Origin and Evolution of Water Masses Near the Antarctic continental Margin: Evidence from H218O/H216O Ratios in Seawater

4 – Large-Scale Physical Oceanography of Polar Oceans

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.

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

Observations near a large tabular iceberg in the Weddell Sea in March and April 2009 show evidence that water from ice melting below the surface is dispersed in two distinct ways. Warm, salty anomalies in T–S diagrams suggest that water from the permanent thermocline is transported vertically as a result of turbulent entrainment of meltwater at the iceberg's base. Stepped profiles of temperature, salinity, and density in the seasonal thermocline are more characteristic of double-diffusive processes that transfer meltwater horizontally away from the vertical ice face. These processes contribute comparable amounts of meltwater–O(0.1 m3) to the upper 200 m of a 1 m2 water column–but only basal melting results in significant upwelling of water from below the Winter Water layer into the seasonal thermocline, suggesting that these two processes may have different effects on vertical nutrient transport near an iceberg.

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Subsurface melting of a free-floating Antarctic iceberg

Seventy-three ostracode species from 38 genera were recovered from the 55 surface sediment samples in Lutzow-Holm Bay, northeastern Antarctica. We investigated bathymetric and geographic distributions of modern benthic ostracode species in the bay and compared this fauna with published modern and fossil ostracode data of Antarctic and southern South American regions. The results show: (1) Four biotopes and three sub-biotopes are recognized based on Q-mode cluster analysis, which suggest distributions of modern ostracodes are mainly controlled by water-mass structure, ice scouring, and light availability. (2) Comparison between the Lutzow-Holm Bay fauna and other ostracode faunas from Antarctica and southern South America shows high endemism and homogeneity of Antarctic ostracode fauna, suggesting in situ evolution of most extant Antarctic species. (3) Most species are endemic to the Antarctica, a few species also inhabit South American waters.

Modern benthic ostracodes from Lützow-Holm Bay, East Antarctica: paleoceanographic, paleobiogeographic, and evolutionary significance

Six years of quasi-meridional crossings of the Antarctic Ocean from Antarctica to southern Africa between 4°W and 16°E longitude are analyzed, with emphasis on temperature and salinity distributions Data are sparse, but (particularly in the southern part of the sections) adequate to show significant variations in the latitude/depth distributions and T-S characteristics of the principal water masses In particular, the cross-sectional area occupied by Warm Deep Water (Weddell Deep Water) dropped by about one third in 1968 Positions of major features associated with the Antarctic Divergence varied by up to 5 degrees of latitude The surface temperature gradient often observed at the Antarctic Divergence appears to be the surface expression of a shallow (possibly seasonal) thermocline which sometimes slopes upward to intersect the sea surface There is a clear need for more closely spaced and consistently planned data from Antarctic cruises, particularly those on which physical oceanography is an ancillary activity As a bare minimum, much more closely spaced surface temperature (and if possible also salinity) measurements would be useful

http://faculty.petra.ac.id/dwikris/docs/cvitae/docroot/html/www.terrapub.co.jp/journals/JO/pdf/5601/56010001.pdf

Interannual Variability of Antarctic Hydrographic Structure and Frontal Zones along Meridional Sections between Syowa Station and Southern Africa

A pressure‐dependent numerical model of vertical mixing in the surface mixed layer is developed, and applied to the problem of Antarctic Bottom Water (AABW) formation in the Weddell Sea. The results are used to formulate a new model of AABW formation. The surface salinity of water circulating around the southern edge of the Weddell Gyre gradually increases, possibly owing to horizontal exchange with water on the continental shelf and/or freezing along the edges of ice shelves. This permits surface mixed layer convection to penetrate deeper and deeper as the water drifts westward. By the time the water reaches the southwest corner of the Weddell Sea, the combined driving of cooling and salt release during freezing, both of which are important, can drive the convection to the bottom in deep water. This forms a water mass called Deep Mixed Water (DMW: θ = ‐0.6 to ‐0.8°C, S = 34.64‰). Upward mixing of Warm Deep Water causes intermittent melting (usually followed by refreezing) at all stations; at the...

Harold Solomon

Papers

Interannual Variability of Antarctic Hydrographic Structure and Frontal Zones along Meridional Sections between Syowa Station and Southern Africa

Vertical mixed layer convection in the Weddell Sea 1

Comments on a Study of Sea Level in the Western North Pacific Ocean