Snow-precipitation coupling and related atmospheric feedbacks over North America

Understanding snow-precipitation coupling mechanisms is of great importance both from theoretical and practical considerations. With this in mind, we conducted regional climate model experiments, with and without interactive snow, to get insights into snow-precipitation coupling mechanisms over North America. In early winter (November and December) the coupling hot-spot is situated around the Hudson Bay and the Great Lakes. It then migrates southward and is located over the Great Plains and Mid-west of the U.S. during January before it gets weaker in February (Figure 1). Further analysis of the precipitation-snow depth/snow cover relationship confirmed that, over the hot-spot regions, the feedback is positive, i.e. high amount of snow on the ground leads to enhanced precipitation which further increases the snow depth and snow cover.

The snow-precipitation feedback mechanisms are however different for the two hot-spot regions. Snow-precipitation coupling in the Hudson Bay - Great Lakes region is associated with large-scale atmospheric circulation anomalies (in the case of anomalously high snow on the ground), that resemble the positive phase of North Atlantic Oscillation, which is associated with southerly flow anomaly over the eastern side of the continent and cold northerly flow over central and western Canada. This circulation pattern favours storm activity and enhanced snow over the region. The coupling hot-spot over the Great Plains and Mid-west U.S. during January, on the other hand, is tied to the albedo effect of snow, which leads to cooling of the lower atmosphere, which in turn determines the precipitation phase, favouring snow formation over rain. The results, in general, are informative for sub-seasonal to seasonal prediction of winter precipitation for the two sub-regions.

Figure 1. Snow-precipitation coupling strength as measured by the percentage difference of variance between coupled and uncoupled simulations for November (a), December (b), January (c) and February (d). Only coupling strength significant at the 0.05 level is shown. The grey contour lines represent climatological freezing line (based on the top layer soil temperature) for the respective months.

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