Comparative Influence of Snow and SST Variability on Extratropical Climate in Northern Winter

Abstract

In this study the influence of snow on atmospheric seasonal mean variability in the extratropical latitudes during boreal winter was studied. The motivation for this analysis was to understand the characteristics of low-frequency atmospheric variability in the extratropical latitudes, and to assess if the interannual variations in snow could lead to potential predictability on seasonal timescales. The influence of snow on atmospheric variability was assessed from a suite of atmospheric general circulation model (GCM) simulations where snow depth amount was either prescribed to a seasonally varying climatology, or was allowed to evolve during the model integration. Further, the influence of snow variability was contrasted with the influence of interannual variability in sea surface temperatures (SSTs) on the atmospheric flow. A systematic influence of snow variability on the atmospheric seasonal mean variability was found. For example, for the GCM simulations in which snow amount and its extent were allowed to evolve freely, the interannual variability of surface air temperature was found to be larger. The influence of snow variability, however, was confined to the lower troposphere, and little change in the interannual variability of upper-tropospheric circulation, for example, 200-hPa heights, occurred. This bottom-up vertical structure of the influence of snow on the atmospheric variability was in contrast to the top-down influence of tropical SST variability on the extratropical flow. The cause for the enhancement of atmospheric variability in the lower troposphere was argued to be related to the dependence of surface albedo on snow depth amount. This dependence was such that the interaction between the atmospheric variability and the underlying snow could be viewed as a positive feedback process whereby surface temperature anomalies amplify even further.