On the Role of Air-Sea-Ice Interactions off East Antarctica

Abstract

In some segments of the Antarctic continental shelf, salt released during sea-ice production transforms near-freezing Antarctic surface waters into saltier (S > 34.52) and denser Shelf Water. Near the shelf break of these locations, somewhat attenuated Shelf Water outflows and sinks down the continental slope, entraining relatively warmer Circumpolar Deep Water to form Antarctic Bottom Water. Thus, sea-ice formation plays a climate-relevant role in the global Meridional Overturning Circulation.

Similar geographic and oceanographic characteristics are found off the Sabrina and Adélie coasts in East Antarctica, therefore Shelf Water is unexpectedly found only in the Adélie Depression and lacking in the Sabrina Basin. A combination of in-situ, remote, and reanalysis datasets from 2003-2015 are used in this study to investigate the conditions leading to Sabrina’s relatively passive role. For the first time, sea-ice production rates are estimated combining both the net sea-ice volume exchanged across the shelf break (export) and the volume variability farther inshore (growth).

The Sabrina Basin produces an average of 197 km^3/yr of sea-ice, 97% of which is exported offshore across the western shelf break. Sabrina’s high sea-ice production rate is well correlated (0.95) to divergent sea-ice motion within the interior of the shelf. In contrast, the Adélie Basin produces only 34 km^3/yr of sea-ice. Sea-ice productivity (production per unit area) in Sabrina Basin (4.6 m/yr) is comparable to the Ross Sea (4.5 m/yr), but still higher than the Adélie Basin (1.9 m/yr).

Based on the estimated contributions of salt, from sea-ice production, and freshwater, from both sea-ice and glacial-ice melt, the salinity evolution of a thick subsurface Thermostad Water layer is reconstructed for 2003–2015. A melt rate of 157.5 Gt/yr in the Totten Glacier and Moscow University Ice Shelf system is required to match the Thermostad Water salinities measured during the summers of 2014 and 2015.

Shelf Water (S > 35.52) was initially produced during the winters of 2003-2008. SW formation stopped, however, during 2008-2011 when summer sea-ice divergence and cross shelf break export (~10 km^3) were at a minimum, which resulted in a prominent Thermostad Water freshening of -5.07 ΔS per decade. In contrast, large summer sea-ice exports (>30 km3) since 2011 have significantly increased the Thermostad Water salinity at a rate of 2.16 ΔS per decade. Thus, it is projected that local Antarctic Surface Water will form Shelf Water again by 2017, and the Sabrina Basin could participate more actively in the global Meridional Overturning Circulation than it has during the past decade.