POSSIBLE ROLE OF SYMMETRICAL INSTABILITY IN THE ONSET AND ABRUPT TRANSITION OF THE ASIAN MONSOON

The physical connections between dry/moist (conditional) symmetric ins tability (SI/CSI) off equatorial diabatic thermal forcing, and the ons et of the Asian monsoon are investigated using the Goddard Laboratory for Atmospheres general circulation model (GLA GCM). The objective of this study is two-fold: to elucidate the causal relationship between S I/CSI and monsoon onset, and to investigate the threshold behavior of asymmetric monsoonal thermal forcing under condition of SI/CSI. This w ork corroborates the authors' earlier linear instability analysis resu lts, which show, that SI/CSI in the boreal summer monsoon basic states may be a plausible explanation for the abrupt monsoon transition. Mon soon transitions in the model, as depicted by the abrupt meridional mo vement of the axis of maximum vertical motion from equator to northern latitudes, occur during 16-20 May for the East Asian Monsoon (EAM) an d 1-5 June for the South Asian Monsoon (SAM) regions. The necessary st ability criterion for dry (moist) SI over the EAM and SAM regions reve als a sudden cross equatorial advection of negative dry potential vort icity (DPV) and moist potential vorticity (MPV) into the summer hemisp here five to ten days preceding the model monsoon transition. This cau ses dry and moist SI. Maximum shift of the zero line of DPV and MPV (d ry and moist symmetrically unstable regions) happens subsequent to mon soon transition. Simplified analysis of the potential vorticity (PV) b udget equation reveals that the lower tropospheric negative PV advecti on into the summer hemisphere is largely governed by the dominance of vertical differential diabatic heating over horizontal differential di abatic heating. The diabatic heating also shows an abrupt increase fro m 2-3 K day(-1) before the transition, to 12-14 K day(-1) at the time of monsoon transition. The genesis of pre-monsoon weak heat source ari ses primarily due to unstable SI and CSI of the pre-monsoon basic stat es, which consequently produce moderately large scale lower (upper) tr opospheric convergence (divergence) patterns slightly poleward of the zero line of DPV and MPV. Lower tropospheric conditionally unstable tr opical atmosphere, in the presence of off equatorial large scale lower (upper) tropospheric convergence (divergence), is conducive to exciti ng CISK-like processes, which may eventually release large amounts of latent heat and develop a strong heat source at the time of monsoon tr ansition. We have noted that a fully established model meridional circ ulation originates only when the diabatic forcing magnitude exceeds so me threshold value of around 5 K day(-1) at the time of monsoon transi tion. The model transition is more pronounced over the EAM region than over the SAM region. The linear steady-state dynamical response of a zonally symmetric atmosphere as a consequence of varying the location and magnitude of an idealized asymmetric thermal forcing reveals that the most intense meridional circulation (maximum efficiency of vertica l motion) is accomplished when the thermal forcing is located around 1 0 degrees N. The interrelationship between the location of zero DPV/MP V contour, lower tropospheric maximum convergence versus maximum verti cal velocity of the monsoonal circulation in the summer hemisphere, cl early suggests that SI (CSI) of zonal monsoon flows is a causal mechan ism for the onset of monsoon transition.