The influence of cross-equatorial pressure gradients on the location of near-equatorial convection

Tomas and Webster note that in regions where there is a substantial cross-equator surface pressure gradient, there is locally anticyclonic absolute vorticity on the low pressure side of the equator and thus, the flow in this region meets the parcel criterion for inertial instability. They hypothesize that the atmospheric response to this absolute-vorticity distribution is a convergence-divergence doublet in the boundary layer. The convergence centre results in strong convection and thus is very important in determining the strength and location of convection (i.e. the intertropical convergence zones) whereas the divergence centre results in suppressed convection. In this work, a zonally symmetric model of the boundary layer is developed to investigate this hypothesis further. An analysis of a linearized version of the model indicates that although the observed flow in these regions meets the parcel criterion for instability, it does not satisfy the linear stability criteria, because of the stabilizing influence of dissipation and the finite vertical scale. Much greater shear than that observed is required for linear instability. It is noted, however, that when the parcel criterion is met, one of the nonlinear terms which was neglected in the linear analysis may have an important influence on the flow. Several experiments are performed integrating the full nonlinear model, relaxing back to a pressure distribution having a cross-equatorial gradient, to test whether the observed vorticity, wind and convergence distributions can be well simulated. It is found that the simulations reach a steady state in which there is a region of locally anticyclonic absolute vorticity on the low pressure side of the equator. This absolute-vorticity distribution results in an anomalous acceleration of the meridional wind and a convergence-divergence doublet, similar to that observed by Tomas and Webster. This response is explained as resulting from accelerations that act to bring the zonal flow out of geostrophic balance as it passes through the region of locally anticyclonic absolute vorticity. The response of the meridional wind to the strength of the pressure gradient is quasi-linear for small and somewhat larger values of forcing but, between these values, there is a range where the response is highly nonlinear, i.e. large increases in the strength of the meridional wind result from small changes in the pressure gradient. It is suggested that this nonlinear response may play a role in the observed sudden onset of the monsoon circulations. The nonlinear model is also forced using pressure distributions taken from observations The resulting steady-state simulated vorticity, wind and convergence distributions closely match the observations for the cases of pressure distributions taken from the east Pacific and east Atlantic during July. When forced using the pressure distribution from the Indian Ocean region during July, the model produces an unrealistically strong meridional wind response. Some improvement is obtained by including the latitudinally dependent zonal pressure gradient force, which is moderately strong in this region. Finally, the model is forced using a pressure distribution taken from observations in the central Pacific Ocean during July. In this case the cross-equator pressure gradient is near zero.