Two Stable Modes of Southern Ocean Winter Stratification

Abstract

The Southern Ocean is responsible for most of the world ocean's deep water characteristics. Cold Southern Ocean water masses are produced as buoyancy is removed by the polar atmosphere, usually in association with sea ice formation. Great amounts of sea ice formed within coastal polynyas are continually swept away by coastal winds, exposing the ocean surface to the cold antarctic atmosphere and further ice formation. Salt rejection forms dense shelf water masses, a key ingredient of Antarctic Bottom Water. In recent years a greater appreciation of winter processes over the deep ocean had developed. The extensive winter sea ice cover of the Southern Ocean is limited in its thickness by mixed layer entrainment of relatively warm and salty deep water. The same processes quickly disposes of the ice cover in the austral Spring. A network of negative feedbacks produces quickly disposes of the ice cover in the austral Spring. A network of negative feedbacks produces a relatively stable but thin veneer of sea ice. The sea ice cover and ocean static stability are maintained by salinity: this is referred to as the saline mode. The saline mode can be upset. The Weddell Polynya of the mid-1970's is a dramatic example of another stable mode. In the polynya condition the ocean stratification is destroyed and vigorous convection persists, eliminating the sea ice cover. This configuration is driven by the temperature instability and is referred to as the thermal mode. The conversion of the saline mode of the thermal mode requires the mixed layer salinity to become sufficiently high to force free convection with the deep water. It is unlikely that the Winter can be cold or long enough to force this conversion. Enhanced wind-induced sea ice divergence could accomplish this task, though that is difficult to reconcile a localized atmospheric effect with the repeated occurrences of small site specific polynyas. It is likely that the formation of offshore polynyas is driven by ocean processes, perhaps associated with circulation interaction with bottom topography. The persistent Weddell Polynya of the mid-1970s altered the thermohaline stratification of the water column to 3000 meters. Smaller, recurring polynyas observed since then are likely associated with sporadic convection events. These events slightly alter thermohaline stratification by injection low salinity water into the mid-water column.