Quasi‐4‐Day Waves During the 2018/2019 SSW and Their Coupling to the Ionosphere Based on Whole Atmosphere Data Assimilation

Abstract A strong westward zonal wavenumber‐2 quasi‐4‐day wave (Q4DW) during the 2018/2019 Northern Hemisphere sudden stratospheric warming (SSW) is both captured by Aura Microwave Limb Sounder (MLS) observations and our new whole neutral atmosphere data assimilation system. The Q4DW during this SSW is characterized by a double‐peak altitudinal structure in temperature, geopotential height, and neutral winds ranging from ∼40 km up to the lower thermosphere. The Eliassen‐Palm flux diagnostics show that the wave source at ∼55 km over 45°N–75°N, the excitation, propagation, and amplification of which are controlled by the critical layer and atmospheric barotropic/baroclinic instability in the polar stratosphere related to SSW. The first Hough‐mode decomposition analysis of Q4DW indicates that the enhancement of the Rossby (2, −3) normal mode is mainly responsible for the amplification of Q4DW, the latitudinal structure of which is distorted by the anomalous background winds during this SSW. In the ionosphere, a simultaneous quasi‐4‐day oscillation (Q4DO) is found in the Wuhan University total electron content (TEC) product near 10:00–12:00 LT at magnetic latitudes of ∼+15° and ∼−25° with maximum amplitudes of ∼1.1 TECU and 1.2 TECU, respectively. Besides, the Q4DO also displays significant interhemispheric asymmetry and longitudinal variations. Interestingly, the secondary wave components ( s = 4, T = 10.7 hr and s = 0, T = 13.7 hr) in neutral winds from the nonlinear interactions between the Q4DW and the migrating semidiurnal tide are detected in the dynamo region, which may play a dominant role in generating Q4DO in the F‐region ionosphere.