The tropospheric response to prescribed tropical and subtropical zonally-asymmetric torques: analytical and idealized model results

The atmospheric response to prescribed zonally asymmetric heatings has been well examined, but the response to prescribed torques has been studied only for zonally symmetric forcings, primarily in the extratropics. Here we investigate the atmospheric response to prescribed zonally-asymmetric tropical and subtropical torques in the upper troposphere. Such torques can be thought of as representing the vertical transfer of momentum by orographic waves or convection. The linear analytical and nonlinear numerical responses to the prescribed torques were considered, and the dependence of the response on the existence of midlatitude baroclinic eddies in the basic state was examined.

The linear Matsuno-Gill model response to an imposed equatorial torque differs significantly from the response to an equatorial heating. The response to a westward upper-tropospheric torque involves downwelling to the west of the torque both north and south of the equator, and upwelling to the east of the torque. However, the zonal-mean meridional streamfunction response to the equatorial torque is identical to that for a mid-level equatorial heating. When the forcing is shifted into the northern hemisphere the meridional streamfunction responses differ dramatically: the response to the westward torque involves strengthening and broadening of the northern hemisphere Hadley cell whereas the response to the heating involves the strengthening of the southern hemisphere Hadley cell.

The nonlinear response was explored in idealized general circulation model (GCM) experiments. The response in the absence of moist processes was found to be qualitatively similar to the linear solutions in the vicinity of the torque but involved a significant transient eddy component as well. When a large-scale meridional temperature gradient was included in the basic state the response involved a poleward shift of the jet and the regions of horizontal eddy momentum flux convergence. These poleward shifts were related to similar shifts in the critical surface for baroclinic waves, which were in turn caused by the prescribed torque. We explore the modification of the response by moist processes using an aquaplanet GCM.