q Springer-Verlag 1997 Modelled atmospheric response to changes in Northern Hemisphere snow cover

Abstract. The surface boundary conditions are altered in a numerical simulation of January climate by pre-scribing (a) higher and (b) lower than average snow extent over Northern Hemisphere land masses. The anomalies in snow cover are shown to have quite a strong impact on the mean climatic state. Associated with an increase in the areal extent of the snow, there is a significant reduction in temperature throughout the lower troposphere. There are also large increases in sea-level pressure over most land areas. Significant responses in the mass field are also seen at 500 hPa where reductions in atmospheric thickness lead to sig-nificant negative anomalies in the height field. Re-sponses are also seen non-locally, over both the North Pacific and North Atlantic basins. The impact of in-creased snow on cyclone tracks is also examined. A re-duction in cyclones is noted over both continents and over the western sectors of both ocean basins. Over the North Atlantic basin this reduction extends across over Europe, significantly weakening the storm track. In the North Pacific, cyclone density is reduced in the west while in the east, there is actually a strengthening of the storm tracks. There are corresponding changes in the genesis of cyclones in both of these regions. The change in cyclogenesis, intensity and density is demon-strated to be associated with changes in baroclinicity between the two experiments. The anomalous snow boundary conditions lead to significant changes in the meridional temperature gradients over both ocean ba-sins which impact on the baroclinic zones.