Dynamics of the Northern Annular Mode at Weekly Time Scales
Rivière, Gwendal ; Drouard, Marie
Rapid onsets of positive and negative tropospheric northern annular mode (NAM) events during boreal winters are studied using ERA-Interim datasets. The NAM anomalies first appear in the North Pacific from low-frequency Rossby wave propagation initiated by anomalous convection in the western tropical Pacific around 2 weeks before the peak of the events. For negative NAM, the enhanced convection leads to a zonal acceleration of the Pacific jet, while for positive NAM, the reduced convection leads to a poleward-deviated jet in its exit region. The North Atlantic anomalies, which correspond to North Atlantic Oscillation (NAO) anomalies, are formed in close connection with the North Pacific anomalies via downstream propagation of low-frequency planetary-scale and high-frequency synoptic waves, the latter playing a major role during the last onset week. Prior to positive NAM, the generation of synoptic waves in the North Pacific and their downstream propagation is strong. The poleward-deviated Pacific jet favors a southeastward propagation of the waves across North America and anticyclonic breaking in the North Atlantic. The associated strong poleward eddy momentum fluxes push the Atlantic jet poleward and form the positive NAO phase. Conversely, prior to negative NAM, synoptic wave propagation across North America is significantly reduced and more zonal because of the more zonally oriented Pacific jet. This, together with a strong eddy generation in the North Atlantic, leads to equatorward eddy momentum fluxes, cyclonic wave breaking, and the formation of the negative NAO phase. Even though the stratosphere may play a role in some individual cases, it is not the main driver of the composited tropospheric NAM events.
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