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Interpreting Observed Northern Hemisphere Snow Trends with Large Ensembles of Climate Simulations

n the context of Arctic climate change, the astonishing rate of sea ice decline has frequently occupied the media spotlight. But what about snow? In the Northern Hemisphere, wintertime snow extent not only has a larger area than that of sea ice, but over the last thirty years the observed fractional decrease in June snow cover has outpaced that of September sea ice. To what extent are such observed snow trends captured by state of the art climate models?

I will present an analysis of Northern Hemisphere snow trends from the Community Earth System Model (CESM). I compare two 40-member ensembles of the model driven by historical radiative forcings over the period 1981--2010, one coupled to a dynamical ocean and the other driven by observed sea surface temperatures. Differences between these two ensembles demonstrate that the trends in snow cover extent and snow water equivalent over North America are strongly influenced by both sea surface temperature trends in the North Pacific and by sea level pressure trends in the North Pacific/North Atlantic sectors. Using the large number of realizations in two such contrasting ensembles, I interpret the extent to which the model trends agree or disagree with observed values during three different seasons, each influenced by differing climate processes. Finally, I discuss how the pattern of observed SST trends over the last three decades may have influenced observed snow trends and the implications for snow trends in the coming decades.