Downward Wave Reflection as a Mechanism for the Stratosphere- Troposphere Response to the 11-year Solar Cycle

The atmospheric response to the 11-year solar cycle is investigated based on the ERA-Interim reanalysis data set for the period of 1979-2013. We show that, during the Northern Hemisphere winter, the stratosphere-troposphere system response is instigated by a combined effect of enhanced vertical wind shear on the equator flank of the polar vortex and reduced wind shear within the vortex in the upper stratosphere. During middle winter, the vertical wind shear at high-latitudes of 2-3 hPa becomes negative more frequently under high solar conditions than low solar conditions. The negative wind shear in the upper stratosphere induces downward wave reflection at high latitudes. Following the wave reflection, a wave-1 anomaly pattern emerges in the high latitude troposphere. The wave-1 anomaly propagates westwards and equatorwards, giving rise to increased winds in the high-latitude North Atlantic sector and an anomalous positive North Atlantic Oscillation (NAO). The positive NAO is further strengthened by baroclinic eddy feedback, leading to a poleward shift of the critical latitudes and enhanced vertical wind shear in the upper troposphere and lower stratosphere in mid-winter. In the light of these results, we suggest that a key mechanism for solar UV radiation to affect the near surface weather during Northern winter may be related to downward wave reflection in the high-latitude upper stratosphere and enhanced baroclinic instability in the troposphere.