The Role of Linear Interference in Northern Annular Mode Variability Associated with Eurasian Snow Cover Extent

One of the outstanding questions regarding the observed relationship between October Eurasian snow cover anomalies and the boreal winter northern annular mode (NAM) is what causes the multiple-week lag between positive Eurasian snow cover anomalies in October and the associated peak in Rossby wave activity flux from the troposphere to the stratosphere in December. This study explores the following hypothesis about this lag: in order to achieve amplification of the wave activity, the vertically propagating Rossby wave train associated with the snow cover anomaly must reinforce the climatological stationary wave, which corresponds to constructive linear interference between the anomalous wave and the climatological wave. It is shown that the lag in peak wave activity flux arises because the Rossby wave train associated with the snow cover is in quadrature or out of phase with the climatological stationary wave from October to mid-November. Beginning in mid-November the associated wave anomaly migrates into a position that is in phase with the climatological wave, leading to constructive interference and anomalously positive upward wave activity fluxes until mid-January. Climate models from the Coupled Model Intercomparison Project 3 (CMIP3) do not capture this behavior. This linear interference effect is not only associated with stratospheric variability related to Eurasian snow cover anomalies but is a general feature of Northern Hemisphere troposphere-stratosphere interactions and, in particular, dominated the negative NAM events of the fall-winter of 2009/10.