GLACIER: Oceans- -- Antsurfwater

Antarctic Surface Water originates near the Antarctic continent. It flows to the north until it encounters Subantarctic Surface Water. Because it is more dense than the Subantarctic Surface Water, Antarctic Surface Water begins to sink and mix with underlying Subantarctic Intermediate Water. The region of convergence between these surface water masses is referred to as the Antarctic Convergence or Polar Front. North of the Polar Front, surface waters have winter temperatures warmer than approximately 2.0°C (35.6°F), and they are relatively salty. South of this front, surface waters have winter temperatures below 1.0°C (33.8°F) and are less salty.

Water masses involved in thermohaline circulation around Antarctica. The Antarctic Surface Water is highlighted (modified from Sugden, 1982).

Because of the absence of barriers to atmospheric and oceanic circulation, the flow of Southern Ocean surface waters is very similar to the atmospheric patterns. The atmospheric circulation around Antarctica can be separated into two broad bands divided by the Circumpolar Trough (located approximately 65°S). The trough is characterized by highly variable, mostly cyclonic, atmospheric flow, and it separates regions of divergent surface water flow (Antarctic Divergence). The Circumpolar Trough separates a zone of westerlies to the north from the polar easterlies to the south, close to the continent.

Antarctic surface currents and surface wind patterns. The surface winds drive the currents (modified from Sugden, 1982).

North of the Circumpolar Trough, prevailing westerlies drive surface waters east; this is the West Wind Drift or the Antarctic Circumpolar Current. The Antarctic Circumpolar Current passes through a constriction in the Drake Passage, which causes it to intensify until it becomes one of the strongest currents in the world. South of the Circumpolar Trough, a number of low pressure cells result in several circulation gyres close to the continent. The largest gyres occur in the Ross Sea and Weddell Sea regions. Adjacent to the continent, airflow off the land mass creates the polar easterlies, which drive the westward-flowing surface current, the East Wind Drift, or Antarctic Coastal Current.

Circumpolar Current (modified from the Ocean University Course Team, 1993).

Shelf Waters

Surface waters on the continental shelf have varying densities, mainly because of salinity, not temperature (the variation in temperature is small - cold is cold!). Ultimately, the salinity of the shelf water mass depends on the amount of sea ice made during a season.

When sea ice forms, it freezes freshwater and leaves most of the salt behind in the ocean water, resulting in an increase in salinity of the ocean water. Sea ice takes up only about 15% of the seawater salt. This can increase the density of the surface waters to the point where they flow off the shelf and into deep waters. Other factors controlling the salinity of shelf waters are the on-shelf flow of Circumpolar Deep Water, dilution from melting of sea ice and glacial ice, and precipitation (snow is not very salty!).

Sea ice in Ross Sea, Antarctica. As sea ice forms, it takes up freshwater, and leaves most of the salt behind. This process increases the salinity and the density of the ocean water. Photograph by S. Shipp, Rice University.

The saltiest shelf waters occur in regions of persistent winter leads and polynyas. In these regions, the sea surface freezes many times, with the sea ice being blown away by the wind each time. In contrast, perennial sea ice cover can result in low salinity shelf water. Wide continental shelves provide more surface area for sea ice formation and, therefore, tend to have more saline shelf surface waters than narrow shelves. The more saline shelf surface waters tend to accumulate on the western side of these wide shelves and in deep shelf basins.