Middle atmosphere effects of localized gravity wave forcing
Final Report Abstract
The project aim, concerning LIM, was to analyze the effect of localized gravity wave hotspots on the stratosphere by using a mechanistic numerical global circulation model. To this end, additional localized gravity wave drag and heating was implemented in the model in order to simulate the effect of a gravity wave breaking hotspot in the East Asian/North Pacific region of the lower stratosphere. The chosen strength and position of this hotspot was guided by observations mainly from GNSS radio occultations. The additional drag created an additional planetary wave, which interfered with the existing stationary planetary wave 1. Depending on the phase difference between the gravity wave hotspot and the stationary planetary wave, the additional drag interfered either constructively or destructively with the existing planetary wave and therefore led to either increasing or decreasing of planetary wave activity and consequently to either weakening or even strengthening of the polar vortex. Since the phase and amplitude of the stationary planetary waves are related to circulation patterns like NAO or ENSO, the effect of gravity wave hotspots also depends on the phases of these patterns. Simulations have been performed by shifting the position of the hotspot either in longitude or in latitude. The effect of the additional gravity wave drag again depends on the phase relation between gravity wave forcing and stationary planetary wave, so that, given a climatological mean maximum westward wind at about 180◦ longitude, hotspots near the East Asian/North Pacific region most effectively lead to enhancement of the planetary waves and thus shifting or weakening of the polar vortex. GW hotspots southward of 50◦N and at East Asian/North Pacific longitudes lead to a negative refractive index at midlatitudes, which prevents the PWs from propagating upwards. Thus, less PWs are breaking in the middle atmosphere, which leads to an increased zonal mean ow, especially at lower middle latitudes. At higher midlatitudes, gravity wave hotspots north of 40◦N usually lead to a weakening of the polar vortex. Simulations with two or three hotspots showed that, in general, GW hotspot effects on wind and temperature are reduced and SPW1 is weaker if a second hotspot is active. Thus, GW hotspots usually destructively interfere with each other. The effect of all three hotspots in combination is similar to the one of two hotspots. To summarize, the results give new insights into whether or not gravity wave activity and breaking hotspots may inuence the middle atmosphere dynamics and the polar vortex. Given the mechanistic model used, the conclusions are of qualitative nature. More realistic results can be obtained by analyzing global circulation model output, based on a realistic description of the troposphere and gravity wave sources.
Publications
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(2018). Impact of intermittent gravity wave activity on the middle atmospheric circulation during boreal winter. Rep. Inst. Meteorol. Univ. Leipzig, 56, 31-44
Samtleben, N. and C. Jacobi
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(2019). Effect of latitudinally displaced gravity wave forcing in the lower stratosphere on the polar vortex stability. Ann. Geophys., 37, 507-523
Samtleben, N., C. Jacobi, P. Pišoft, P. Šácha, and A. Kuchar
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(2019). Implementing a whole atmosphere gravity wave parameterization in the Middle and Upper Atmosphere Model: preliminary results. Rep. Inst. Meteorol. Univ. Leipzig, 57 (59-70)
Lilienthal, F., N. Samtleben, C. Jacobi, and E. Yigit
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(2020). Impact of local gravity wave forcing in the lower stratosphere on the polar vortex stability: eect of longitudinal displacement. Ann. Geophys., 38, 95-108
Samtleben, N., A. Kuchar, P. Šácha, P. Pišoft, and C. Jacobi
