Abstract
Abstract. The effects of gravity wave (GW) breaking hotspots in the lower stratosphere, especially the role of their longitudinal distribution, are evaluated through a sensitivity study by using a simplified middle atmosphere circulation model. For the position of the local GW hotspot, we first selected a fixed latitude range between 37.5 and 62.5∘ N and a longitude range from 112.5 to 168.75∘ E, as well as an altitude range between 18 and 30 km. This confined GW hotspot was then shifted in longitude by 45∘ steps, so that we created eight artificial GW hotspots in total. Strongly dependent on the location of the respective GW hotspot with regard to the phase of the stationary planetary wave of wavenumber 1 (SPW 1) generated in the model, the local GW forcing may interfere constructively or destructively with the modeled SPW 1. GW hotspots, which are located in North America near the Rocky Mountains, lead to an increase in the SPW 1 amplitude and EP flux, while hotspots located near the Caucasus, the Himalayas or the Scandinavian region lead to a decrease in these parameters. Thus, the polar vortex is less (Caucasus and Himalayan hotspots) or more weakened (Rocky Mountains hotspot) by the prevailing SPW activity. Because the local GW forcing generally suppresses wave propagation at midlatitudes, the SPWs 1 propagate into the polar region, where the refractive index turned to positive values for the majority of the artificial GW hotspots. An additional source of SPW 1 may be local instabilities indicated by the reversal in the meridional potential vorticity gradient in the polar region in connection with a positive EP divergence. In most cases, the SPWs 1 are breaking in the polar region and maintain the deceleration and, thus, the weakening of the polar vortex. While the SPWs 1 that form when the GW hotspots are located above North America propagate through the polar region into the middle atmosphere, the SPWs 1 in the remaining GW hotspot simulations were not able to propagate further upwards because of a negative refractive index above the positive refractive index anomaly in the polar region. GW hotspots, which are located near the Himalayas, influence the mesosphere–lower thermosphere region because of possible local instabilities in the lower mesosphere generating additional SPWs 1, which propagate upwards into the mesosphere.
Highlights
Atmospheric dynamics is characterized by waves with different spatial and temporal scales (Douville, 2009) mainly forced in the lower part of the atmosphere, i.e., in the troposphere and stratosphere
As an extension of the results of Samtleben et al (2019) we performed a continuing sensitivity study, which investigates the effect of breaking gravity wave (GW) hotspots in the stratosphere and their impact on the middle atmosphere dynamics, which is mainly determined by the polar vortex
Because the results are mostly determined by the phase of the stationary planetary wave of wavenumber 1 (SPW 1), we compare the stationary planetary wave (SPW) 1 phase reproduced by the model (Fig. 10) to the SPW 1 phase extracted from SABER temperature measurements taken between 2002 and 2007 (Mukhtarov et al, 2010)
Summary
Atmospheric dynamics is characterized by waves with different spatial and temporal scales (Douville, 2009) mainly forced in the lower part of the atmosphere, i.e., in the troposphere and stratosphere. In connection with an intensified PW activity, this process can produce a preconditioning of the polar vortex (Šácha et al, 2016; Samtleben et al, 2019) or even a sudden stratospheric warming (SSW) (Albers and Birner, 2014) Such an effect of breaking GWs in the LS has been observed in several model studies (e.g., Plougonven et al, 2008; Constantino et al, 2015) as well as in satellite measurements showing enhanced GW drag in the stratosphere before SSW events (Ern et al, 2016).
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