Monthly variability of satellite-derived cyclonic activity for the southern hemisphere winter



A satellite-derived climatology of cloud vortices for the Southern Hemisphere winter (June to September) indicates that significant within-season changes occur in the areal frequencies of extratropical depressions. This variability is found to be most strongly associated with the climatic frontal zones, while on a zonally-averaged basis, the semi-annual oscillation of the circumpolar vortex appears to dominate the latitudinal changes in cyclonic activity between June and September. Surface-atmosphere interactions evidently play a key role in the wintertime circulation at middle and high latitudes. Cyclogenesis over the Southern Oceans is observed to be linked closely to the baroclinity associated with the Oceanic Polar Front, while on a regional scale, the presence of a time-averaged zone of in situ cyclogenesis over sub-antarctic latitudes of the South Pacific is found to be augmented by the influence of the unique winter season sea ice regime in that sector.