Axisymmetric and asymmetric monsoons in idealized experiments

The physical processes behind monsoons have been identified, but the conditions that make the monsoon of any one region unique are still poorly understood. Recent studies (Dirmeyer 1998, Xie and Saiki 1999) have for instance made use of full physics GCMs to explore how variations in land-sea distribution determine and affect the mean monsoonal circulations. In this study, we perform idealized experiments with a dry primitive equation model in which the meridional circulation is forced by Newtonian relaxation to a specified background state. The advantage of this approach relies in that it allows to isolate the influence of dynamical constraints on the nature of the forced response from other mechanisms at play in a full physics GCM. Both axisymmetric and eddy-permitting experiments are performed with a heating localized in the subtropics.

Axisymmetric, eddy-non-resolving simulations show that the flow forced by a heating localized in the subtropics exhibits a quasi-angular-momentum-conserving, monsoonal overturning response only when the forcing amplitude exceeds a critical threshold. This threshold behavior is the manifestation of the dynamical constraint of zero absolute vorticity in the upper branch of the meridional circulation. The sensitivity of these results to the width and the center of the thermal forcing is explored.

We investigate how this dynamical constraint of zero absolute vorticity is met when eddies are allowed to interact with the forced flow by performing 3D eddy-resolving simulations with both axysimmetric and asymmetric heating. Preliminary results from axysimmetric cases suggest that the threshold behavior from a thermally balanced state to a non-linear meridional circulation carries to 3D simulations and is relatively unaffected by the presence of eddies.