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Acoustic Doppler Current Profiler-derived acoustic backscatter data were compared with physical and biological data collected during leg 2 of the PRIME cruise in the northeastern Atlantic during July 1996. The leg 2 transect from 59 to 37°N ran approximately along the 20°W meridian and crossed a range of hydrographic conditions from near subarctic in the north to subtropical in the south. A front at 52.5°N was an extension of a southern branch of the North Atlantic Current system. A stronger frontal feature was observed at 48°N and was considered to be the northern limit of Eastern North Atlantic Water that had been ventilated at the eastern boundary. At 37°N a Lagrangian time-series study was performed. High acoustic backscatter was observed within the PRIME eddy at 59°N and at the southern site. Other changes in backscatter intensity, or more correctly, in ‘mean volume backscattering strength’ (MVBS), coincided with two main fronts crossed during the transect and upon arrival at the southern site. An increased MVBS in the surface 100 m in the north was not present south of 52.5°N. Between 48 and 37°N the MVBS approached its lowest of the transect. At 37°N marked changes in the backscatter were observed, with a strong signal seen in the top 75 m. As well as changes in the MVBS there were differences in the diel migratory patterns observed in the backscatter. Scattering at 37°N was concentrated above a strong pycnocline at 75 m. In the north the scattering signal was generally noisier and less well defined than in the south. Biomass has been estimated from the MVBS by comparing it with biomass estimates derived from Optical Plankton Counter data and carbon/nitrogen measurements. Although such calibrations have their limitations, the results give important information concerning net sampling strategies and are a good test of the representativeness of such sampling.

The PRIME cruise to the North Atlantic during June/July 1996 surveyed and sampled an extremely vigorous and deep-reaching eddy with a significant barotropic component. Although it exhibited anticyclonic flow 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 significantly affected 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 first being detected in late 1995 and that it almost certainly formed locally, probably as a result of an instability in the current flow around the northwest of the Hatton Bank. Comparisons between satellite SLAs and hydrographic estimates of sea-surface elevation confirm that the eddy had a substantial barotropic flow. Both the altimeter data and the sea-surface height derived from the acoustic Doppler current profiler agree that the PRIME eddy had a sea-surface elevation of about 20 cm and that its diameter was about 120 km.

Hydrographic data from the World Ocean Circulation Experiment (WOCE) and South Atlantic Ventilation Experiment (SAVE) in the region of transition between the Scotia Sea and the Argentine Basin are examined to determine the composition of the deep water from the Southern Ocean that enters the Atlantic, and to describe the pathways of its constituents. The deep current that flows westward against the Falkland Escarpment is formed of several superposed velocity cores that convey waters of different origins: Lower Circumpolar Deep Water (LCDW), Southeast Pacific Deep Water (SPDW), and Weddell Sea Deep Water (WSDW). Different routes followed by the WSDW upstream of, and through, the Georgia Basin, lead to distinctions between the Lower-WSDW (σ4>46.09) and the Upper-WSDW (46.04