On the Role of Flow Regime Behavior and Stratosphere-Troposphere Response to the 11-year Solar Cycle during Boreal Winter

The "Top-down" pathways by which the effects of solar forcing in the upper atmosphere are transmitted downward to influence the near surface weather have been a subject of intensive study in the recent decades. The basic mechanism of this Sun-climate connection is known to involve planetary-scale Rossby waves and their interaction with the winter stratospheric polar vortex but the details remain obscure.

The winter stratospheric polar vortex apparently become stronger during solar maximum years during the last few decades. One known effect is the solar ultra-violet (UV) radiance and its interaction with the stratospheric ozone that enhances the equator-to-pole temperature gradient over an 11-year solar cycle. Direct radiative effect is however very small and limited in the upper stratosphere where the air density is miniscule in comparison to the troposphere. To account for the observed change in winds and temperatures both in the stratosphere and the troposphere, dynamical amplification is required.

Here, the stratosphere-troposphere coupling is examined in terms of flow regime behaviour. We demonstrate that a wide-jet regime defined in the upper stratosphere during early winter favours nonlinear wave interaction including resonant behaviour, in contract to normal wave mean-flow interaction, which often leads to stratospheric sudden warmings and downward movement of zonal mean anomalies in middle winter. We show that the wide-jet regime is associated with significant changes in atmospheric transience, whereby the stratospheric westerly jet is enhanced via downward wave reflection/propagation and scale-interaction in the lower stratosphere. Those changes are further projected onto tropospheric westerlies at the jet exit region and to affect storm tracks there. The extent to which solar UV and/or energetic particle precipitation may contribute to such flow regime behaviour is discussed. The reason why the 11-year solar cycle signal has waxed and waned over an extended period is also conjectured.