Mechanism of the intraseasonal oscillation in the South Asian summer monsoon region

Abstract

The mechanism of the intraseasonal oscillation in the South Asian summer monsoon region (ISO) is examined with a zonally averaged, atmospheric model (2D model), a three dimensional, atmospheric intermediate model (3D model). In both models an ocean mixed layer model is added to examine the influence of air-sea interactions on the characteristics of the ISO. Without the ocean mixed layer, an interaction between the baroclinic and barotropic modes of atmosphere can produce the ISO in both 2D and 3D models. The propagation of precipitation is caused by the phase relationship between convection and the barotropic divergence in the atmosphere. Most importantly, in the northern hemisphere, the vertical advection of July-mean easterly wind shear in regions of convection induces barotropic divergence (convergence) to the north (south) of convection. The resulting moisture convergence in the boundary layer induces the northward propagation of precipitation. The initiation of convection is also produced by the barotropic divergence in the atmosphere. Especially, the strong July-mean vertical motion at IDS causes convergence in the boundary layer between IDS and the equator. The baroclinic mode, on the other hand, acts to enhance existing convection. The differences between the ISO simulated by the 2D model and 3D models are caused by the zonal variation of winds, and atmospheric waves in the 3D model. The zonal divergence of barotropic winds enhances the westward propagation of convection along 18N, and the barotropic mode of zonal advection drives the continuous northward movement of convection across the equator. The continuous northward propagation across the equator is also enhanced by the atmospheric waves, since the Rossby wave response to the heating source in both hemispheres creates a divergence in the baroclinic mode near the equator. The inclusion of air-sea interactions in the 2D and 3D models improves the continuity in the northward propagation of convection. The meridional variation of SST enhances the boundary layer moisture convergence in front of the convection, thereby facilitating the northward propagation of convection. In addition, the SST gradient induced by the dipole type of Rossby-wave-like convection in the Indian ocean may increase the development of convection near the equator.