The Role of Oscillating Southern Hemisphere Westerly Winds: Southern Ocean Coastal and Open-Ocean Polynyas

Abstract

An oscillation in intensity of the Southern Hemisphere westerly winds is a major characteristic of the southern annular mode. Its impact upon the sea ice-ocean interactions in the Weddell and Ross Seas is investigated by a sea ice-ocean general circulation model coupled to an energy balance model for three temporal scales and two amplitudes of intensity. It is found that the oscillating wind forcing over the Southern Ocean plays a significant role both in regulating coastal polynyas along the Antarctic margins and in triggering open-ocean polynyas. The formation of coastal polynya in the western Weddell and Ross Seas is enhanced with the intensifying winds, resulting in an increase in the salt flux into the ocean via sea ice formation. Under intensifying winds, an instantaneous spinup within the Weddell and Ross Sea cyclonic gyres causes the warm deep water to upwell, triggering open-ocean polynyas with accompanying deep ocean convection. In contrast to coastal polynyas, open-ocean polynyas in the Weddell and Ross Seas respond differently to the wind forcing and are dependent on its period. That is, the Weddell Sea open-ocean polynya occurs earlier and more frequently than the Ross Sea open-ocean polynya and, more importantly, does not occur when the period of oscillation is sufficiently short. The strong stratification of the Ross Sea and the contraction of the Ross gyre due to the southward shift of Antarctic Circumpolar Current fronts provide unfavorable conditions for the Ross Sea open-ocean polynya. The recovery time of deep ocean heat controls the occurrence frequency of the Weddell Sea open-ocean polynya.