Diagnosis of the Surface Momentum Balance over the Tropical Pacific Ocean

Abstract

The purpose of this study is to evaluate the suitability of using linear drag as a proxy for surface friction in the observed climatological-mean momentum balance over the tropical Pacific Ocean. The linear drag parameterization of kinetic energy dissipation in the planetary boundary layer is widely used in simplified models of the tropical atmosphere, and in numerous observational studies of the surface momentum balance. Climatological seasonal-mean fields of sea level pressure and surface wind from the Comprehensive Ocean-Atmosphere Data Set are used to calculate the pressure gradient, Coriolis, and acceleration terms in the momentum budget; friction is derived as a residual. It is found that when friction is parameterized as a linear dissipation of kinetic energy, the damping time scale for the meridional wind is ?2?3 times faster than the damping time for the zonal wind. The preceding formulation fits the observations well, especially in the trade-wind regions. It is suggested that the different damping coefficients for the zonal (u) and meridional (v) winds are, in part, a reflection of the different vertical profiles of u and v in the planetary boundary layer. A realistic simulation of the tropical surface wind field from the observed sea level pressure field is obtained using a linear momentum balance with unequal damping lime scales for u and v. With equal damping times, the meridional component of the surface flow is too strong. Nonlinear advection improves the zonal wind simulation in limited regions of the northeast trades equatorial easterlies, and off South America, but only by ?0.5 m S1.