Influences of surface air temperature and atmospheric circulation on winter snow cover variability over Europe

The relationships between snow cover (SC) variability in Europe, the local surface air temperature (SAT), and the associated atmospheric circulation changes are studied. This investigation indicates that the European winter SC is closely correlated with SAT. Higher (lower) SC is coincident with strong and large-scale surface cooling (warming). Similar but weaker temperature signals are observed in the middle and upper troposphere. Periods of enhanced (reduced) SC are characterized by surface heat loss (gain), partly due to dampened (enhanced) sensible heat fluxes towards the ground surface, which is in turn related to the lower (higher) SAT. Higher (lower) SC is also accompanied by reduced (enhanced) downward longwave irradiance. Consistent with previous studies, our analysis demonstrates that variations in the atmospheric circulation in the North Atlantic/European sector, including those associated with the North Atlantic Oscillation, are accompanied by changes in horizontal heat advection and SC over Europe. The circulation changes modulate the water vapour transport towards the European continent, and thereby influence the available water vapour there and lead to fluctuations in downward longwave irradiance and cloud cover. The wind anomalies associated with these variations also drive surface heat flux changes in the North Atlantic, which in turn lead to well-defined sea surface temperature (SST) tendencies. The above characteristic patterns exhibit notable variability in different calendar months of the winter season. The monthly averaged circulation anomalies are evidently related to changes in the tracks of atmospheric disturbances with synoptic time scales. Overall, there is no strong evidence supporting a principal role for the North Atlantic SST or the El Nino Southern Oscillation in driving inter-annual SC variability over Europe.