Surface oceanography of BROKE-West, along the Antarctic margin of the south-west Indian Ocean (30–80∘E)

Abstract

Abstract Hydrographic CTD and ADCP data were collected during the BROKE-West research voyage (January–March 2006) in the south-west Indian Ocean sector of the Antarctic margin. These data describe the large-scale circulation, water masses, fronts and summertime stratification in the surface layer over the continental shelf, slope and rise region between 30 and 80 ∘ E that forms CCAMLR Statistical Area 58.4.2. The surface circulation matched the full-depth circulation and consisted of the eastward flowing southern Antarctic Circumpolar Current front to the north, and the westward flowing Antarctic Slope Current associated with the Antarctic Slope Front along the continental slope to the south. Two sub-polar gyres were detected south of the Southern Boundary of the Antarctic Circumpolar Current: the eastern Weddell Gyre in the Cosmonaut Sea ( 30 – 50 ∘ E ) and the greater Prydz Bay Gyre in the Cooperation Sea ( 60 – 80 ∘ E ). In the eastern Weddell Gyre, the seasonal mixed layer depths were shallower, warmer and fresher relative to the regions to the east which were deeper, cooler and more saline. This spatial variability is found to be strongly correlated to the large-scale pattern of sea ice melt/retreat in the months preceding the voyage and the accumulated wind stress thereafter. Areas of upwelling warm deep waters into the surface layer are presented from positive anomalies of potential temperature and nutrient concentrations (nitrate and silicate). These anomalies were strongest in the eastern Weddell Gyre in the vicinity of the Cosmonaut Polynya/Embayment, north of Cape Anne and near the Southern Boundary of the Antarctic Circumpolar Current in the eastern sector of the survey. The summertime stratification (seasonal mixed layer, seasonal pycnocline and Tmin layer) are discussed relative to the distributions of chl a and acoustically determined Antarctic Krill (Euphausia superba) densities. Elevated chl a concentrations were found in the surface layer of the marginal ice zone and it is proposed that these are retained south of the fast, narrow jet of enhanced Antarctic Slope Current on the upper continental slope. There is qualitative evidence of these maxima being subducted and transported north in the seasonal pycnocline in response to Ekman convergence from the easterly winds in this region. The seasonal mixed layer within the sub-polar gyres had relatively low chl a concentrations with sub-surface maxima in the seasonal pycnocline and the top of the Tmin layer. Surface concentrations increased once again north of the Southern Boundary in the north-east of the survey. Krill and chl a concentrations were both co-located and decoupled at different locations across the survey. There was no clear oceanographic boundary influencing the distribution of the krill surveyed, though further work is necessary to properly synthesize this and other biological patterns with the oceanographic processes, given the varying time and length scales and intrinsic sampling limitations.