Roles of forced and inertially unstable convection development in the onset process of Indian summer monsoon

The NCEP/NCAR R1 reanalysis data are employed to investigate the impact of forced and inertial instability in the lower troposphere over the Arabian Sea on the onset process of Indian summer monsoon (ISM), and to reveal the important role of zonal advection of zonal geostrophic momentum played in the forced unstable convection. Results show that during the ISM onset the zero absolute vorticity contour (η= 0) shifts northward due to the strong cross-equatorial pressure gradient in the lower troposphere over southern Arabian Sea. Thus a region with negative absolute vorticity is generated near the equator in the northern hemisphere, manifesting the evident free inertial instability. When a southerly passes through this region, under the influence of friction a lower convergence that facilitates the convection flourishing at the lower latitudes appears to the north of zero absolute vorticity contour. However, owing to such a traditional inertial instability, the convection is confined near the equator which does not have direct influence on the ISM onset. On the contrary in the region to the north of the zero absolute vorticity contour and to the south of the low pressure center near the surface, although the atmosphere there is inertially stable, the lower westerly jet can develop and bring on the apparent zonal advection of zonal geostrophic momentum. Both theoretical study and diagnosing analysis present that such a zonal advection of geostrophic momentum is closely associated with the zonal asymmetric distribution of meridional land-sea thermal contrast, which induces a convergence center near and further north of the westerly jet in the lower troposphere over the southwestern coast of the Indian Peninsula, providing a favorable lower circulation for the ISM onset. It illustrates that the development of convection over the Arabian Sea in late spring and early summer is not only due to the frictional inertial instability but also strongly affected by the zonal asymmetric distribution of land-sea thermal contrast. Moreover, before the ISM onset due to the eastward development of the South Asian High (SAH) in the upper troposphere, high potential vorticity is transported to the region over the Arabian Sea. Then a local trumpet-shaped stream field is generated to cause the evident upper divergence-pumping effect which favors the ISM onset. When the upper divergence is vertically coupled with the lower convergence resulted from the aforementioned forced unstable convection development near the southwestern coast of Indian Peninsula, the atmospheric baroclinic unstable development is stimulated and the ISM onset is triggered.