Roles of Gravity Waves in Preconditioning of a Stratospheric Sudden Warming

As well as strong upward propagation of planetary waves from the troposphere, the state of the stratospheric mean flow has been recognized as a key factor for the occurrence of stratospheric sudden warmings (SSWs). The modification of the mean flow to a suitable state for an SSW occurrence is called “vortex preconditioning.” Recently, increasing attention has been paid to the role of gravity waves (GWs) in the preconditioning mechanism. However, because of the limited availability of data sets covering the whole neutral atmosphere, much uncertainty still exists in the role of GWs in the preconditioning. The aim of this study is to investigate the mechanism of modification of the mean flow in the stratosphere and mesosphere before SSWs from a climatological viewpoint and elucidate the role of GWs in it. We use two state‐of‐the‐art data sets covering the whole neutral atmosphere: a 17‐year medium‐resolution reanalysis data set and the output data from hindcast simulations performed with a GW‐permitting general circulation model. It is shown that the second principal component of the zonal‐mean zonal wind in the stratosphere and mesosphere tends to show a maximum prior to an SSW, characterizing preconditioning. GW forcing alters the structure of the upper part of the jet and contributes to the preconditioning along with planetary waves. Comparison of GW forcing between the reanalysis and GW‐permitting model suggests that the magnitude of parameterized GW forcing is approximately half that of the GW forcing in the polar upper stratosphere where the forcing is responsible for the preconditioning. Plain Language Summary Stratospheric sudden warmings (SSWs) are a rapid increase in temperature in the polar winter stratosphere in winter. SSWs are mainly attributed to strong westward forcing caused by planetary waves in the polar stratosphere. To extend the lead time for prediction of an SSW, enhancing our understanding of SSW precursors is of significant importance. One of the possible precursors is so‐called “vortex preconditioning,” in which the zonal‐mean flow in the stratosphere gets tuned to a suitable state for strong planetary waves to converge in the polar stratosphere. In this study, using a 17‐year reanalysis data set and high‐resolution general circulation model, both of which cover from the surface to the lower thermosphere (∼110 km in altitude), a statistical analysis and a case study on vortex preconditioning are performed. It is shown that not only p