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

Abstract

Atmospheric tides are global-scale waves whose periods are an integer fraction of a solar day (Chapman & Lindzen, 1970). The tides are forced primarily by radiative and latent heating effects in the lower atmosphere (Hagan, 1996), but obtain their largest amplitudes in the mesosphere-lower-thermosphere (MLT) region (80-120 km altitude). There they are expressed as pronounced oscillations in a broad range of atmospheric fields, such as density, pressure, and wind. The migrating tides are those tides which follow the apparent motion of the sun, having a longitudinal zonal wavenumber (S) and latitudinal spherical harmonic (Hough mode) structure. In the current work, the focus lies on the migrating semidiurnal (SW2; for Semidiurnal, Westward S = 2) tide. The SW2 tidal winds maximize in the mid-and high-latitude MLT (Manson et al., 2002;Wu et al., 2011), where they form a major source of day-to-day and inter-seasonal variability of the MLT-ionosphere system (Arras et al., 2009;G. Shepherd et al., 1998;Smith, 2012). The SW2 tide is recognized as an important vertical coupling mechanism (Forbes, 2009;Pedatella & Forbes, 2010), and as a contributing factor to the vertical mixing and energy budget of the upper atmosphere (Becker, 2017;Forbes et al., 1993).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 (

Seasonality of the Migrating Semidiurnal Tide in the Tropical Upper Mesosphere and Lower Thermosphere and Its Thermodynamic and Momentum Budget

“…(2021) and van Caspel et al. (2022) found that the seasonality of SW2 in the middle to high latitudes can be explained by the background atmosphere’s distortion of SW2 modes originating from the stratosphere. However, these do not mean that our results contradict with their results.…”

“…These suggested mechanisms for the seasonality of SW2 in the tropics are very different from the suggested mechanisms for the seasonality of SW2 in the middle to high latitudes. Pedatella et al (2021) and van Caspel et al (2022) found that the seasonality of SW2 in the middle to high latitudes can be explained by the background atmosphere's distortion of SW2 modes originating from the stratosphere. However, these do not mean that our results contradict with their results.…”

“…SW2 in the tropical upper mesosphere and lower thermosphere (UMLT) region (between 85 and 100 km) is largely ignored because most studies focus on SW2’s variability in the middle‐ to high‐latitude UMLT where its amplitudes are known to be the largest (A. H. Manson et al., 2002; Q. Wu et al., 2011). These studies often analyze the interactions of SW2 with winter‐prominent phenomena like sudden stratospheric warmings and stationary planetary‐scale waves (Angelats i Coll & Forbes, 2002; Forbes & Wu, 2006; Forbes et al., 2008; Limpasuvan et al., 2016; Palo et al., 1999; Pedatella & Forbes, 2010; Teitelbaum & Vial, 1991; Teitelbaum et al., 1989; van Caspel et al., 2022; Yamashita et al., 2002; J. Zhang et al., 2021).…”

“…However, based on the studies using ground‐based instruments, linear models, and 3D models, there are altitudes in the tropical UMLT where SW2’s amplitudes may be significant enough that it could be involved in wave–wave interactions with other planetary waves. In the middle to high latitudes, SW2 has been shown to interact with these planetary‐scale waves and form nonmigrating tides (Angelats i Coll & Forbes, 2002; Forbes & Wu, 2006; Forbes et al., 2008; Limpasuvan et al., 2016; Palo et al., 1999; Pedatella & Forbes, 2010; Teitelbaum & Vial, 1991; Teitelbaum et al., 1989; van Caspel et al., 2022; Yamashita et al., 2002; J. Zhang et al., 2021). Thus, understanding this complexity in SW2’s amplitudes over the tropical UMLT is important in knowing more of these other wave–wave interactions.…”

“…Its long wavelength allows it to vertically propagate up to the ionosphere/thermosphere (I/T) region (Chapman & Lindzen, 1970). While it propagates, it has been shown to interact with the background atmosphere and/or atmospheric waves such as planetary‐scale waves and gravity waves (Angelats i Coll & Forbes, 2002; Forbes, 1982; Forbes & Wu, 2006; Forbes et al., 1995, 2008; Lindzen & Hong, 1974; Palo et al., 1999; Pedatella & Forbes, 2010; Teitelbaum & Vial, 1991; Teitelbaum et al., 1989; van Caspel et al., 2022; Walterscheid & Venkateswaran, 1979a, 1979b; Yamashita et al., 2002; J. Zhang et al., 2021).…”