Tropical stationary waves in a shallow water model with realistic zonal-mean winds

A nonlinear shallow water model is used to study the stationary waves in the tropical upper troposphere. Earth-like zonal-mean winds are generated by imposing a zonally-symmetric, global-scale topography distribution underneath a thin fluid layer, and relaxing the fluid depth towards its initial, global-mean value. Zonally-symmetric basic states with zero mean meridional flow are also constructed by balancing the total height field with the zonal-mean zonal winds. Both hemispherically-symmetric (equinoctial) and hemispherically-asymmetric (solstitial) basic states are considered. Stationary waves are generated by adding a mass source-sink distribution either along the equator or shifted into one hemisphere. The thin fluid depth promotes realistic tropical wave structures and prevents a midlatitude resopnse.

The model produces a Gill-like response when a resting basic state is used. Westerly zonal-mean flow in the subtropics amplifies the stationary wave response and shifts the centers of the eddy height maxima and rotational flow anomalies poleward and eastward. These changes bring the simulated stationary waves into better agreement with the observed seasonally-varying large-scale eddy circulations in the tropical upper troposphere. Moving the eddy forcing off the equator leads to a substantial increase in the equilibrium eddy response in the forced hemisphere, but the response in the opposite hemisphere only decreases slightly because the eddy divergent winds force rotational anomalies across a wide latitudinal range. The zonal-mean zonal winds in the solstitial basic state enhance the eddy features in the winter hemisphere and reduce the eddy response in the summer hemisphere. Hemispheric asymmetry in either the eddy forcing or the basic state also leads to eddy momentum fluxes that are directed across the equator.

When the eddy forcing is located in the summer hemisphere of the solstitial basic state, the cross-equatorial mean meridional flow enhances the propagation of wave activity across the equator, leading to stronger cross-equatorial eddy momentum fluxes and an eddy response with similar amplitude in both hemispheres. These results suggests that the anti-correlation between the mean meridional flow and eddy momentum fluxes over the equator and the surprising hemispheric symmetry of the observed tropical stationary waves over the course of the seasonal cycle can both be attributed to the tendency for the maximum eddy and zonal-mean diabatic forcing to occur in the same latitude band.