Seasonal and Interannual Variation of the Interhemispheric Coupling During the Austral Winter in WACCM6

We investigate interhemispheric coupling (IHC) during the austral winter by examining global characteristics of the dynamical fields initiated by stratospheric warming in the Southern Hemisphere using output of the Whole Atmosphere Community Climate Model version 6 covering 195 simulation years. About 100 events were extracted and analyzed, focusing on the seasonal and interannual variation of the IHC response in the Northern Hemisphere. The seasonal variation can be explained by differences in the background atmosphere. The upward shift and weakening of the gravity wave forcing during the IHC in September are opposite to the characteristics of July and August. However, the mean zonal wind and the momentum deposition of gravity waves in the northern middle atmosphere in September are also opposite to those in July and August. Together, these result in the same sign of the anomaly. Additionally, an interannual variation in the altitude of the warming in the northern polar region is also observed, with warmings split at 100 km. A warming that appears above 100 km is explained by an additional mechanism of gravity wave modulation, which drives the winter-to-summer circulation in the lower thermosphere, while a warming below 100 km corresponds to the weakening of the summer-to-winter circulation in the upper mesosphere. Correspondingly, no warming is observed near 100 km, where the circulation and its modulation are absent. This interannual variation of the IHC is influenced by the intraseasonal oscillation, which represents a similar meridional temperature structure in the global middle atmosphere.