Spatial and temporal variability of nonlinear wave processes during SSWs of various types

The study is dedicated to the analysis of the internal dynamics of sudden stratospheric warmings (SSWs) of various types from the point of view of explicit calculation of nonlinear interactions of planetary waves with each other and with the mean flow, as well as wave activity. Wave processes were analyzed using the approach consisting of studying the variability of perturbed potential enstrophy. This approach allows a detailed study of the evolution of the interaction of waves with the mean flow, to estimate the contribution of divergence and advection of the potential enstrophy flux to the change in wave activity. Using available climatological descriptions of SSWs observed in recent decades and processing of hydrometeorological parameters data from the MERRA-2 reanalysis database, SSWs accompanied by a shift and splitting of the stratospheric polar vortex were selected and divided into groups. A separate group of SSWs with an atypical position and shape of the stratospheric polar vortex, not falling into the first two groups, was identified. During such SSWs, not only the displacement of the stratospheric polar vortex was observed, but also its extension over the territory of the Northern Hemisphere, and a short-term splitting. As a result, five SSW events were selected for each group. For each studied event, the terms in the balance equation of perturbed potential enstrophy for stationary planetary waves with zonal wave numbers 1 and 2 (SPW1 and SPW2) were calculated. An analysis of a large number of SSWs of various types showed that nonlinear wave-wave interactions, which are terms of the third order of smallness and are usually not taken into account in studies, make a significant contribution to changes in wave activity before and during the development of SSWs with a displacement of the stratospheric polar vortex. A similar contribution of the terms responsible for the interaction between SPWs was obtained for the SSWs from the last group with an atypical position and shape of the stratospheric polar vortex. Wave-wave interactions are observed before the SSW onset mainly above 45 km and north of 50° N. The interaction of SPW1 with the mean flow is maximal 1-3 weeks before the splitting SSW, and SPW2 – during the week before the SSW. At the same time, during and 5 days before the SSW, the wave activity of SPW2 is proportional to or exceeds the wave activity of SPW1. The exchange terms of SPW1 and SPW2 with the mean flow increase 1-3 weeks before the SSW with a shift of the stratospheric polar vortex. Analysis of the terms responsible for the interaction of the wave with the mean flow during the development of SSWs from the third group showed that the development of these SSWs is accompanied by variations in the exchange terms characteristic of both SSWs with splitting and with a shift of the stratospheric polar vortex. Further statistical processing of the obtained results can help to identify predictors of the formation of the SSW. The study was supported by St. Petersburg University (project 116234986).