Reply on CC1

Abstract

The role of surface ocean anomalies for the continental Northern Hemisphere snow cover is investigated, together with the interactions between snow cover and atmosphere. Four observational datasets and two large multi-model ensembles of atmosphere-only simulations are used, with prescribed sea surface temperature (SST) and sea ice concentration (SIC). A first ensemble uses observed interannually varying SST and SIC conditions for 1979–2014, while a second ensemble is identical except for SIC where a repeated climatological cycle is used. SST and external forcing typically explain 10 to 25 % of the snow cover variance in model simulations, with a dominant forcing from the tropical and North Pacific SST, while no robust influence of the SIC is found. In observations, the Ural blocking is the main driver of the November and April snow cover over Eastern Eurasia, while the North Atlantic Oscillation (NAO) dominates the snow cover forcing in January. In November and more robustly in January, dipolar anomalies of snow cover over Eurasia, with positive anomalies over Europe and negative anomalies over Southern Siberia, also precede the Arctic Oscillation (AO) by one month. In models, snow cover over western Eurasia in January also precedes by one or two months a negative AO phase. The detailed outputs from one of the models suggest that both the western Eurasia snow cover and polar vortex are generated by Ural blocking, and that both snow cover and polar vortex anomalies act to generate the AO one or two months later.