[P-EM13] Coupling Processes in the Atmosphere-Ionosphere System

[PEM13-22] Migrating Semidiurnal Tide During the September Equinox Transition in the Northern Hemisphere

★Invited Papers

*Nicholas M Pedatella¹, Hanli Liu¹, J. Fede Conte², Jorge Chau², Christopher Hall³, Christoph Jacobi⁴, Nicholas Mitchell^5,6, Masaki Tsutsumi⁷ (1.High Altitude Observatory, National Center for Atmospheric Research, 2.Leibniz Institute of Atmospheric Physics, Rostock University, 3.Tromso Geophysical Observatory, The Arctic University of Norway, 4.Institute for Meteorology, Universitat Leipzig, 5.Department of Electronic and Electrical Engineering, University of Bath, 6.British Antarctic Survey, 7.National Institute of Polar Research)

Keywords:semidiurnal tide, whole atmosphere modeling , mesosphere and lower thermosphere

Specified Dynamics Whole Atmosphere Community Climate Model with thermosphere- ionosphere eXtension simulations are used to investigate the solar migrating semidiurnal tide (SW2) around September equinox at middle to high latitudes in the Northern Hemisphere. A pronounced minimum in SW2 occurs around September equinox, and is characterized by a ∼50% reduction in tidal amplitudes for 20–30 days. Analysis of the simulation results indicates that the SW2 minimum occurs due to the seasonal transition of the zonal mean zonal winds, which alter the generation and propagation of different symmetric and antisymmetric modes of SW2. In particular, the antisymmetric modes notably decrease due to the more hemispherically symmetric zonal winds around equinox. It is further demonstrated that interannual variability in the timing of the SW2 minimum is related to the timing of the seasonal transition of the zonal mean zonal winds in the middle atmosphere. This leads to an earlier occurrence of the SW2 minimum during years when the seasonal transition occurs earlier, such as the recent 2019 September equinox which saw an earlier transition of the Southern Hemisphere zonal mean zonal winds following the occurrence of a sudden stratosphere warming. The connection between the timing of the SW2 minimum in the Northern Hemisphere and the timing of the seasonal transition in the middle atmosphere winds is confirmed by seasonal variability of 12-h tides deduced from specular meteor radar observations at middle to high latitudes in the Northern Hemisphere.