RESPONSES OF CLIMATE AND CYCLONES TO REDUCTIONS IN ARCTIC WINTER SEA-ICE

A perpetual January simulation of the global atmosphere has been used to study the climatology of the northern hemisphere winter, with particular reference to its synoptic characteristics and to changes induced by reductions in the concentration of Arctic sea ice. Lows were tracked using an objective scheme and were analyzed for a number measures of cyclone behavior. The qualitative features of the cyclone density and flux distributions compare well with observations, except in certain respects noted. Cyclone velocities and intensities are also presented and discussed. Decreases in sea ice concentration produced a monotonic but nonlinear warming in the lower troposphere and weakening and southward contraction of the midlatitude westerlies. Mean sea level pressure reductions were biased toward the western Arctic. There was a significant decrease in the speeds and intensities of cyclonic systems north of 45 degrees N but little overall change in areal densities or in the arrangement of the major storm tracks. Some density maxima were displaced toward regions of opened up sea ice or in response to circulation changes. A number of aspects of the response are interpreted as being due to changes in thermal steering and baroclinicity or to nonlinear effects. Surprisingly, no significant expansion of cyclonic activity occurred in the central Arctic. The relative constancy of cyclone numbers and storm tracks is a response very different from that found in a similar study of the southern hemisphere.