The Mechanism of Interhemispheric Coupling Revealed by a Gravity Wave-Permitting General Circulation Model

Interhemispheric coupling (IHC) is the positive correlation between temperatures in the polar winter stratosphere and the polar summer upper mesosphere and the lower thermosphere. Over the past two decades, several mechanisms have been proposed to explain the IHC. However, a consensus on the mechanism is yet to be fully reached, particularly regarding the role of gravity waves (GWs). We conduct hindcast simulations for seven boreal winters using a GW-permitting general circulation model encompassing the whole neutral atmosphere. The model is initialized through spectral nudging to state-of-the-art reanalysis data of identical vertical coverage. Treating the 7-year averages of the model outputs as a climatology, the IHC is investigated as a sequential evolution of anomalies relative to it. The results showed that consequential interplay of GWs and quasi-two-day waves in the summer mesosphere is the key driver of the IHC, as is consistent with Yasui et al. (2021, doi: 10.1175/jas-d-21-0045.1) except for the final step toward the summer pole facilitated by primary GWs in the Antarctic. These waves are filtered by an anomalously weak westward jet due to a temperature gradient caused by planetary wave forcing, considered to be quasi-two-day waves. The mechanism is mostly applicable to both types of IHC events: those associated with sudden stratospheric warming and those with vortex intensification. Meanwhile, comparisons across the seven boreal winters indicate vulnerabilities in specific processes underlying the mechanism. In the reanalysis, it is suggested that GW parameterizations underestimate the GW forcing anomalies, but analysis increments reduce this discrepancy.