Migrating Semidiurnal Tide During the September Equinox Transition in the Northern Hemisphere

Abstract

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.

Spring‐Fall Asymmetry in VLF Amplitudes Recorded in the North Atlantic Region: The Fall‐Effect

“…This is confirmed in Figure 9 of Pedatella et al. (2021), who analyzed Specified Dynamics Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension (SD‐WACCMX) simulations. In addition, Conte et al.…”

“…Even though the wind reversal time in Figure 3a corresponds to mesospheric altitudes, it is reasonable to expect it to occur some days earlier than at stratospheric altitudes. This is confirmed in Figure 9 of Pedatella et al (2021), who analyzed Specified Dynamics Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (SD-WACCMX) simulations. In addition, Conte et al (2018) reported that meteor radar observations show an increase in gravity wave activity during the fall equinox.…”

Determining the Origin of Tidal Oscillations in the Ionospheric Transition Region With EISCAT Radar and Global Simulation Data

“…While the transition from predominant semidiurnal to diurnal tide also takes place at and around 110 km altitude, there is an upper band of strong semidiurnal oscillations especially during the first half of September. This apparent weakening of the upper 12 hr band around equinox is an important feature since atmospherically forced semidiurnal tides have been shown to undergo such an autumn transition (Pedatella et al, 2021). Whether this upper band is generated in situ or forced by some atmospheric tidal mode that propagates unusually far up remains to be investigated in more detail.…”

The Mid‐ to High‐Latitude Migrating Semidiurnal Tide: Results From a Mechanistic Tide Model and SuperDARN Observations

“…The numerical study of the SW2 tide has a long history (e.g., Forbes & Garrett, 1979). Nevertheless, open questions remain about the mechanisms governing the tide's seasonal and short‐term variability (Conte et al., 2018; G. Liu et al., 2021; Pedatella et al., 2020; Zhang et al., 2021). Many recent studies are in part driven by the increasing availability of high‐altitude and tide‐resolving general circulation models.…”