Laterally Driven Stochastic Motions in the Tropics

A two-layer dry atmospheric model containing non-geostrophic effects and parametrized dissipation is used to investigate the large-scale tropical eddies as perturbations on a basic state which has empirical static stability and symmetric zonal current. The perturbed motion is treated as a stationary random process driven by stochastic forcing at ±30° latitude. The latter is inferred from observations at 30N. In the predicted internal statistics, 1) variance of horizontal velocity components is much larger at the upper level and decreases equatorward, 2) variances of horizontal divergence and of temperature decrease markedly equatorward, and 3) the eddies transport sensible heat and wave energy equatorward, and transport zonal momentum poleward. The eddies gain kinetic energy from pressure work on the boundaries and lose it by dissipation and conversions to zonal kinetic energy and eddy available potential energy, which is in turn depleted by radiative cooling and conversion to zonal available potential energy. The observed tropical statistics which are available agree with the corresponding theoretical values. Therefore, the large-scale eddies in low latitudes seem to be driven by the unstable baroclinic processes in higher latitudes. Much of the predicted meridional velocity variance in the equatorial region at the upper level arises from motions of periods around 5 days, wavelength around 10,000 km, with westward phase propagation. These are similar to the disturbances in the equatorial lower stratosphere recently discovered by Yanai and Maruyama.