It has often been reported that warming at high latitudes in the Southern Hemisphere (SH) summer mesosphere and lower thermosphere (MLT) appears during Arctic sudden stratospheric warming (SSW) events. This phenomenon, which is called “interhemispheric coupling (IHC)”, has been thought to occur because of the modulation of mesospheric meridional circulation driven by forcing of gravity waves (GWs) originating in the troposphere. However, quasi-two-day waves (QTDWs) develop during SSWs and result in strong wave forcing in the SH mesosphere. Thus, this study revisits IHC following Arctic SSWs from the viewpoint of wave forcing, not only by GWs and Rossby waves (RWs) originating in the troposphere but also by GWs, RWs, and Rossby-gravity waves generated in situ in the middle atmosphere, and elucidates the causes of warm anomalies in the SH MLT region. During SSWs, westward wind anomaly forms because of cold equatorial stratosphere, GW forcing is then modulated, and barotropic/baroclinic and shear instabilities are strengthened in the SH mesosphere. These instabilities generate QTDWs and GWs, respectively, which cause significant anomalous westward wave forcing, forming a warm anomaly in the SH MLT region. The intra-seasonal variation in QTDW activity can explain seasonal dependence of the time lag in IHC. Moreover, it is revealed that the cold equatorial stratosphere is formed by middle-atmosphere Hadley circulation, which is strengthened by wave forcing associated with stationary RW breaking leading to SSWs. The IHC mechanism revealed in this study indicates that waves generated in the middle atmosphere contribute significantly to the meridional circulation, especially during SSWs.

A momentum budget is examined in the stratosphere, mesosphere, and lower thermosphere using simulation data over ~11 years from a whole-atmosphere model in terms of the respective contributions of gravity waves (GWs), Rossby waves (RWs), and tides. The GW forcing is dominant in the mesosphere and lower thermosphere (MLT), as indicated in previous studies. However, RWs also cause strong westward forcing, described by Eliassen–Palm flux divergence (EPFD), in all seasons in the MLT and in the winter stratosphere. Despite the relatively coarse model resolution, resolved GWs with large amplitudes appear in the MLT. The EPFD associated with the resolved GWs is eastward (westward) in the summer (winter) hemisphere, similar to the parameterized GW forcing. A pair of positive and negative EPFDs are associated with the RWs and GWs in the MLT. These results suggest that the RWs and resolved GWs are generated in situ in the MLT. Previous studies suggested that a possible mechanism of RW generation in the MLT is the barotropic/baroclinic instability. This study revisits this possibility and examines causes of the instability from a potential vorticity (PV) viewpoint. The instability condition is characterized as the PV maximum at middle latitudes on an isentropic surface. Positive EPFD for RWs is distributed slightly poleward of the PV maximum. Because the EPFD equals the PV flux, this feature indicates that the RW radiation acts to reduce the PV maximum. The PV maximum is climatologically maintained in both the winter and summer mesospheres, which is caused by parameterized GW forcing.

The contributions of gravity waves to the momentum budget in the mesosphere and lower thermosphere (MLT) is examined using simulation data from the Ground-to-Topside Model of Atmosphere and Ionosphere for Aeronomy (GAIA) whole-atmosphere model. Regardless of the relatively coarse model resolution, gravity waves appear in the MLT region. The resolved gravity waves largely contribute to the MLT momentum budget. A pair of positive and negative Eliassen–Palm flux divergences of the resolved gravity waves are observed in the summer MLT region, suggesting that the resolved gravity waves are likely in situ generated in the MLT region. In the summer MLT region, the mean zonal winds have a strong vertical shear that is likely formed by parameterized gravity wave forcing. The Richardson number sometimes becomes less than a quarter in the strong-shear region, suggesting that the resolved gravity waves are generated by shear instability. In addition, shear instability occurs in the low (middle) latitudes of the summer (winter) MLT region and is associated with diurnal (semidiurnal) migrating tides. Resolved gravity waves are also radiated from these regions. In Part I of this paper, it was shown that Rossby waves in the MLT region are also radiated by the barotropic and/or baroclinic instability formed by parameterized gravity wave forcing. These results strongly suggest that the forcing by gravity waves originating from the lower atmosphere causes the barotropic/baroclinic and shear instabilities in the mesosphere that, respectively, generate Rossby and gravity waves and suggest that the in situ generation and dissipation of these waves play important roles in the momentum budget of the MLT region.

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