A New Method to Estimate Three-Dimensional Residual Mean Circulation in the Middle Atmosphere

We propose a new method to estimate three-dimensional (3-d) material circulation driven by waves based on recently-derived formulas by Kinoshita and Sato (JAS, 2013) that are applicable both to Rossby waves and gravity waves. As the formulas use time mean instead of zonal mean, stationary waves cannot be treated. Taking it into consideration that the average is inherently needed to remove twice phase-oscillation of flux quadratics, the 3-d wave activity flux associated with stationary waves and their induced residual mean flow can be estimated by an extended Hilbert transform. Using simulation data for July by a gravity-wave resolving general circulation model, 3-d structure of the Brewer-Dobson circulation (BDC) is examined. The residual mean flow is divided into unbalanced flow due to momentum flux divergence and Stokes drift due to heat flux divergence. The southward flow in the winter circulation of the middle and upper stratosphere is composed of three parts: Stokes drift by planetary waves (PW) in high latitudes of the southern hemisphere (SH), which is partly canceled by northward unbalanced flow by PW, the unbalanced flow by PW in SH middle and low latitudes, and the unbalanced flow by gravity waves in the northern hemisphere (NH). In SH high latitudes, contribution of stationary PW is more dominant than that of transient PW. The BDC is not zonally uniform. The southward flow in NH is strong in the Asian and African monsoon region. The southward Stokes drift and northward unbalanced flow in SH high latitudes are distributed in different longitudes, resulting in the existence of net northward flow in SH in winter.