Modelling low frequency variability in Southern Hemisphere extra‐tropical cyclone characteristics and its sensitivity to sea‐surface temperature

Abstract

The outflow regions of the western boundary currents of the Southern Hemisphere and their extensions into the South Indian Ocean, South Pacific and South Atlantic Currents are prominent areas of low frequency variability in sea-surface temperature (SST). One mechanism of decadal/multidecadal variability involves modulation of surface heat flux exchange and the track and intensity of extra-tropical cyclones and associated frontal systems in response to a zonal mid-latitude SST anomaly. Here, the influence of this type of SST pattern on cyclone properties is investigated by applying a vortex tracking scheme to atmospheric general circulation model integrations. In order to assess the subdecadal to multidecadal scale response, these integrations were conducted for a 21-year period with forcing consisting of a warm SST anomaly stretching across the Southern Hemisphere mid-latitudes. In response to this forcing, a large-scale trough at mid-latitudes and ridging at higher latitudes are generated in the model. Associated with this change in Southern Hemisphere baroclinicity are increases in the density and flux of cyclonic systems in the 40°–55°S zone but decreases poleward of this latitude belt. It is found that the model response weakens throughout the Southern Hemisphere mid- to high-latitudes in the second decade compared to the first but that there is less evidence of a hemispheric weakening in the second quinquennium relative to the first. Implications of these results for mechanisms of decadal/multidecadal climate variability are discussed. Copyright © 2001 Royal Meteorological Society.