Stratosphere-mesosphere coupling during stratospheric sudden warming events

In this review article we summarize recent results in the coupling of the stratosphere–mesosphere during stratospheric sudden warming (SSW) events. We focus on the role of planetary and gravity waves in driving the middle atmosphere circulation and illustrate the stratosphere–mesosphere coupling during undisturbed wintertime circulation, during an SSW event, and after an SSW event during the formation of an elevated stratopause using simulations of past Arctic and Antarctic winters from the Specified Dynamics version of the Whole Atmosphere Community Climate Model (SD-WACCM). We illustrate the transition of the polar stratopause from being a gravity wave driven phenomena to a planetary wave driven phenomena during SSW events and its subsequent reestablishment and control by gravity waves. We also examine the synoptic structure of the stratosphere, mesosphere, and lower thermosphere using SD-WACCM data fields that show the structure of the vortex during specific dynamical events in both hemispheres. We illustrate the longitudinal asymmetry in the thermal structure in the stratosphere and mesosphere driven by differences in circulation over the polar cap regions during an SSW event. We complement this analysis of the middle atmosphere circulation with a classification of both the Arctic and Antarctic winters since 1979 into major, minor, elevated stratopause or quiet winters based on the level of disturbance using the Modern Era-Retrospective Analysis for Research and Applications (MERRA) reanalysis data. From the MERRA data we find that the combined occurrences of both major and minor warmings in the Arctic have remained constant over the past three decades while we find a minor increase in their occurrences in the Antarctic.