Inter-annual variations in pan-Arctic snowmelt onset and their relationship to atmospheric conditions

Snowmelt onset on all components of the Arctic cryosphere (land ice, sea ice and seasonal snow cover) was mapped by combining previously published results derived from QuikSCAT and the Special Sensor Microwave Imager (SSM/I) data. Large inter-annual variations of up to one month were observed in some regions, which are likely related to variations in large scale atmospheric circulation. We examined the relationships between climate indices and the annual melt onset date over both land and sea ice, and between the location and strength of the spring 500hPa circumpolar vortex and melt onset date over ice caps. To identify connections between regional melt onset timing and atmospheric circulation, we examined the association between melt onset date and regional synoptic patterns for cases of extreme early/late melt onset. Since clouds strongly affect the radiation balance at the surface, their role in snowmelt onset was also explored. At the regional scale, extreme early/late melt onset are closely related to surface flow patterns determined by the location and strength of cyclonic and anti-cyclonic systems during the spring melt period. In general, early melt onset is associated with early occurrence of southerly airflow, while late onset occurs when there is persistent northerly airflow throughout the melt season. This is usually associated with the cold sector of a cyclone and/or the downstream effects of a high pressure/ridge. In the Eurasian Arctic, melt onset is often associated with deep cyclonic systems, which is consistent with the correlation between melt onset date and the Arctic Oscillation index. In the North American Arctic, melt onset timing is often related to the strength of the western North American ridge, and correlated with the Pacific North-American index. The position of the 500hPa circumpolar vortex has a strong influence on melt onset timing on ice caps in the Canadian Arctic.