Role of Gravity Waves in a Vortex-Split Sudden Stratospheric Warming in January 2009

Abstract

Sudden stratospheric warmings (SSWs) are dramatic events accompanying enormous changes in temperatures and winds in the high-latitude winter stratosphere. SSWs have been known to be triggered by large-scale planetary waves (PWs). Small-scale gravity waves (GWs) can contribute to the evolution of SSWs by inducing anomalous PW activities in the stratosphere. The roles of GWs in modulating PWs during SSWs have mainly been studied in terms of changes in the wave propagation condition or triggering the instability and resonance of PWs. In addition, GW drag (GWD) can generate PWs as a nonconservative source of the potential vorticity equation. In this study, we examine the role of GWs on a vortex-split type SSW event that occurred in January 2009 (SSW09), focusing on the nonconservative GWD (NCGWD) forcing during the pre-SSW period, using the MERRA-2 reanalysis dataset. A ZWN-2 pattern of the NCGWD forcing is developed in the upper stratosphere and lower mesosphere (USLM; 0.3-0.1 hPa in the present dataset) just before the PWs of ZWN-2 in the stratosphere began to enhance. Downward-propagating PWs of ZWN-2 with a period of 11 days are generated in the USLM by the NCGWD forcing about 10 days before the central date of the SSW09 and then gradually descend to the mid-stratosphere (~5 hPa), The downward-propagating PWs have magnitudes of approximately 10-20% of the upward-propagating stationary PWs of ZWN-2 from the troposphere, which contribute to strong wave amplification in the mid-stratosphere during the evolution of SSW09.