Tropical Stationary Waves in a Nonlinear Shallow-Water Model with Realistic Basic States

Abstract

Abstract The nonlinear shallow-water equations are used to study the tropical stationary wave response to steady thermal forcing near the equator in earthlike zonally symmetric basic states. A thin (200 m) fluid layer is superimposed over a large (1500 m) zonally symmetric topography distribution that decreases smoothly from the Tropics to the Poles, thus providing the large meridional height gradients required to maintain realistic zonal-mean zonal winds without introducing unrealistically large tropical wave speeds. A mean meridional circulation is maintained by relaxing the fluid toward its initial, global-mean depth. Both hemispherically symmetric (equinoctial) and hemispherically asymmetric (solstitial) basic states are considered. Stationary waves are generated by adding a fixed mass source/sink distribution near the equator. The presence of westerly zonal-mean winds in the subtropics amplifies the steady eddy response to the tropical mass forcing and shifts the Rossby gyres poleward and eastward, relative to their position in a resting basic state. When either the mass forcing or the center of the topography distribution is moved off the equator, the eddy response develops considerable hemispheric asymmetry. When the eddy forcing is centered in the “summer” hemisphere of the solstitial basic state, the eddy response exhibits a similar amplitude in both hemispheres, which suggests that the considerable hemispheric symmetry of the observed seasonally varying eddy circulations in the tropical upper troposphere may be attributed to the tendency for the maximum zonal-mean and eddy diabatic heating to occur in the same latitude band throughout the seasonal cycle. Hemispheric asymmetry in either the basic state or the eddy forcing also leads to cross-equatorial eddy momentum fluxes. Experiments performed using a balanced basic state with zero-mean meridional flow indicate that the cross-equatorial mean meridional winds play a significant role in promoting the propagation of wave activity across the equator under solstitial conditions.