Abstract
Abstract. In this study we explore the seasonal variability of the mean winds and diurnal and semidiurnal tidal amplitude and phases, as well as the Reynolds stress components during 2019, utilizing meteor radars at six Southern Hemisphere locations ranging from midlatitudes to polar latitudes. These include Tierra del Fuego, King Edward Point on South Georgia island, King Sejong Station, Rothera, Davis, and McMurdo stations. The year 2019 was exceptional in the Southern Hemisphere, due to the occurrence of a rare minor stratospheric warming in September. Our results show a substantial longitudinal and latitudinal seasonal variability of mean winds and tides, pointing towards a wobbling and asymmetric polar vortex. Furthermore, the derived momentum fluxes and wind variances, utilizing a recently developed algorithm, reveal a characteristic seasonal pattern at each location included in this study. The longitudinal and latitudinal variability of vertical flux of zonal and meridional momentum is discussed in the context of polar vortex asymmetry, spatial and temporal variability, and the longitude and latitude dependence of the vertical propagation conditions of gravity waves. The horizontal momentum fluxes exhibit a rather consistent seasonal structure between the stations, while the wind variances indicate a clear seasonal behavior and altitude dependence, showing the largest values at higher altitudes during the hemispheric winter and two variance minima during the equinoxes. Also the hemispheric summer mesopause and the zonal wind reversal can be identified in the wind variances.
Highlights
IntroductionThe primary forcing of the mesosphere–lower thermosphere (MLT) at small scales is by gravity waves arising from various tropospheric sources, among them flow over orography (mountain waves), deep convection (convective gravity waves), frontal systems, and jet stream imbalances and shear instabilities (Fritts and Nastrom, 1992; see the review by Fritts and Alexander, 2003, and Plougonven and Zhang, 2014)
Gravity waves (GWs) originating at the lower atmosphere by a number of sources are an essential driver of the mesosphere–lower thermosphere (MLT) dynamics, forcing a meridional flow due to a zonal drag, which drives the mesopause temperature up to 100 K away from the radiative equilibrium (e.g., Lindzen, 1981; Becker, 2012), introducingPublished by Copernicus Publications on behalf of the European Geosciences Union.G
We analyze the data with the adaptive spectral filter (ASF) technique (Baumgarten and Stober, 2019) to obtain daily mean winds, as well as diurnal and semidiurnal tides
Summary
The primary forcing of the MLT at small scales is by gravity waves arising from various tropospheric sources, among them flow over orography (mountain waves), deep convection (convective gravity waves), frontal systems, and jet stream imbalances and shear instabilities (Fritts and Nastrom, 1992; see the review by Fritts and Alexander, 2003, and Plougonven and Zhang, 2014) These various GWs typically have horizontal phase speeds comparable to the mean winds at higher altitudes; they are strongly influenced by varying winds along their plane of propagation. GWs can propagate upward until they become dynamical unstable or they are filtered by critical levels, where they undergo breaking and dissipation, resulting in local mean flow accelerations that act as sources of non-primary GWs
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