Instability of the axisymmetric monsoon flow and intraseasonal oscillation

A linear model of intermediate complexity based on quasi-equilibrium theory is used in a zonally symmetric aquaplanet configuration to investigate the stability of the mean summer monsoon flow in the south Asian region. In the control case, the mean state has one linearly unstable mode that corresponds in period and structure to the variability in the nonlinear model described by Bellon and Sobel ( 2008) and to the observed 30- to 60-d mode of intraseasonal variability. This mode features propagation of the precipitation anomalies from the equator to the monsoon trough around 20 degrees N. Both the period of this mode and its direction of propagation are determined by the direction and magnitude of the mean meridional baroclinic flow. The wind-induced surface heat fluxes associated with the surface westerlies in the northern tropics are an important contributor to the instability of the mode. The mechanisms of propagation and reinitiation of convection are further described. ( 1) Meridional advection of vorticity by the mean flow is responsible for the creation of free-tropospheric vorticity north of the convection center. This vorticity in turn creates boundary layer convergence via Coriolis acceleration and subsequent northward movement of the convection. ( 2) Meridional advection of zonal momentum creates equatorial free-tropospheric easterlies on the equator. These easterlies in turn generate low-level convergence via the beta term of the Coriolis acceleration and cause the reemergence of convection at the equator.