Formation and export of dense shelf water from the Adélie Depression, East Antarctica

Abstract

Coastal polynyas in the Adélie Depression are an important source of Antarctic Bottom Water to the Australian‐Antarctic Basin. We present time series (April 1998 to May 1999 and August 1999 to February 2000) of data from temperature‐salinity sensors, in both the Adélie Depression and the known outflow region of the Adélie Sill, to describe the annual cycle of shelf water densities. From April through September, salinification beneath the polynya produces dense shelf waters. During September–October, shelf water densities in the depression peak at 27.94 kg m−3, and the cooling and freshening signature of Ice Shelf Water is observed north of Buchanan Bay. In November–December, shelf water densities decrease as intrusions of warm and relatively fresh modified Circumpolar Deep Water enter east of the Adélie Sill. From January–March the surface layer is conditioned by the cooling of the atmosphere, which overturns the upper water column. At the Adélie Sill, observed daily mean currents were approximately 10 cm s−1 with intense instantaneous currents greater than 50 cm s−1 at the sill depth. Using an idealized outflow region with a rectangular cross‐sectional area (6 × 106 m2), we present the first estimates of shelf water export by potential density class. Assuming shelf water with a minimum density of 27.88 kg m−3 has sufficient negative buoyancy for downslope mixing and a fourfold volume increase (1:3 mixing ratio) from entrainment, the dense shelf water export of 0.1–0.5 Sv results in an annual average production of bottom water in this region of between 0.4 and 2.0 Sv. The wide range in bottom water estimate results from data limitations, and a narrowing of this range requires further mooring observations.