggest that the PRIME eddy is not a unique feature. 4.2. Tracking the PRIME eddy (mapping) Altimeter SLA values provided every 7 km along track were gridded to form maps indicating dynamic topography due to transient features. For the ERS satellites, data were gridded onto a 0.1253 grid using a search radius of 70 km, and weighting values using a Gaussian curve of full-width at half-maximum of 35 km. For the more widely spaced T/P tracks, data were gridded onto a 0.253 grid with a search radius of 140 km and a full-width at half-maximum value of 70 km. The 10-day repeat orbit of the T/P mission was not particularly suited to the horizontal mapping of anomalies in the size of the PRIME eddy due to its coarse spatial resolution. The track spacing of this satellite at this latitude, about 100 km, was close to the diameter of the eddy, so the mapping continuity from one cycle to the next was compromised as the eddy drifted onto and in between the tracks giving SLA signals of varying strength (Fig. 4a). Maps of T/P SLA for cycles 140 and 141 corresponding approximately to leg 2 of the PRIME cruise do show the eddy (Figs. 4b and c), although the track spacing was problematic on other cycles. The PRIME eddy was reasonably clear due to the position of track 70 in relation to the eddy. Due to their increased spatial resolution, the ERS satellites were far better suited to this type of mapping (Fig. 5a). Figs. 5b}l show successive 35-day "elds for the combined ERS-1 and ERS-2 SLAs. These "elds covered the time period from May 1995 to June 1996; so the last "eld (Fig. 5l) corresponds to leg 1 of the PRIME cruise. The two ERS satellites could be successfully merged in this way as the time lag between "elds was only 1 day (a negligible temporal di!erence over a 35-day "eld). This merging of the data allowed us to complete spatial coverage for all "elds. A seasonal steric e!ect due to warming of the upper ocean was observed over the one year of this data. The PRIME eddy is clearly observed as a positive SLA in Figs. 5h}l and there was some
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Fig. 5. Combined ERS-1 and ERS-2 sea-level anomalies (35-day "elds) from the year preceding the PRIME cruise with corresponding track spacing. Solid lines represent positive anomalies, contour interval is 2 cm. The PRIME eddy is observed in the later half of the time series. (l) The time of the PRIME cruise.
evidence for its presence in Figs. 5e}g. Fig. 5h corresponds to the time period from mid-December 1995 to mid-January 1996. The PRIME eddy had therefore existed in approximately the same position within the Iceland Basin for at least 6 months prior to the cruise.
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Fig. 6. Ascending ERS-1 track sea-level anomalies o!set from one another by 25 cm for clarity. The ordinant in this "gure is the cycle number and corresponds to time.
Fig. 7. Descending ERS-1 track sea-level anomalies o!set from one another by 25 cm for clarity. The ordinant in this "gure is the cycle number and corresponds to time.
4.3. Tracking the PRIME eddy (track-by-track analyses) SLAs along the eight ERS-1 tracks (Fig. 1) crossing over or passing near to the PRIME eddy for the year prior to the PRIME cruise are shown in Figs. 6 (ascending) and 7 (descending). In both ascending and descending tracks the SLA signature of the eddy was most clearly observed in the centre two tracks as expected. In the majority of the cases, the ERS-2 data closely resembled that of ERS-1 and showed broadly similar features (not shown). In the case of the ERS data, consistency over time was sacri"ced somewhat for the increased spatial resolution. However, the PRIME eddy was still clearly observed from late 1995. A strong PRIME eddy signal was seen in the middle of the time series for track 173 and was also seen in the latter half of the track 631 time series (Fig. 7), implying a slight eastward propagation. Track 631 showed a slight northward propagation. Ascending tracks 906 and 362 (Fig. 6) also implied a slow northeastward movement of the eddy. A net northeastward drift agreed with the prevailing currents of the region, although the magnitude of the eddy drift was considerably smaller than would be expected due to currents alone. It is possible that this anticyclonic eddy's expected self-propagation (west/southwest with respect to
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Fig. 8. Hofmuller diagram of TOPEX/POSEIDON Sea-level anomalies for tracks 248, 70 and 146 across the Iceland Basin from southeast to northwest, from cycle 2 (October 1992) to 149 (September 1996). Anomalies greater than #10 cm are shown in bold, anomalies less than }10 cm in grey. Vertical dashed lines represent the positions of the Rockall Channel (&553N) and Iceland Basin (&593N). The PRIME eddy is observed from the beginning of 1996 on track 70 within the Iceland Basin.
lines of planetary or topographic b; see McWilliams, 1985; Martin et al., 1998) partially countered the prevailing drift. T/P track-by-track analysis o!ered superior temporal resolution. The 10-day repeat T/P time series extended back as far as October 1992, allowing us to determine the climatology of eddies in this area. Track 70 passed close to the eddy centre (Fig. 1). In order to track the eddy back in time, sea-level anomalies greater than $10 cm for this track were plotted, along with adjacent descending tracks 248 and 146, on space}time axes in the form of a Hofmuller diagram (Fig. 8). Fig. 8 covers four years of data (cycle 2 (October 1992) to cycle 149 (October 1996)). Anomalous features were clearly observed within the Rockall Channel and Iceland Basin on all three tracks. Both are areas of high eddy activity (Heywood et al., 1994; Bersch, 1995). The doming caused by the PRIME eddy was seen between 59 and 603N from at least the start of 1996 (cycle 121) to the cruise (cycles 138}141). Fig. 8 (track 70) shows that the eddy was present prior to the usual winter mixing period, although it is possible that winter mixing could have occurred earlier in 1996. The winter mixing depths (the maximum depth at which the potential temperature is within 0.53C of its surface value) calculated for the decade 1964}1974 at Ocean Weather Ship India (593N, 193W) are shown in Fig. 9. This is close to the location of the PRIME eddy. Fig. 9 suggests that winter mixing in this region could occur anytime between January and April, and that this can often extend to depths in
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Fig. 9. Winter mixing depths at Ocean Weather Ship INDIA (593N, 193W) between 1964 and 1974 using a *T"0.53C criterion. Maximum winter mixing typically occurs around March each year but may occur anytime between January and April. The continuous line represents the mean winter mixing depth averaged over successive 3-day time intervals and linearly interpolated over missing data.
excess of 500 m. As the PRIME eddy exhibits a pycnostad at 600 m, it is possible that the stad formed when winter storms mixed the upper ocean down to this depth. No evidence of the PRIME eddy is seen along adjacent tracks 248 or 146 at times prior to its signal on track 70, suggesting, within the limitations of the T/P spatial resolution, that it formed locally in the Iceland Basin, probably from instabilities in the current #ow around the northwestern #ank of the Hatton Bank. The eddy appears to have moved little along track 70 during 1996 in agreement with the ERS analyses. 4.4. Sea-level variability Fig. 10 shows the sea-level variability for the "rst nine months of the years 1993}1996. Only variabilities greater than 5 cm are shown in order to emphasise the main patterns. These data were derived from the T/P altimetry measurements. Higher variability is seen in all plots within the Iceland Basin and the Rockall Channel for all the four years. Di!erences in the sea}level variability from year to year were observed, with 1993 and 1995 showing similar patterns, 1994 showing low variability and 1996 high variability within the Iceland Basin. It should be noted that a truly stationary eddy would not exhibit a signal in the sea-level variability. However, the PRIME eddy was observed to rotate, as a whole, with a period of approximately 3.5 days (Martin et al., 1998, Fig. 6). This complex anticyclonic precession would result in high sea-level variability in the region of the eddy without the eddy translating a great distance. There is some evidence for this in Fig. 10d in the vicinity of the eddy.
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Fig. 10. Sea-level variability (cm) for the "rst 9 months of the years 1993}1996. Only variabilities greater than 5 cm are contoured for clarity. Subsequent contour interval is 1 cm.
5. Conclusions The PRIME eddy has been clearly observed in the altimetry. T/P and ERS tracks bisected the PRIME eddy close to its centre revealing an SLA of almost 20 cm relative to a 3-year mean. A comparison of altimeter-derived SLA and hydrographic estimations of sea-surface elevation con"rmed the "ndings of Martin et al. (1998) that there was a signi"cant non-baroclinic component of #ow. Estimations of sea-surface elevation from the ADCP-derived total velocities matched the altimetry, giving justi"cation to the techniques used by Martin et al. (1998) of referencing the geostrophic shear to ADCP absolute velocities. Small di!erences in height can be attributed to centrifugal ageostrophic (cyclostrophic) components of #ow and/or the satellite tracks not crossing the eddy centrally. The altimeter data showed that the eddy had moved very little (less than the PRIME eddy diameter (120 km) in any direction) and that it had existed in the Iceland Basin since at least the start of 1996, some 6 months prior to the cruise, and possibly even earlier. This means that the eddy had most likely formed prior to the last deep winter mixing period, usually around March. Track-by-track analyses show that the eddy formed locally in the Iceland Basin and was not advected in from outside this region, although some motion is evident within the basin itself. Temperature and salinity characteristics show the core of the eddy to contain water expected of the region (Martin et al., 1998). Although we did not have the opportunity to sample either of the other two possible eddies located in the AVHRR images, their temperature/SLA characteristics suggest that the PRIME eddy was not unique. More work is required to determine the mechanisms forming the cold surface
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caps to these warm core eddies. The increased sea-level variability in the Iceland basin during 1996 suggests an increase in eddy (mesoscale) activity in the region during the PRIME year, but whether this is due to more PRIME-type eddies or more regular eddies cannot be ascertained with the present data.
Acknowledgements The SLA products are supplied by the CLS Space Oceanography Division, Toulouse, France (AVISO/Altimetry, 1996; Le Traon et al., 1995; Le Traon and Ogor, 1998). The ERS products were generated as part of the proposal `Joint analysis of ERS-1, ERS-2 and TOPEX/POSEIDON altimeter data for oceanic circulation studiesa selected in response to the Announcement of Opportunity for ERS-1/2 by the European Space Agency (proposal code: A02.F105). Thanks to Steve Groom at the Remote Sensing Data Analysis Service (RSDAS) at Plymouth Marine Laboratory (PML) for supplying the AVHRR image. Data processing was undertaken by the RSDAS using data received and archived at the Satellite Receiving Station, Dundee. Thanks also to the O$cers, Scientists and Crew of RRS Discovery cruise 221. This work was supported by the Natural Environment Research Council PRIME Grant GST/02/1084 and is PRIME contribution 56.
References Arhan, M., de Verdiere, A.C., Memery, L., 1994. The eastern boundary of the subtropical North Atlantic. Journal of Physical Oceanography 24 (6), 1295}1316. AVISO, 1996. User Handbook, Corrected Sea Surface Heights (CORSSHs), AVI-NT-011-311-CN, 2nd Edition, 21pp. AVISO, 1997. User Handbook, Sea Level Anomalies (SLAs), AVI-NT-011-312-CN, 2nd Edition, 24pp. Bersch, M., 1995. On circulation of the north-eastern North Atlantic. Deep-Sea Research 42, 1583}1607. Fu, L.-L., Christenson, E.J., Yamarone Jr., A., Lefebvre, M., MeH nard, Y., Dorrer, M., Escudier, P., 1994. TOPEX/POSEIDON mission overview. Journal of Geophysical Research 99 (C12), 24369}24381. Heywood, K.J., McDonagh, E.L., White, M.A., 1994. Eddy kinetic energy of the North Atlantic subpolar gyre from satellite altimetry. Journal of Geophysical Research 99 (C11), 22525}22539. Le Traon, P.Y., Gaspar, P., Bouyssel, F., Makhmara, H., 1995. Using TOPEX/POSEIDON data to enhance ERS-1 orbit. Journal of Atmospheric and Oceanic Technology 12, 161}170. Le Traon, P.Y., Ogor, F., 1998. ERS-1 Orbit improvement using TOPEX/POSEIDON: the 2 cm challenge, Journal of Geophysical Research 103 (C4), 8045}8057. Martin, A.P., Wade, I.P., Richards, K.J., Heywood, K.J., 1998. The PRIME Eddy. Journal of Marine Research 56 (2), in press. McWilliams, J.C., 1985. Submesoscale coherent vortices in the ocean. Reviews of Geophysics 23 (2), 165}182. White, M., Heywood, K.J., 1995. Seasonal and interannual changes in the North Atlantic Subpolar Gyre from Geosat and TOPEX/POSEIDON altimetery. Journal of Geophysical Research 100 (C12), 24931}24941.
Tracking the PRIME eddy using satellite altimetry Ian P. Wade*, Karen J. Heywood School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK Received 11 September 1997; received in revised form 6 May 1998; accepted 5 August 1998
Abstract The PRIME cruise to the North Atlantic during June/July 1996 surveyed and sampled an extremely vigorous and deep-reaching eddy with a signi"cant barotropic component. Although it exhibited anticyclonic #ow and featured a warm core at depth, it had been capped at some point during its lifetime, so appeared as a cold feature in the upper 500 m. Satellite-derived sea-surface temperatures (SST) showed it to have moved little during the few weeks prior to the cruise. In this paper we discuss the origin of the PRIME eddy including where and when it is likely to have formed. Consistently large amounts of cloud cover restrict the use of SST imagery to track such features. Altimetry provides a better method to trace this eddy back in time and space since microwave radiation is not signi"cantly a!ected by cloud cover. Sea-level anomaly (SLA) data from the TOPEX/POSEIDON and European Remote Sensing (ERS) satellites were used. Results show that the eddy remained almost stationary in the Iceland Basin since "rst being detected in late 1995 and that it almost certainly formed locally, probably as a result of an instability in the current #ow around the northwest of the Hatton Bank. Comparisons between satellite SLAs and hydrographic estimates of sea-surface elevation con"rm that the eddy had a substantial barotropic #ow. Both the altimeter data and the sea-surface height derived from the acoustic Doppler current pro"ler agree that the PRIME eddy had a sea-surface elevation of about 20 cm and that its diameter was about 120 km. 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction As part of the UK Plankton Reactivity In the Marine Environment (PRIME) research programme, a cruise on RRS Discovery (D221) was conducted in the Northeast Atlantic during June and July 1996. The cruise was divided into two legs. The major aim of leg 1 was to perform a Lagrangian time series of sampling for biology and chemistry in the Iceland Basin. It was decided that an eddy would make a suitable site as the water contained within the eddy would remain * Corresponding author. Now at: SEADATA Division, Fugro GEOS Ltd., Southampton Oceanography Centre, Empress Dock, Southampton SO14 3ZH, UK. E-mail address: [email protected] (I.P. Wade). 0967-0645/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 0 0 ) 0 0 0 9 4 - 1
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Fig. 1. Bathymetry of the study area showing isobaths at 1000 m intervals with eddy location (E) and relevant TOPEX/POSEIDON tracks (labelled) and ERS tracks (unlabelled). Ascending ERS tracks (southwest to northeast) are 448, 906, 362 and 820 (respectively west to east). Descending ERS tracks (northwest to southeast) are 717, 173, 631 and 87 (respectively west to east). H"Hatton Bank, R"Rockall Bank. (Inset) AVHRR sea-surface temperature image of the PRIME eddy taken on 6 July 1996 during the early stages of leg 2 of the cruise. Enlargement of area marked E 59}603N, 22}203W. White areas represent cloud cover. The centre of the eddy has an SST of approximately 10.33C compared to an ambient SST of approximately 11.53C. (Images supplied by Steve Groom, RSDAS, PML; from data supplied by the Satellite Receiving station, Dundee.).
coherent for longer periods of time relative to the surrounding waters. In order to locate a suitable feature, infra-red images of sea-surface temperature (SST) obtained using the advanced very high resolution radiometer (AVHRR) from the National Ocean and Atmosphere Administration (NOAA) satellites were examined for the six weeks preceding the cruise. They showed a suitable feature within the Iceland Basin with a cold surface core for several weeks prior to the cruise. An example of SST image from the time of the cruise (03:53 GMT on 6 June 1996) is shown in Fig. 1 (inset). A decrease in temperature of approximately 13C between the centre of the eddy and the surrounding water matched that observed during the SeaSoar surveys of the feature (Martin et al., 1998). The formation of the PRIME eddy is a topic of interest, not only because of its particularly strong and deep-reaching circulation, but also because the location and timing of its formation will determine the biological and chemical characteristics of the water contained within the eddy. This will include its nutrient concentration and the planktonic species within it. Knowing the age of the eddy should enable more accurate modelling of the productivity within this semi-enclosed system.
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The PRIME eddy was chosen as it was the most coherent of three possible cold-core eddies located sporadically using AVHRR SST images. Unfortunately cloud cover is frequent in the Northeast Atlantic, so tracking this eddy through time using SST data proved unsatisfactory. Tracking the eddy by other satellite-based methods appears to be the answer to the problem of extensive cloud cover. Eddies have marked e!ects on the sea-surface height, appearing as either depressions (cyclonic rings) or elevations (anticyclonic rings) relative to the surrounding waters. Satellite altimetry, therefore, o!ers an alternative tracking method that is una!ected by cloud cover. Satellites capable of measuring sea-surface height to within a few cm over the entire globe are routinely used. In this paper we use TOPEX/ POSEIDON and ERS altimeter data to track the PRIME eddy back in time and space.
2. Physical oceanography of the PRIME Eddy Cold-core rings are cyclonic and appear as depressions in the sea surface. However, despite having a cooler SST, the PRIME eddy rotated anticyclonically at the surface (shown in the shipborne acoustic Doppler current pro"ler (ADCP) data during the initial SeaSoar surveys of the region; Fig. 4c of Martin et al., 1998). Analysis of the deeper hydrographic structure explained this apparent anomaly. A zonal section comprising seven full-depth CTDs was undertaken from 59.13N, 19.43W to 59.13N, 21.73W across the eddy at the end of leg 1 (27}29 June). The section was approximately 130 km in length, with a mean station spacing of approximately 21 km and sea bed depths between 2800 and 2900 m. The density anomaly (p ) section through the eddy (Fig. 2) F showed a reversal in the slope of the isopycnals above and below a large warm-core pycnostad centred around 600 m. The expected cyclonic contribution to the #ow in the surface 600 m was countered by a stronger anticyclonic #ow due to the isopycnal slopes below this depth. Because of its strong anticyclonic circulation, we expected the eddy to manifest itself in satellite altimeter data as a doming of the sea surface. Winter mixing depths in this region are typically in the order of 500} 600 m. Because there is decreased strati"cation through the upper few hundred meters of the eddy after formation, such mixing may preferentially cool the surface water within the eddy by enhanced entrainment at the base of the mixed layer, thus creating, and enhancing, a surface cold signature (Arhan et al., 1994). The physical oceanographic characteristics of the PRIME eddy are discussed further by Martin et al. (1998), who show that the temperature and salinity properties of the water inside the eddy were not inconsistent with local formation, possibly as a result of an instability in the #ow to the northwest of the Hatton Bank. Knowing the location at which the eddy was shed should assist in determining the processes by which it formed, and its relationship to the larger scale circulation.
3. Methods 3.1. Estimation of sea-level anomalies (SLAs) Satellite altimeter data from the TOPEX/POSEIDON (henceforth T/P), ERS-1 and ERS-2 missions were analysed. These missions use microwave altimetry to make precise observations of
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Fig. 2. Density anomalies (p ) across the eddy centre at the end of leg 1 of the PRIME cruise. An extensive pycnostad is F observed centred at 600 m with a doming upwards of isopycnals above this depth. Flow above 600 m is driven anticyclonically by the deeper #ow.
sea level, with an accuracy of 2}3 cm . The T/P satellite mission is jointly run by the United States, National Aeronautics and Space Administration (NASA) and the French Space Agency, Centre National d'Etudes Spatiales (CNES). The T/P satellite has been on a 10-day Exact Repeat Mission (i.e. the satellite over#ies the same location every 10 days) from its launch in August 1992 until present (Fu et al., 1994). The ERS-1 and ERS-2 missions are run by the European Space Agency (ESA). ERS-1 was launched in July 1991 and deployed into its second series of 35-day repeat orbits (Phase G) in March 1995 to coincide with the launch of ERS-2 (April 1995), which also operates with a 35-day repeat on the same ground tracks but o!set by 1 day. The ERS 35-day repeat orbits o!er superior spatial resolution over the T/P 10-day repeat orbit. Data used in these analyses have been generated by the Collecte Localisation Satellites (CLS) Space Oceanography Division and are distributed on CD-ROMs by Archivage, Validation et InterpreH tation des Donnees des Satellites OceH anographiques (AVISO). Altimeter measurements require a number of geophysical corrections. The data have been corrected, by CLS, for instrumental errors, environmental perturbations, tidal in#uences and inverse barometer e!ects to give a corrected sea-surface height (CORSSH). Full details of these corrections are available (AVISO, 1996). CORSSH is dominated by the geoid, which is not known accurately on scales less than basin-scale. Therefore, sea-level anomaly (SLA) "les are generated from several CORSSH "les. The T/P anomaly is taken as the deviation from a 3-year mean of cycles 11}121 (January 1993 to December 1995). The ERS anomaly is taken from a mean derived from ERS-1 phases C and G
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("rst and second 35-day repeat orbits, respectively) using T/P data to correct for orbit errors (AVISO, 1997). This yields an ERS 3-year mean compatible with the T/P 3-year mean. Because SLA data are calculated relative to a 3-year mean, quasi-constant slopes in sea level, associated with permanent currents, are e!ectively removed and temporary, mobile features, such as eddies, remain. This type of data is therefore well suited to the study of eddies. The PRIME eddy was located at approximately 59.53N, 21.03W (Fig. 1). The inset to this "gure shows the PRIME eddy clearly identi"ed during the cruise by AVHRR SST. Six T/P tracks (241, 165 and 89 ascending, 146, 70 and 248 descending) and eight ERS tracks (448, 906, 362 and 820 ascending and 717, 173, 631 and 87 descending) that pass closest to the eddy are shown. T/P 10-day repeat track spacing at the latitude of the PRIME eddy is approximately 100 km. ERS 35-day repeat track spacing is approximately 40 km. Processed T/P SLA data were analysed between cycle 2 (October 1992) and cycle 149 (September 1996, the latest processed data available). These cycles cover the time of the June}July 1996 PRIME cruise (cycles 138}141). ERS-1 and ERS-2 data from March 1995 to June 1996 were used to complement the T/P data. 3.2. Sea-level variability For a number of altimeter passes along a certain track, i"1 to n, the sea-level anomaly is de"ned as *HG"SLA"HG!HM , where H is the sea-surface height, and the sea-level variability (i.e. the rms of the SLA) as p "( L (SLA)/n. 1* G Sea-level variabilities were calculated from T/P altimeter data for the years 1993}1996. Because the 1996 data only extended to September of that year (cycle 149), sea-level variability for the years 1993}1995 was calculated only using data from January to September. This results in directly comparable variabilities between the years that are una!ected by any seasonal changes (White and Heywood, 1995).
4. Results 4.1. Comparisons of sea-surface height between altimetry and hydrography The currents at 200 m associated with the eddy were large, of order 50 cm s\. This was likely to be associated with a signi"cant doming of the sea surface, which should be detectable by satellite altimeters. The dynamic height relative to 2500 m (the baroclinic contribution to the sea-surface elevation) is plotted in Fig. 3. Since the reference level was arbitrary, the dynamic heights were adjusted so that the height at the edge of the eddy (station 1) was zero. The CTD section passed through the centre of the eddy (station 5), where the dynamic height was about 8 cm. This assumed a zero velocity at 2500 m. Fig. 3 also shows the T/P SLA along track 70 (see Fig. 1 for the location of the track in relation to the eddy) for cycle 141 at the end of the PRIME cruise in July 1996. Although the satellite track did
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Fig. 3. Sea-surface height from the TOPEX/POSEIDON altimeter along track 70 (cycle 141, end of PRIME cruise) compared with dynamic height (referenced to 2500 m) and ADCP current-derived height along the zonal CTD section. The abscissa is the distance (km) to allow comparisons of the di!erent sampling strategies that crossed the eddy from di!erent orientations. It has a purely arbitrary starting point.
Fig. 4. TOPEX/POSEIDON sea-level anomalies (10-day "elds) from cycles 140 and 141 (leg 2) with corresponding track spacing. Solid lines represent anomalies greater than 10 cm, contour interval is 2 cm.
not cross the eddy zonally, a comparison with the zonal CTD section was justi"ed if we assumed the eddy to be radially symmetrical. The altimetry showed a positive anomaly of almost 18 cm (dashed line), considerably larger than the dynamic height relative to 2500 m (circles). This was not surprising since Martin et al. (1998) showed that the eddy had a substantial barotropic component and also, as is always present in eddying or meandering #ow, a small ageostrophic cyclostrophic (or rotational) component. In anticyclonic #ow the cyclostrophic component augments the horizontal pressure gradient. Using the ADCP to provide a reference level for the geostrophic shear showed that the currents at 2500 m were about 20 cm s\, far from zero (Martin et al., 1998). Our comparison of the satellite altimetry with the hydrographic estimate con"rms that the eddy has a large component of #ow that is not baroclinic. Although we believe that altimeter track 70 passed close to the eddy centre at the time of the cruise, any deviation from it would result in an underestimate of the sea-level anomaly associated with the eddy.
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The di!erence in sea-surface height between two stations is directly proportional to the surface geostrophic #ow (< ) between them g tan(i) g*H " , < " f¸ f where g is the acceleration due to gravity, f is the Coriolis parameter, i is the angle between the sea surface and a level surface, *H is the height di!erence and L the horizontal distance between the stations. Using this relationship and the absolute current measurements taken with the shipborne ADCP, we derived the sea-surface height di!erence between station pairs and therefore a height estimate across the eddy including the non-baroclinic terms. These ADCP-derived sea-surface heights (crosses) are shown integrated from station 1 (Fig. 3). The agreement between these ADCP-derived sea-surface heights and the altimetric sea-level anomaly was encouraging. Both datasets showed the eddy to be approximately 120 km in diameter and to exhibit a doming of the sea surface of almost 20 cm. Assuming a comparatively small cyclostrophic component of #ow, this suggested that the T/P altimeter track 70 did indeed pass close to eddy centre. It is interesting to note that all three cold-surface eddies considered for sampling, from the initial AVHRR images, had positive SLAs in the altimetry data sets. The other features are not discussed in detail here due to a lack of hydrographic data. However, the repeated cold-core/positive SLA combination does suggest that the PRIME eddy is not a unique feature. 4.2. Tracking the PRIME eddy (mapping) Altimeter SLA values provided every 7 km along track were gridded to form maps indicating dynamic topography due to transient features. For the ERS satellites, data were gridded onto a 0.1253 grid using a search radius of 70 km, and weighting values using a Gaussian curve of full-width at half-maximum of 35 km. For the more widely spaced T/P tracks, data were gridded onto a 0.253 grid with a search radius of 140 km and a full-width at half-maximum value of 70 km. The 10-day repeat orbit of the T/P mission was not particularly suited to the horizontal mapping of anomalies in the size of the PRIME eddy due to its coarse spatial resolution. The track spacing of this satellite at this latitude, about 100 km, was close to the diameter of the eddy, so the mapping continuity from one cycle to the next was compromised as the eddy drifted onto and in between the tracks giving SLA signals of varying strength (Fig. 4a). Maps of T/P SLA for cycles 140 and 141 corresponding approximately to leg 2 of the PRIME cruise do show the eddy (Figs. 4b and c), although the track spacing was problematic on other cycles. The PRIME eddy was reasonably clear due to the position of track 70 in relation to the eddy. Due to their increased spatial resolution, the ERS satellites were far better suited to this type of mapping (Fig. 5a). Figs. 5b}l show successive 35-day "elds for the combined ERS-1 and ERS-2 SLAs. These "elds covered the time period from May 1995 to June 1996; so the last "eld (Fig. 5l) corresponds to leg 1 of the PRIME cruise. The two ERS satellites could be successfully merged in this way as the time lag between "elds was only 1 day (a negligible temporal di!erence over a 35-day "eld). This merging of the data allowed us to complete spatial coverage for all "elds. A seasonal steric e!ect due to warming of the upper ocean was observed over the one year of this data. The PRIME eddy is clearly observed as a positive SLA in Figs. 5h}l and there was some
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Fig. 5. Combined ERS-1 and ERS-2 sea-level anomalies (35-day "elds) from the year preceding the PRIME cruise with corresponding track spacing. Solid lines represent positive anomalies, contour interval is 2 cm. The PRIME eddy is observed in the later half of the time series. (l) The time of the PRIME cruise.
evidence for its presence in Figs. 5e}g. Fig. 5h corresponds to the time period from mid-December 1995 to mid-January 1996. The PRIME eddy had therefore existed in approximately the same position within the Iceland Basin for at least 6 months prior to the cruise.
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Fig. 6. Ascending ERS-1 track sea-level anomalies o!set from one another by 25 cm for clarity. The ordinant in this "gure is the cycle number and corresponds to time.
Fig. 7. Descending ERS-1 track sea-level anomalies o!set from one another by 25 cm for clarity. The ordinant in this "gure is the cycle number and corresponds to time.
4.3. Tracking the PRIME eddy (track-by-track analyses) SLAs along the eight ERS-1 tracks (Fig. 1) crossing over or passing near to the PRIME eddy for the year prior to the PRIME cruise are shown in Figs. 6 (ascending) and 7 (descending). In both ascending and descending tracks the SLA signature of the eddy was most clearly observed in the centre two tracks as expected. In the majority of the cases, the ERS-2 data closely resembled that of ERS-1 and showed broadly similar features (not shown). In the case of the ERS data, consistency over time was sacri"ced somewhat for the increased spatial resolution. However, the PRIME eddy was still clearly observed from late 1995. A strong PRIME eddy signal was seen in the middle of the time series for track 173 and was also seen in the latter half of the track 631 time series (Fig. 7), implying a slight eastward propagation. Track 631 showed a slight northward propagation. Ascending tracks 906 and 362 (Fig. 6) also implied a slow northeastward movement of the eddy. A net northeastward drift agreed with the prevailing currents of the region, although the magnitude of the eddy drift was considerably smaller than would be expected due to currents alone. It is possible that this anticyclonic eddy's expected self-propagation (west/southwest with respect to
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Fig. 8. Hofmuller diagram of TOPEX/POSEIDON Sea-level anomalies for tracks 248, 70 and 146 across the Iceland Basin from southeast to northwest, from cycle 2 (October 1992) to 149 (September 1996). Anomalies greater than #10 cm are shown in bold, anomalies less than }10 cm in grey. Vertical dashed lines represent the positions of the Rockall Channel (&553N) and Iceland Basin (&593N). The PRIME eddy is observed from the beginning of 1996 on track 70 within the Iceland Basin.
lines of planetary or topographic b; see McWilliams, 1985; Martin et al., 1998) partially countered the prevailing drift. T/P track-by-track analysis o!ered superior temporal resolution. The 10-day repeat T/P time series extended back as far as October 1992, allowing us to determine the climatology of eddies in this area. Track 70 passed close to the eddy centre (Fig. 1). In order to track the eddy back in time, sea-level anomalies greater than $10 cm for this track were plotted, along with adjacent descending tracks 248 and 146, on space}time axes in the form of a Hofmuller diagram (Fig. 8). Fig. 8 covers four years of data (cycle 2 (October 1992) to cycle 149 (October 1996)). Anomalous features were clearly observed within the Rockall Channel and Iceland Basin on all three tracks. Both are areas of high eddy activity (Heywood et al., 1994; Bersch, 1995). The doming caused by the PRIME eddy was seen between 59 and 603N from at least the start of 1996 (cycle 121) to the cruise (cycles 138}141). Fig. 8 (track 70) shows that the eddy was present prior to the usual winter mixing period, although it is possible that winter mixing could have occurred earlier in 1996. The winter mixing depths (the maximum depth at which the potential temperature is within 0.53C of its surface value) calculated for the decade 1964}1974 at Ocean Weather Ship India (593N, 193W) are shown in Fig. 9. This is close to the location of the PRIME eddy. Fig. 9 suggests that winter mixing in this region could occur anytime between January and April, and that this can often extend to depths in
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Fig. 9. Winter mixing depths at Ocean Weather Ship INDIA (593N, 193W) between 1964 and 1974 using a *T"0.53C criterion. Maximum winter mixing typically occurs around March each year but may occur anytime between January and April. The continuous line represents the mean winter mixing depth averaged over successive 3-day time intervals and linearly interpolated over missing data.
excess of 500 m. As the PRIME eddy exhibits a pycnostad at 600 m, it is possible that the stad formed when winter storms mixed the upper ocean down to this depth. No evidence of the PRIME eddy is seen along adjacent tracks 248 or 146 at times prior to its signal on track 70, suggesting, within the limitations of the T/P spatial resolution, that it formed locally in the Iceland Basin, probably from instabilities in the current #ow around the northwestern #ank of the Hatton Bank. The eddy appears to have moved little along track 70 during 1996 in agreement with the ERS analyses. 4.4. Sea-level variability Fig. 10 shows the sea-level variability for the "rst nine months of the years 1993}1996. Only variabilities greater than 5 cm are shown in order to emphasise the main patterns. These data were derived from the T/P altimetry measurements. Higher variability is seen in all plots within the Iceland Basin and the Rockall Channel for all the four years. Di!erences in the sea}level variability from year to year were observed, with 1993 and 1995 showing similar patterns, 1994 showing low variability and 1996 high variability within the Iceland Basin. It should be noted that a truly stationary eddy would not exhibit a signal in the sea-level variability. However, the PRIME eddy was observed to rotate, as a whole, with a period of approximately 3.5 days (Martin et al., 1998, Fig. 6). This complex anticyclonic precession would result in high sea-level variability in the region of the eddy without the eddy translating a great distance. There is some evidence for this in Fig. 10d in the vicinity of the eddy.
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Fig. 10. Sea-level variability (cm) for the "rst 9 months of the years 1993}1996. Only variabilities greater than 5 cm are contoured for clarity. Subsequent contour interval is 1 cm.
5. Conclusions The PRIME eddy has been clearly observed in the altimetry. T/P and ERS tracks bisected the PRIME eddy close to its centre revealing an SLA of almost 20 cm relative to a 3-year mean. A comparison of altimeter-derived SLA and hydrographic estimations of sea-surface elevation con"rmed the "ndings of Martin et al. (1998) that there was a signi"cant non-baroclinic component of #ow. Estimations of sea-surface elevation from the ADCP-derived total velocities matched the altimetry, giving justi"cation to the techniques used by Martin et al. (1998) of referencing the geostrophic shear to ADCP absolute velocities. Small di!erences in height can be attributed to centrifugal ageostrophic (cyclostrophic) components of #ow and/or the satellite tracks not crossing the eddy centrally. The altimeter data showed that the eddy had moved very little (less than the PRIME eddy diameter (120 km) in any direction) and that it had existed in the Iceland Basin since at least the start of 1996, some 6 months prior to the cruise, and possibly even earlier. This means that the eddy had most likely formed prior to the last deep winter mixing period, usually around March. Track-by-track analyses show that the eddy formed locally in the Iceland Basin and was not advected in from outside this region, although some motion is evident within the basin itself. Temperature and salinity characteristics show the core of the eddy to contain water expected of the region (Martin et al., 1998). Although we did not have the opportunity to sample either of the other two possible eddies located in the AVHRR images, their temperature/SLA characteristics suggest that the PRIME eddy was not unique. More work is required to determine the mechanisms forming the cold surface
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caps to these warm core eddies. The increased sea-level variability in the Iceland basin during 1996 suggests an increase in eddy (mesoscale) activity in the region during the PRIME year, but whether this is due to more PRIME-type eddies or more regular eddies cannot be ascertained with the present data.
Acknowledgements The SLA products are supplied by the CLS Space Oceanography Division, Toulouse, France (AVISO/Altimetry, 1996; Le Traon et al., 1995; Le Traon and Ogor, 1998). The ERS products were generated as part of the proposal `Joint analysis of ERS-1, ERS-2 and TOPEX/POSEIDON altimeter data for oceanic circulation studiesa selected in response to the Announcement of Opportunity for ERS-1/2 by the European Space Agency (proposal code: A02.F105). Thanks to Steve Groom at the Remote Sensing Data Analysis Service (RSDAS) at Plymouth Marine Laboratory (PML) for supplying the AVHRR image. Data processing was undertaken by the RSDAS using data received and archived at the Satellite Receiving Station, Dundee. Thanks also to the O$cers, Scientists and Crew of RRS Discovery cruise 221. This work was supported by the Natural Environment Research Council PRIME Grant GST/02/1084 and is PRIME contribution 56.
References Arhan, M., de Verdiere, A.C., Memery, L., 1994. The eastern boundary of the subtropical North Atlantic. Journal of Physical Oceanography 24 (6), 1295}1316. AVISO, 1996. User Handbook, Corrected Sea Surface Heights (CORSSHs), AVI-NT-011-311-CN, 2nd Edition, 21pp. AVISO, 1997. User Handbook, Sea Level Anomalies (SLAs), AVI-NT-011-312-CN, 2nd Edition, 24pp. Bersch, M., 1995. On circulation of the north-eastern North Atlantic. Deep-Sea Research 42, 1583}1607. Fu, L.-L., Christenson, E.J., Yamarone Jr., A., Lefebvre, M., MeH nard, Y., Dorrer, M., Escudier, P., 1994. TOPEX/POSEIDON mission overview. Journal of Geophysical Research 99 (C12), 24369}24381. Heywood, K.J., McDonagh, E.L., White, M.A., 1994. Eddy kinetic energy of the North Atlantic subpolar gyre from satellite altimetry. Journal of Geophysical Research 99 (C11), 22525}22539. Le Traon, P.Y., Gaspar, P., Bouyssel, F., Makhmara, H., 1995. Using TOPEX/POSEIDON data to enhance ERS-1 orbit. Journal of Atmospheric and Oceanic Technology 12, 161}170. Le Traon, P.Y., Ogor, F., 1998. ERS-1 Orbit improvement using TOPEX/POSEIDON: the 2 cm challenge, Journal of Geophysical Research 103 (C4), 8045}8057. Martin, A.P., Wade, I.P., Richards, K.J., Heywood, K.J., 1998. The PRIME Eddy. Journal of Marine Research 56 (2), in press. McWilliams, J.C., 1985. Submesoscale coherent vortices in the ocean. Reviews of Geophysics 23 (2), 165}182. White, M., Heywood, K.J., 1995. Seasonal and interannual changes in the North Atlantic Subpolar Gyre from Geosat and TOPEX/POSEIDON altimetery. Journal of Geophysical Research 100 (C12), 24931}24941.