Predictability of snow-depth anomalies over Eurasia and associated circulation patterns

Abstract

This study investigates the variability and predictability of snow depth anomalies over the Eurasian continent at the end of winter, as represented in 12 ensembles of General Circulation Model simulations performed at the European Centre for Medium-Range Weather Forecasts. Each ensemble includes nine integrations performed with the same prescribed sea surface temperature, but started from time-lagged initial conditions. An empirical orthogonal function (EOF) analysis shows that the leading EOF of Eurasian snow depth in March has a zonallyoriented dipole structure, with a band of positive anomalies covering northem Europe and Siberia, and negative anomalies over central Europe, the Himalayas and north China. A significant relationship is found between the positivehegative phase of this snow-depth anomaly and warm/cold El Niiio Southern Oscillation events. The positive phase of the snow-depth EOFl is associated with a wintertime circulation characterized by a strengthening of the westerly winds over Europe and Siberia; in the upper troposphere, this westerly anomaly is accompanied by negative zonal wind anomalies over Eurasia around 3 W N and positive zonal wind anomalies between the equator and 25N over Africa and south-east Asia. A good degree of predictability is found in the snowrelated circulation anomalies: considering 500-hPa height, 850-hPa zonal wind and 200-hPa zonal wind, the interannual variations of the ensemble-mean fields show a correlation of 48%. 56% and 65% (respectively) with the corresponding observed anomalies over the eastern half (0 to 180'E) of the northern hemisphere. The tropical component of the zonal wind anomaly associated with snow-depth EOFl is strongly predictable; it shows a marked persistence from winter to the early summer, and affects the large-scale circulation over south Asia in the early and central periods of the monsoon season.