Evolution of Atmospheric Response to Early-Season Eurasian Snow Cover Anomalies

Abstract

Recently it has been shown that the area extent of Eurasian snow cover during September?October?November (SON) and the principal component of the leading mode of extratropical Northern Hemisphere (NH) climate variability in the following winter are statistically correlated. In this paper, physical linkages between SON Eurasian snow cover and the wintertime climate variability in the NH atmosphere are postulated. And in order to test the proposed hypotheses, comprehensive analyses of satellite-based observations for snow cover and reanalysis data for geopotential heights and sea level pressure are used. The magnitude of the correlation between snow cover and climate variability is found to be inversely proportional to the height suggesting that snow cover may act as a lower boundary forcing to the tropospheric circulation. Conversely, however, an index constructed to capture the downward propagating circulation anomaly from the lower stratosphere to the middle troposphere is shown to be as highly correlated with snow cover variability as the Arctic oscillation derived from sea level pressure. A mechanism involving the vertical propagation of Rossby waves is proposed to explain this apparent contradiction. Anomalous fall snow cover variability not only alters near-surface temperatures but also impacts upward propagating Rossby waves. Changes forced in the stratosphere by anomalous snow cover are not realized until later in the winter season when the troposphere and stratosphere are actively coupled.