On the physical structure of a front in the Bellingshausen Sea

Abstract

The structure of a front observed at 67°S in Austral spring in the Bellingshausen Sea is described. The front extended to full depth, and has been identified by Read et al. (1995) as the southernmost major front of the Antarctic Circumpolar Current where Circumpolar Deep Water (CDW) outcrops. In this paper the structure of the front is examined on scales down to 10 km using a closely spaced CTD section and down to 3 km from underway surveys with SeaSoar and an Accustic Doppler Current Profiler (ADCP) which resolved the density and velocity structure, respectively, in the top 400 m. A seasonal halocline was present throughout the survey area on both sides of the front, at the base of a mixed layer 50–70 m deep. The signature of the previous winter's mixed layer depth could be seen in the weak stratification between 70 and 150 m. The subsurface temperature minimum within the winter mixed layer was significantly colder south of the front (−1.7°C) than north of it (−1.1°C). The front, defined by eastward velocities greater than 10 cm s −1, was 70–80 km wide with flow at speeds of up to 50 cm s 1 in the surface layer. The zone within which water mass changes occurred was narrower, about 40 km, contained within the zone of eastward flow and extended down to 1000 m. The strong velocities in the frontal jet caused differential advection of surface properties from upstream (west) of the survey area, resulting in sharp cross-frontal gradients of salinity and chlorophyll a. Chlorophyll was largest (over 5 mg m −3) in a band barely 10 km wide that extended along-front for well over 100 km and was tightly constrained along the southern flank of the front where isopycnals outcropped from the winter mixed layer into the surface layer across the seasonal halocline. The large values of chlorophyll could only be maintained by advection into the survey area from an unknown source region upstream (Boyd et al., 1995). Patches of high chlorophyll were found only south of the front, and it is hypothesised that productivity south of the front is a consequence either of some property of the upwelling CDW or of water that had been under ice, identified by the −1.7°C subsurface temperature minimum. Temperature anomalies on the 27.4 kg m −3 isopycnal indicate that patches of water south of the front had broken off from the southern flank of the front, so it is probable that the bloom was spreading southwards from the front within the survey area rather than northwards from the ice edge.