Stability Characteristics of the Mesopause Region Above the Andes

Abstract

It is widely known that gravity waves (GWs) transport their energy and momentum from the lower atmosphere to the mesosphere and lower thermosphere (MLT). As these waves reach large amplitudes, they dissipate and deposit energy and momentum in this region, impart significant forcing to the global atmospheric circulation. GWs are dissipated primarily through instability processes: convective instability (CI) that occurs when large-amplitude waves create a negative vertical potential temperature gradient (Hodges, 1967), and dynamic (shear) instability (DI) when a large vertical gradient of horizontal wind is created by wave motion or momentum deposition (Fritts & Rastogi, 1985). Other instability processes could happen under specific conditions, such as vortical-pair instability (Dong & Yeh, 1988), parametric instability (Klostermeyer, 1991, slantwise dynamic instability (Hines, 1971), and resonant instability (Phillips, 1977).Multiple theories have been proposed to describe the links between waves and instabilities. Lindzen (1981)' linear GW saturation theory provided a simple way to explain the formation of GW breaking, instabilities, and turbulence (Fritts, 1984;Tsuda, 2014). Lindzen (1981) applied the concept of GW breaking due to instabilities to formulate a self-consistent theory that successfully explained the zonal wind reversal and summer-to-winter meridional circulation in the mesosphere. The linear GW saturation theory predicted that high frequency components of GWs mainly contribute to CI (Fritts, 1984;Tsuda, 2014). Multiple nonlinear saturation theories were also proposed (Dunkerton, 1987;Fritts & Alexander, 2003;Hines, 1991;Klostermeyer, 1991) to explain the relationships between instabilities and nonlinear wave-wave and wave-mean flow interactions that are not accounted for in a linear theory. Both types of theories helped to understand the wave breaking processes and instabilities.Numerical simulations and observational studies have investigated the contributions to the formation of instabilities based on both linear and nonlinear saturation theories. Simulations suggested that inertia GWs might lead to Kelvin-Helmholtz instability (KHI) (Andreassen et al., 1994(Andreassen et al., , 1998Fritts & Yuan, 1989). Sonmor and Klaassen (1997) used a Floquet analysis of a monochromatic wave propagating in a uniformly stratified background and found that the generation of different types of instabilities are related to the internal GWs with variable frequencies. Small-amplitude GWs in the absence of environmental variations can lead to instabilities (Fruman & Achatz, 2012;Walterscheid et al., 2013). Yue et al. (2010) found that about 60% of the large wind shear formation is driven by long − period waves such as tidal-period perturbations. Fritts et al. ( 2018) suggested