C. Derksen, R. Brown and L. Wang
Climate Research Division, Environment Canada
A combination of low winter snow accumulation and warm spring temperatures created a new record low spring snow cover duration (SCD) over the Arctic in 2010, since satellite observations began in 1966. The 2009/10 winter was characterized by an atmospheric circulation pattern that contributed to cold, dry conditions and below-average snow accumulation over large areas of Siberia, but to enhanced snowfall in the eastern Canadian Arctic. Atmospheric circulation conditions in March and April helped pull warm southerly air into the western North American Arctic, which contributed to early snow melt.
Reliable information on snow cover across the Arctic and sub-Arctic is needed for climate monitoring, for understanding the Arctic climate system, and for the evaluation of snow cover and (associated feedbacks) in climate models. Monitoring snow cover across the Arctic region is complicated by strong local controls on snow cover, frequent cloud cover, and large gaps and biases in surface observing networks. Therefore, it is beneficial to consider multiple sources of snow information (for example, from satellite observations, analyses of surface measurements, and output from atmospheric reanalysis) both to address the uncertainties associated with individual datasets, and to understand how various snow cover related variables are inter-related. These variables include snow cover extent (SCE: the area covered by snow), snow cover duration (SCD: how long snow is on the ground), and snow water equivalent (SWE: the amount of liquid water stored in the form of snow).
SCD was computed separately for the first (Fall) and second (Spring) halves of the snow year to provide information on changes in the start and end dates of snow cover (Fig. S1). A new record low spring SCD was observed over both the North American and Eurasian sectors of the Arctic during 2010 (Fig. S2a) continuing the trend to earlier spring snow melt over the Arctic identified from multiple datasets by Brown et al. (2010). Fall SCD (or snow cover onset date) continues to show no sign of any trends (Fig. S2b). Northern Hemisphere spring SCE, for months when snow cover is confined largely to the Arctic (Fig. S2c), continue to show decreasing trends previously linked to the poleward amplification of SCE sensitivity to warming air temperatures (Dery and Brown, 2007).
Spatial patterns of fall, spring, and seasonal SCD anomalies for 2009/10 are shown in Fig. S1. Negative (shorter duration) spring SCD anomalies were evident over much of the Arctic land area in 2010 (with the exception of Scandinavia). Early spring melt was particularly strong over sub-Arctic Canada and Alaska. Mean monthly snow depth anomalies for April and May 2010 from the Canadian Meteorological Centre (CMC) global snow depth analysis (Brasnett, 1999) are shown in Fig. S3. The pre-melt Arctic winter snowpack is characterized by unusually shallow snow over Alaska, central Siberia, and western Russia with positive (deeper snow) anomalies over the eastern Canadian Arctic and eastern Eurasia. Large areas of the Eurasian Arctic show snow depth anomalies getting more negative from April (Fig. S3a) to May (Fig. S3b), illustrating an early and rapid snow melt across this region. This melt in the Eurasian sector is consistent with warm air temperature anomalies in this region during these two months.
A multi-dataset Arctic SWE time series, derived for 1999 through 2010, indicate that regionally average April SWE anomalies (the month of maximum accumulation) were positive (more liquid water in the form of snow) in North America, and near normal in Eurasia (Fig. S4). Taken together with the early spring snow melt that characterized much of the Arctic in 2010, this is consistent with a more dynamic snowmelt regime (see Francis et al., 2009), and illustrates the potentially disconnected nature of SWE and SCE anomalies in the Arctic. .
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