Satellite climatology of ‘polar lows’ and broadscale climatic associations for the Southern Hemisphere

Abstract

AbstractA climatology of polar air cloud vortices (so‐called ‘polar lows’) is derived for the Southern Hemisphere using sets of medium resolution (5–4 km) DMSP (Defense Meteorological Satellite Program) imagery. The climatology is for the winter season (June through to September) and covers the 7–year period 1977–1983. Both comma cloud and spiraliform polar air signature types are considered, as identified in recent satellite studies for the Northern Hemisphere. The comma clouds dominate over a wide range of ocean latitudes, whereas the spiraliform systems occur less frequently (about 1:10 for the hemisphere), and show maximum frequencies in sea ice latitudes around mid‐winter (about 1:3).Within‐season variations in the locations of maximum occurrence of polar air systems are consistent with the large‐scale changes in the longwaves associated with the semi‐annual oscillation of surface pressures. Interannual variations in polar low occurrence for the seven winters appear connected, at least in part, with changes in the amplitude of sea‐level pressure (SLP) wavenumber one. When the seasonal cycle is amplified, as in the year of a ‘warm’ ENSO (El Nino/Southern Oscillation) event (e.g. 1982), large numbers of polar lows are observed south‐east of Australia and around New Zealand. This contrasts with the situation for year (−1) of a warm event (e.g. 1981), when the annual cycle of the trough is suppressed. At that time, reduced frequencies of cold air outbreaks in the New Zealand area are accompanied by fewer polar lows.On a seasonally averaged basis, and in most winters, there is a positive relationship between the regional extent of the Antarctic sea ice, the longitudes of preferred occurrence of cold air outbreaks and the incidence of polar lows. This effect may be enhanced just downstream of areas of strongest oceanic heat loss to the atmosphere.