Abstract
Abstract. In order to investigate the impact of a locally confined gravity wave (GW) hotspot, a sensitivity study based on simulations of the middle atmosphere circulation during northern winter was performed with a nonlinear, mechanistic, general circulation model. To this end, we selected a fixed longitude range in the East Asian region (120–170∘ E) and a latitude range from 22.5 to 52.5∘ N between 18 and 30 km for the hotspot region, which was then shifted northward in steps of 5∘. For the southernmost hotspots, we observe a decreased stationary planetary wave (SPW) with wave number 1 (SPW 1) activity in the upper stratosphere and lower mesosphere, i.e., fewer SPWs 1 are propagating upwards. These GW hotspots lead to a negative refractive index, inhibiting SPW propagation at midlatitudes. The decreased SPW 1 activity is connected to an increased zonal mean zonal wind at lower latitudes. This, in turn, decreases the meridional potential vorticity gradient (qy) from midlatitudes towards the polar region. A reversed qy indicates local baroclinic instability, which generates SPWs with wave number 1 in the polar region, where we observe a strong positive Eliassen–Palm (EP) divergence. As a result, the EP flux increases towards the polar stratosphere (corresponding to enhanced SPW 1 amplitudes), where the SPWs with wave number 1 break, and the zonal mean zonal wind decreases. Thus, the local GW forcing leads to a displacement of the polar vortex towards lower latitudes. The effect of the local baroclinic instability indicated by the reversed qy also produces SPWs with wave number 1 in the lower mesosphere. The effect on the dynamics in the middle atmosphere due to GW hotspots that are located northward of 50∘ N is negligible, as the refractive index of the atmosphere is strongly negative in the polar region. Thus, any changes in the SPW activity due to the local GW forcing are quite ineffective.
Highlights
During winter, the dynamics of the middle atmosphere are mainly dominated by the polar vortex
Positive differences can be observed equatorward from an imaginary line connecting the subtropical and polar-night jet centers, with a maximum difference of 8 to 10 m s−1. These zonal wind anomalies are consistent with a polar vortex that is shifted towards lower latitudes, and the wind reversal in the mesosphere is shifted upwards at lower latitudes
The polar vortex is shifted towards lower latitudes but remains very strong (Baldwin and Holton, 1988), which leads to a suppression of stationary planetary wave (SPW) according to the Charney–Drazin criterion (Charney and Drazin, 1961)
Summary
The dynamics of the middle atmosphere are mainly dominated by the polar vortex. To capture the global distribution of GWs, the potential energy (Epot), momentum flux (MF) or stability indicators (Pišoft et al, 2018) can be estimated using satellite data (Ern et al, 2004; Fröhlich et al, 2007; Hoffmann et al, 2013; Schmidt et al, 2016) These numerous observational studies highlight a number of different local GW hotspots, which are mainly generated by orography and convection. E.g., by Smith (2003), Lieberman et al (2013) or Matthias and Ern (2018), Šácha et al (2016) observed a forcing of additional stationary planetary waves (SPWs) due to a longitudinally variable GW drag We pursue this idea by shifting the EA/NP hotspot meridionally while keeping its longitude range fixed to obtain information about its impact on the middle atmosphere at different latitudinal positions.
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