Possible Role of Symmetric Instability in the Onset and Abrupt Transition of the Asian Monsoon

Abstract

The physical connections between dry/moist (conditional) symmetric instability (SI/CSI) off equatorial diabatic thermal forcing, and the onset 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 SI/CSI and monsoon onset, and to investigate the threshold behavior of asymmetric monsoonal thermal forcing under condition of SI/CSI. This work corroborates the authors' earlier linear instability analysis results, which show, that SI/CSI in the boreal summer monsoon basic states may be a plausible explanation for the abrupt monsoon transition.
Monsoon transitions in the model, as depicted by the abrupt meridional movement of the axis of maximum vertical motion from equator to northern latitudes, occur during 16-20 May for the East Asian Monsoon (EAM) and 1-5 June for the South Asian Monsoon (SAM) regions. The necessary stability criterion for dry (moist) SI over the EAM and SAM regions reveals a sudden cross equatorial advection of negative dry potential vorticity (DPV) and moist potential vorticity (MPV) into the summer hemisphere five to ten days preceding the model monsoon transition. This causes dry and moist SI. Maximum shift of the zero line of DPV and MPV (dry and moist symmetrically unstable regions) happens subsequent to monsoon transition. Simplified analysis of the potential vorticity (PV) budget equation reveals that the lower tropospheric negative PV advection into the summer hemisphere is largely governed by the dominance of vertical differential diabatic heating over horizontal differential diabatic heating.
The diabatic heating also shows an abrupt increase from 2-3K day^-1 before the transition, to 12-14K day^-1 at the time of monsoon transition. The genesis of pre-monsoon weak heat source arises primarily due to unstable SI and CSI of the pre-monsoon basic states, which consequently produce moderately large scale lower (upper) tropospheric convergence (divergence) patterns slightly poleward of the zero line of DPV and MPV. Lower tropospheric conditionally unstable tropical atmosphere, in the presence of off equatorial large scale lower (upper) tropospheric convergence (divergence), is conducive to exciting CISK-like processes, which may eventually release large amounts of latent heat and develop a strong heat source at the time of monsoon transition. We have noted that a fully established model meridional circulation originates only when the diabatic forcing magnitude exceeds some threshold value of around 5K day^-1 at the time of monsoon transition. 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 vertical motion) is accomplished when the thermal forcing is located around 10°N. The interrelationship between the location of zero DPV/MPV contour, lower tropospheric maximum convergence versus maximum vertical velocity of the monsoonal circulation in the summer hemisphere, clearly suggests that SI (CSI) of zonal monsoon flows is a causal mechanism for the onset of monsoon transition.