Generation of planetary waves by gravity-wave drag in the middle atmosphere

Large-scale atmospheric circulation has been represented mostly by interaction between the mean flow and planetary waves (PWs). Although the importance of gravity waves (GWs) has been recognized for long time, contribution of GWs to the large-scale circulation is receiving more attention recently, with conjunction to GW drag (GWD) parameterizations for climate and global weather forecasting models that extend to the middle atmosphere. As magnitude of GWD increases with height significantly, circulations in the middle atmosphere are determined largely by interactions among the mean flow, PWs and GWs. Classical wave theory in the middle atmosphere has been represented mostly by the Transformed Eulerian Mean (TEM) equation, which include PW and GW forcing separately to the mean flow. Recently, increasing number of studies revealed that forcing by combined PWs and GWs is the same, regardless of different PW and GW forcings, implying a compensation between PWs and GWs forcing. There are two ways for GWs to influence on PWs: (i) changing the mean flow that either influences on waveguide of PWs or induces baroclinic/brotropic instabilities to generate in situ PWs, and (ii) generating PWs as a source of potential vorticity (PV) equation when asymmetric components of GWD exist. The fist mechanism has been studies extensively recently associated with stratospheric sudden warmings (SSWs) that are involved large amplitude PWs and GWD. The second mechanism represents more directly the relationship between PWs and GWs, which is essential to understand the dynamics in the middle atmosphere completely (among the mean flow, PWs and GWs). In this talk, a recently reported result of the generation of PWs by GWs associated with the strongest vortex split-type SSW event occurred in January 2009 (Song et al. 2020, JAS) is presented focusing on the second mechanism.