Calculating Tropical Winds from Time Mean Sea Level Pressure Fields

Abstract

The time-mean tropical surface momentum balance is investigated with a simple model that calculates tropical surface winds from time mean sea level pressure fields. The model domain is the global tropical strip centered on the equator with lateral boundaries at ±30° latitude. Steady state surface winds are numerically calculated from the nonlinear horizontal momentum equations, with forcing from observed climatological monthly mean sea level pressures and prescribed lateral boundary winds. Dissipation is parameterized by linear damping and diffusion. Comparisons of model winds with observed climatological monthly mean winds show realistic simulations in most regions and in all months. The poorest simulations occur in the meridional component of the wind in near-equatorial areas of strongly convergent or weak winds. In these areas, and in the near-equatorial region generally, diffusion processes make a significant positive contribution to the realism of the model winds. Horizontal nonlinear advection also improves the simulation near the equator, though to a smaller degree. The generally skillful model winds refute the conventional idea that weak gradients make the tropical pressure field a poor tool for calculating tropical winds. To the contrary, tropical pressure fields contain substantial information about associated winds. Thus, a relatively complete momentum balance can be identified for the major features of the time-mean tropical wind field.