Cyclone behaviour response to changes in winter southern hemisphere sea‐ice concentration

Abstract

Abstract The sensitivity of climatological cyclone behaviour in the southern hemisphere to reductions in winter antarctic sea‐ice concentration is studies in general‐circulation‐model (GMC) experiments. An increase in the number of cyclones over most of the Weddell an Ross Seas is a response common to all cases of reduction of sea‐ice concentration from 100%. There is a tendency, particularly at lower ice concentrations, for more cyclones to be found in the latitude band of about 10 to 15° of latitude width centered on the antarctic coast, and a belt of fewer cyclones to the north. The structure of the changes in cyclogenesis assumes a much lesser zonally oriented form, and all experiments induce enhanced cyclogenesis in the western Weddell Sea. For the most part thee is an increase in cyclogenesis south of about 65°S, although some longitudinal sectors show up as regions of decreased cyclogenesis even through the sea‐ice concentration has been reduced there. The intensity of the extratropical cyclones has been quantified as the difference between the Mean depth of the systems and the climatological average pressure for each particular sea‐ice configuration. Perhaps contrary to what might have been expected, this ‘relative’ central pressure of lows exhibits an increase over most of the sea‐ice area (i.e. the cyclones become weaker in a relative sense). In an earlier study it was found that the climatological response to imposed anomalies of sea‐ice concentration was that the lowering of the concentration of antarctic sea ice resulted in a lowering of sea‐level pressure, particularly close to the continent. The results presented here strongly suggest that the climatological decreases in pressure result essentially from increases in cyclone numbers as opposed to the possibility that the cyclones are more intense. We have attempted to relate the regions of cyclogenesis with a measure of baroclinicity in the control simulation. We have also examined the relationship between changes in these parameters in response to anomalous sea‐ice concentration. Despite some similarity between theses, our study emphasizes the important role being played by other factors, including surface heat fluxes and topography, in these experiments. This study demonstrates the utility of GCMs in the study of high‐latitude cyclone sensitivity and suggests it is a valuable tool to be used in tandem with observational‐based analyses.