Monsoon Regime Transitions With Zonally Asymmetric Forcing in a 3-D Idealized Atmosphere

Using idealized atmospheric models, previous studies showed that the response to a zonally symmetric heating centered off the equator is of two distinct types. In one type, below a threshold forcing, the atmosphere maintains a relatively weak circulation with dynamics dominated by viscosity or horizontal eddy momentum transports. In the other type, beyond a critical forcing, a strong nonlinear meridional circulation results. Previous studies about these two regimes assumed a zonally symmetric thermal forcing, but observed forcings can have strong zonal asymmetries, such as in monsoon regions. In this project, we study the response of the Hadley circulation in an idealized atmosphere to a zonally asymmetric subtropical heating. Specifically, the equilibrium temperature distribution has a local subtropical peak and is flat elsewhere, including on the equator. We examine how the strength of the monsoon circulation varies with the heating and determine how the regime transitions and vorticity balance differ for zonally symmetric and asymmetric heatings. Simulations show that the critical forcing needed to achieve a nonlinear circulation is stronger for an asymmetric forcing than for a symmetric forcing, consistent with previous results for shallow water models. Furthermore, by adding a time dependence to the strength of the forcing, we also investigate how the strength of the circulation and its linearity change with a seasonally varying heating like that which occurs in the real atmosphere.