Tuesday, 9 June 2009
Stowe Room (Stoweflake Resort and Confernce Center)
Observational studies indicate that the correlation between autumnal snow cover anomalies over Siberia and wintertime circulation anomalies projects significantly onto the Northern Annular Mode (NAM). Years in which there is anomalously high Siberian snow cover in autumn correlate with the negative phase of the wintertime NAM pointing to the potential utility of snow cover data as a seasonal forecasting tool. Forced general circulation model (GCM) experiments reveal that a large autumnal Siberian snow forcing generates a negative NAM stratosphere-troposphere coupling response that propagates down to the surface within several weeks. Contrastingly, this response is not found in unforced GCM runs with seasonally varying snow cover. Potential dynamical reasons for this discrepancy are probed further by imposing an idealized Siberian snow forcing in a relatively simple GCM. The forcing generates a weak yet positive NAM response. A decomposition of the meridional eddy heat flux response over the first several weeks indicates that while the nonlinear contribution produces a positive response in the stratosphere, consistent with the observed increase in vertical propagation of wave activity; destructive interference with the background state induces a negative linear term that dominates that overall meridional eddy heat flux response to the surface cooling, suppressing vertical wave activity propagation. When the snow forcing wave response constructively interferes with the background wave field, particularly the wave 1 component, the linear heat flux term continues to dominate but becomes positive leading to a negative NAM response in the stratosphere consistent with other GCM studies.
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