The boreal summer intraseasonal oscillation over the South Asian monsoon region

The spatial and temporal structures of the northward-propagating boreal summer intraseasonal oscillation (BSISO) are studied based on the analysis of both the ECHAM Atmospheric General Circulation Model (AGCM) simulation and the NCEP/NCAR reanalysis. The BSISO structure and evolution characteristics simulated by the model bear close similarities to those derived from the NCEP/NCAR reanalysis, suggesting that the internal atmospheric dynamics may play an important role in the northward propagating BSISO. The most notable features are remarkable meridional asymmetries, relative to the BSISO convection, in the vorticity and specific humidity fields. A positive vorticity perturbation with an equivalent barotropic structure appears a few degrees north of the convection center. The maximum specific humidity also shows a clear northward shift in the lower troposphere.; Two mechanisms are proposed for the northward propagation of the BSISO, due to the vertical shear of mean flow and moisture-convection feedback. The coupling between the free-atmosphere baroclinic and barotropic modes in the presence of the vertical shear of the mean flow is important for the first mechanism.; The characteristic features of the BSISO during its reinitiation period are studied and a possible mechanism is elucidated based on the simulation by a simplified AGCM. The transition for the active to suppressed convection phase is associated with the divergence and reduced specific humidity in the boundary layer. This low-level divergence takes place to balance the low-level horizontal temperature advection, which is induced by active convection over the eastern equatorial Indian Ocean. The summer mean state is found important for the horizontal temperature advection and low-level divergence.; The seasonal migration of the thermal equator over the south Asian monsoon region may play an important role for the seasonality of the ISO behavior. During boreal summer, the thermal equator exhibits a significant northward shift away from the geographic equator, and this shift acts to dampen the Kelvin wave and destroy the coupled Kelvin-Rossby ISO wave-packet, giving rise to the meridional bifurcation of the ISO convection over south Asian monsoon region. This hypothesis is supported by both an eigen-value analysis and numerical simulation based on a simplified system.