The role of gravity waves (GWs) in a sudden stratospheric warming (SSW) event that occurred in January 2009 (SSW09) is investigated using the MERRA-2 dataset. Nearly 2 weeks prior to the central date (lag 5 0), at which the zonal-mean zonal wind at 10 hPa and 608N first becomes negative, westward GW drag (GWD) is significantly enhanced in the lower mesosphere and stratosphere. At 5 days before lag 5 0, planetary waves (PWs) of zonal wavenumber 2 (ZWN-2) in the stratosphere are enhanced, while PWs of ZWN-1 are weakened, which are evident from the amplitudes of the PWs and their Eliassen-Palm flux divergence (EPD). To examine the relationship between PWs and GWs, a nonconservative GWD (NCGWD) source term of the linearized quasigeostrophic potential vorticity equation is considered. A ZWN-2 pattern of the NCGWD forcing is developed around z 5 55-60 km with a secondary peak around z 5 40 km just before the PWs of ZWN-2 in the stratosphere began to enhance. A significant positive correlation between the NCGWD forcing in the upper stratosphere and lower mesosphere (USLM; 0.3-0.1 hPa in the present data) and the PWs of ZWN-2 in the stratosphere (5-1 hPa) exists. This result demonstrates that the amplification of the PWs of ZWN-2 in the stratosphere before the onset of SSW09 is likely related to the generation of PWs by GWD in the USLM, which is revealed by the enhanced downward-propagating PWs of ZWN-2 into the stratosphere from above.
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Acknowledgments. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2017R1A2B2008025) and by the Korea Polar Research Institute (KOPRI; PE19020/PE20100). The MERRA-2 dataset was obtained from https://gmao.gsfc.nasa.gov/ reanalysis/MERRA-2/data_access/. The CERA-SAT dataset was obtained from https://www.ecmwf.int/en/forecasts/datasets/ reanalysis-datasets/cera-sat.
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All Science Journal Classification (ASJC) codes
- Atmospheric Science