Zonation and geostrophic flow of the Antarctic circumpolar current at Drake Passage

Abstract

The Antarctic Circumpolar Current in Drake Passage comprises four regimes characterized by distinct temperature-salinity relationships in the near-surface waters and by different depths of common temperature-salinity characteristics in deeper water. Between the Subantarctic Zone in the northern passage and the Antarctic Zone farther south is a transition region, the Polar Frontal Zone. The southernmost regime is the Continental Zone, which is restricted to a narrow band near the southern continental slope. Historical hydrographic data are used to determine the mean positions of the fronts that separate the zones. Expandable bathythermograph data show that the Subantarctic and the Polar fronts fluctuate about 100 km north and south of their mean positions. No seasonal differences in the mean frontal positions are evident. Baroclinic geostrophic transport calculations relative to 2500 dbar were made from synoptic hydrographic station pairs that span the passage. For sections that encompass the Subantarctic and Continental zones, the mean summer transport is 78.7 × 10 6 m 3 s −1, about 10% higher than the winter mean of 70.6 × 10 6 m 3 s −1. Transport calculations were also made from the average values of dynamic height and transport function within the Subantarctic and Continental Zones. The mean summer transport of 80.9 × 10 6 m 3 s −1 is significantly different from the winter mean of 73.4 × 10 6 m 3 s −1. The largest flow in summer is between the Subantarctic and Polar Frontal Zones, whereas in winter the flow between the Polar Frontal Zone and the Antarctic Zone is predominant. The lower winter transport is associated with shallower isopycnals throughout the water column in the Subantarctic Zone. Seasonal dynamic height averages are used to estimate the baroclinic component of the pressure signals from gauges deployed at each side of the passage between 1976 and 1978. The total transport (baroclinic plus barotropic) is estimated to be on the order of 26 × 10 6 m 3 s −1 greater in winter than in summer.