Atmospheric Coupling Through Gravity Waves During Stratospheric Sudden Warmings: Gravity Wave Variations, Generation Mechanisms, and Impacts

Full understanding of gravity wave influences on the middle and upper atmosphere remains an unresolved research topic. The goals of this work are two-fold. First, gravity wave sources and propagation characteristics are explored using assimilated meteorological analyses from the European Centre for Medium-Range Weather Forecasting (ECMWF) during the 2009 stratospheric sudden warming (SSW). Second, gravity wave impacts on polar temperatures in the middle and upper atmosphere are examined by modulating the gravity wave parameterization scheme in the Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model (TIME-GCM). Gravity waves that are resolved in ECMWFT799 are validated with satellite and lidar observations. ECMWF gravity wave potential energy density (GW-Ep) shows two enhancements, on January 5 and 15--22, prior to the peak 2009 SSW on January 23--24. The two gravity wave enhancements are associated with the amplifications of planetary wave 1 and wave 2, respectively, and there is a sudden decay of GW-Ep after the peak 2009 SSW. GW-Ep enhancements prior to the SSW correspond well with the positive vertical gradients of total perturbation energy flux (FE), indicating an in-situ energy source. The spatial and temporal distributions of gravity wave activities correlate with those of the residual tendencies introduced by Snyder et al. [2009]. These results suggest that the two peaks of GW-Ep are caused by the enhancements of the wave excitation in the stratosphere due to the residual tendency forcings. The sudden decay of gravity wave amplitudes correlates well with the suppressions of gravity wave propagation from the troposphere to the stratosphere obtained from the ray-tracing model. In addition, the vertical derivatives of FE decay after January 22. These results indicate that the sudden decay of gravity waves after the wind reversal is likely due to suppressions of gravity wave propagation from the troposphere along with the reductions of in-situ gravity wave excitation by the polar night jet. The responses of the mesosphere and lower thermosphere (MLT) temperatures to gravity waves during SSWs are investigated using TIME-GCM through modifying gravity wave parameters. This study confirms that the height of gravity wave forcing region is mainly determined by gravity wave amplitude and wavelength, and the vertical depth is closely tied to the spectral width of gravity wave phase speed. The gravity wave forcings control the pattern and strength of residual circulation and thereby the characteristics of MLT cooling and warming regions. The planetary wave forcings in the MLT also affect the vertical depth and magnitude of MLT temperature anomalies through further modifying the residual circulation. These planetary wave forcings are likely generated in-situ by the gravity wave forcings at high latitudes. Therefore, the mechanisms of gravity wave controlling the MLT temperature during a SSW are directly through gravity wave forcing and indirectly through generating planetary waves in-situ. Realistic gravity wave variations during the 2009 SSW obtained from ECMWF-T799 are implemented in TIME-GCM. The following two simulations are examined. Case 1 includes the enhanced gravity waves with longitudinal variations. Case 2 suppresses gravity waves with horizontal wavelength longer than 150 km. Both cases improve the TIME-GCM simulations of the MLT temperature responses to the 2009 SSW, indicating that realistic gravity wave variations have impacts on the MLT thermal structure