Impact of Weak and Strong Stratospheric Polar Vortices in the Northern and Southern Hemispheres on Solar-Migrating Semidiurnal Tides in UA-ICON

This study uses a 60-year free-run simulation by the upper atmospheric extension of the ICOsahedral Non-hydrostatic (UA-ICON) general circulation model to explore the influence of stratospheric polar vortex variability on the mesosphere and lower thermosphere (MLT). The Northern Annular Mode (NAM) index and the Southern Annular Mode (SAM) index are derived from the model output to represent the strengths of the polar vortices in the Northern Hemisphere (NH) and Southern Hemisphere (SH), respectively. During weak polar vortex events in the NH, there is an increase in zonal-mean zonal winds in the MLT at the NH high latitudes and a warming of the lower thermosphere, accompanied by a cooling of the mesosphere. Conversely, the wind and temperature anomalies are reversed during strong NH polar vortex events. During weak polar vortex events in the SH, similar but generally weaker anomalies are observed in the corresponding hemisphere. This study also elucidates the response of the solar-migrating semidiurnal tide (SW2) to variations in the strength of the polar vortices. A weak NH polar vortex is associated with an increase in SW2, while a strong NH vortex results in a decrease in SW2. The response of SW2 to changes in the SH polar vortex is similar, although considerably weaker. The NH polar vortex variability can explain around (Formula presented.) of the variability in the SW2 during NH winter. The SH polar vortex, however, accounts for only a small fraction of the variability (up to (Formula presented.)) in SW2.