ALBERT

Description: ABSTRACT [1]  We provide statistical evidence of the effect of the solar wind dynamic pressure (P sw ) on the northern winter and spring circulations. We find that the vertical structure of the Northern Annular Mode (NAM), the zonal mean circulation and Eliassen-Palm (EP)-flux anomalies show a dynamically consistent pattern of downward propagation over a period of ~45 days in response to positive P sw anomalies. When the solar irradiance is high, the signature of P sw is marked by a positive NAM anomaly descending from the stratosphere to the surface during winter. When the solar irradiance is low, the P sw signal has the opposite sign, occurs in spring, and is confined to the stratosphere. The negative P sw signal in the NAM under low solar irradiance conditions is primarily governed by enhanced vertical EP-flux divergence and a warmer polar region. The winter P sw signal under high solar irradiance conditions is associated with positive anomalies of the horizontal EP-flux divergence at 55-75°N and negative anomalies at 25-45°N, which corresponds to the positive NAM anomaly. The EP-flux divergence anomalies occur ~15 days ahead of the mean-flow changes. A significant equatorward shift of synoptic-scale Rossby wave breaking (RWB) near the tropopause is detected during Jan-Mar, corresponding to increased anti-cyclonic RWB and a decrease in cyclonic RWB. We suggest that the barotropic instability associated with asymmetric ozone in the upper stratosphere and the baroclinic instability associated with the polar vortex in the middle and lower stratosphere play a critical role for the winter signal and its downward propagation.