Energetics and monsoon bifurcations

Monsoons involve increases in dry static energy (dse), with primary contributions from increased shortwave radiation and condensation of water vapor, compensated by dse export via horizontal fluxes in monsoonal circulations. we introduce a simple box-model characterizing evolution of the dse budget to study nonlinear dynamics of steady-state monsoons. horizontal fluxes of dse are stabilizing during monsoons, exporting dse and hence weakening the monsoonal circulation. by contrast latent heat addition (lha) due to condensation of water vapor destabilizes, by increasing the dse budget. these two factors, horizontal dse fluxes and lha, are most strongly dependent on the contrast in tropospheric mean temperature between land and ocean. for the steady-state dse in the box-model to be stable, the dse flux should depend more strongly on the temperature contrast than lha; stronger circulation then reduces dse and thereby restores equilibrium. we present conditions for this to occur. the main focus of the paper is describing conditions for bifurcation behavior of simple models. previous authors presented a minimal model of abrupt monsoon transitions and argued that such behavior can be related to a positive feedback called the ‘moisture advection feedback’. however, by accounting for the effect of vertical lapse rate of temperature on the dse flux, we show that bifurcations are not a generic property of such models despite these fluxes being nonlinear in the temperature contrast. we explain the origin of this behavior and describe conditions for a bifurcation to occur. this is illustrated for the case of the -mean monsoon over india. the default model with mean parameter estimates does not contain a bifurcation, but the model admits bifurcation as parameters are varied.