Connection between the Asian Summer Monsoon and Stratosphere-Troposphere Circulation over the Asian Region

Abstract

We examined the connection between the Asian summer monsoon and stratosphere-troposphere circulation over the Asian region during a 25-year period (1980-2004) in boreal summer (June-August) using NCEP/ NCAR reanalysis data. Strong monsoon years (6 years) were extracted from the 25-year sample using the monsoon index of Webster and Yang (1992), and composite maps of selected variables were created. Analyses revealed significant positive geopotential height anomalies over Japan and over Iran around 100 hPa in strong monsoon years. Both stretching and horizontal advection terms were dominant in the vorticity budget of high pressure anomalies over Japan, whereas only horizontal advection term was dominant over Iran. Wave activity flux was calculated for the strong monsoon years. Upward wave fluxes occurred over the Aral Sea and over Mongolia, in association with precipitation in northern India and in the Philippines. These correspondences are likely to show the Rossby wave response to diabatic heating in the Asian monsoon regions. Energy conversion was estimated in both East Asia and Central Asia using an equation for the rate of change of perturbation total energy. In East Asia, barotropic energy conversion was positive in the low-latitude lower troposphere. In mid-latitudes, baroclinic energy conversion was positive in the troposphere. These results correspond to northward wave fluxes in the tropical lower troposphere and upward wave fluxes in mid-latitudes. In Central Asia, barotropic energy conversion was more dominant than baroclinic energy conversion in mid-latitudes. Upward fluxes occur in mid-latitudes, whereas northward fluxes do not occur in the tropics, unlike in East Asia. The estimation of the total wavenumber of stationary Rossby waves indicates that precipitation in the Philippines is strongly connected with high pressure anomalies over Japan through internal Rossby waves. In contrast, high pressure anomalies over Iran do not seem to be deeply related to precipitation in India. These results suggest that lower stratospheric high pressure anomalies over Japan are connected with both Rossby waves generated through precipitation in the Philippines and baroclinic energy conversion over Mongolia, whereas high pressure anomalies over Iran are controlled profoundly by internal dynamical processes near the Aral Sea.