Wave Driving in the Tropical Lower Stratosphere as Simulated by WACCM. Part I: Annual Cycle

Abstract

This study explores the climatological annual cycle of temperature, circulation, and wave driving distributions in the tropical lower stratosphere as produced in a 50-yr simulation of the Whole Atmosphere Community Climate Model (WACCM). The simulation is forced with a climatological sea surface temperature and sea ice condition. The present diagnoses verify the primary balances of the annual cycle in this region, consistent with lower temperatures, stronger residual circulation (upwelling and local meridional outflow), and nearby stronger wave driving for Northern Hemisphere (NH) winter. An in-detail analysis on the wave driving further reveals that the stronger driving, occurring mostly in the northern tropics and subtropics, is contributed by northward and upward propagation (associated with meridional and vertical fluxes of zonal momentum, respectively) of equatorial Rossby waves forced by convective heating, and also by equatorward propagation of NH extratropical planetary and synoptic waves. The results are used to discuss implications about possible factors that may affect the different observations of the wave driving. The present framework and results will be extended to investigate ENSO-induced changes in this region during NH winter in a forthcoming paper.