Atmospheric meridional circulation impacts on contrasting winter sea ice extent in two years in the Pacific sector of the Southern Ocean

Abstract

An explanation is sought for the marked variation in maximum sea ice extent observed between 2 years in the 2 areas of greatest interannual variability in winter ice extent in the South Pacific sector of the Southern Ocean. The rôle of ice recession in controlling ice extent is highlighted, and the adjustments in the near-surface atmospheric meridional circulation and air temperature that attend winter periods of ice retreat and advance are noted. Distinct meridional flow and air temperature adjustments attend periods of sea ice retreat that limit ice to higher than normal latitudes as well as ice advance to lower than normal latitudes. Ice advance leading to above-normal ice extent, for instance, takes place only when a lowering of air temperatures is accompanied by equatorward flow. A lack of warm air advection is, however, needed to adequately account for the development and maintenance of above-normal ice extent. Systematic meridional circulation changes also take place during the development and over the duration of ice extent anomalies. These are shown to emanate from adjustments of the semi-annual cycle in the extra-tropical South Pacific atmospheric circulation.