Water mass distribution and circulation west of the Antarctic Peninsula and including Bransfield Strait

Abstract

Historical hydrographic data from Bransfield Strait and the region west of the Antarctic Peninsula were analyzed to provide descriptions of water mass distributions and circulation patterns. Circumpolar Deep Water (CDW), which is characterized by temperatures above 1.0øC, salinities of 34.6 to 34.73, and oxygen values below 4.5 ml 14, is the most prominent water mass in this region, is found between 200 and 700 m, and is present in all seasons throughout the region examined. Below 200 m this water mass floods the continental shelf west of the Antarctic Peninsula. CDW is also found in Bransfield Strait, but the distribution is limited to the northern side of the Strait near the South Shetland Islands. Mixing of CDW results in reduction of the oxygen content of the overlying waters by 25 to 45%, which suggests an average annual entrainment rate for the west Antarctic Peninsula of 0.7 to 1.43 x 10 '6 m s 4. The freshwater input needed to balance the salinity input from CDW is on the order of 0.63 m y-i, which can be supplied by local precipitation and advection of ice into the region from the Bellingshausen Sea, which then melts. The annual heat flux associated with CDW is 12 W m '2, which is sufficient to melt this amount of ice. A second prominent water mass, Bransfield Strait Water (< 0øC, 34.45 to 34.6), is found throughout the central and southern portions of the Strait. The circulation pattern, constructed from historical data sources, for the region west of the Antarctic Peninsula shows that the large-scale geostrophic flow may be composed of one or more clockwise gyres. This mesoscale variability is likely the result of the rugged bottom topography and has implications for the transport and retention of physical and biological properties. Surface drifters indicate that the circulation in Bransfield Strait is clockwise and may be continuous with the circulation west of the Antarctic Peninsula above 500 m. The circulation pattern inferred from historical temperature distributions suggests that the westward flowing Polar Slope Current, which has been observed north of the South Shetland Islands, does not extend beyond Smith Island. was done in this region until the 1970s when some stations were occupied along the axis of the Bransfield Strait [Gordon and Nowlin, 1978] and along the northern flank of the South Shetland Islands [Nowlin and Clifford, 1982; Sievers and Nowlin, 1984] as part of the International Southern Ocean Studies (ISOS) program. More hydrographic measurements were made in the Bransfield Strait region in the 1980s as part of the First International BIOMASS Experiment (FIBEX) and other Antarctic krill research activities [e.g., Stein, 1983; Stein and Rakusa-Suszczewski, 1983; 19-84]. These studies yielded primarily measurements of temperature distributions in the upper 500 m, but are still useful for characterizing the water column structure in this region. During the Second International BIOMASS Experiment (SIBEX), multidisciplinary cruises were conducted in an expanded study area that included ...

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