Surface Wind over Tropical Oceans: Diagnosis of the Momentum Balance, and Modeling the Linear Friction Coefficient

John C. H. Chiang Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York

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Stephen E. Zebiak International Research Institute for Climate Prediction, Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York

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Abstract

Previous diagnostic studies of surface wind momentum balances over tropical oceans showed that, under a linear friction assumption, the meridional friction coefficient is two to three times larger than the zonal friction coefficient, and that both friction coefficients exhibit a pronounced meridional dependence. The authors’ diagnosis of a global marine surface dataset confirms these results. Furthermore, it is shown that to first approximation the friction coefficients are independent of longitude and season in the tropical band between ∼20°S and ∼20°N. Poleward of 20°N and 20°S, the coefficients are no longer solely a function of latitude. To explain these empirical results, a simple analytical model of the friction coefficient is formulated based on the simplest K-theory mixed-layer parameterization, assuming constant viscosity. The model does a good job of reproducing the observed zonal friction coefficient, but does poorly for meridional friction. The poor result is thought to be from model sensitivity to the specified planetary boundary layer (PBL) thickness. By reversing the calculation, using observed meridional friction coefficients, and assuming no meridional winds at PBL top, model PBL heights were derived that compared favorably with zonally averaged inversion heights for June–August over the tropical Atlantic.

This model suggests that both coefficients increase away from the equator because of the decrease in PBL thickness. Furthermore, the zonal friction coefficient is smaller than the meridional coefficient because strong zonal winds at the top of the boundary layer mixes down, reducing the retarding influence of surface zonal momentum fluxes. The results also suggest that the boundary layer top winds and height are important components in modeling surface winds over the tropical oceans.

Corresponding author address: Mr. John C. Chiang, Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964.

Email: jchiang@ldeo.columbia.edu

Abstract

Previous diagnostic studies of surface wind momentum balances over tropical oceans showed that, under a linear friction assumption, the meridional friction coefficient is two to three times larger than the zonal friction coefficient, and that both friction coefficients exhibit a pronounced meridional dependence. The authors’ diagnosis of a global marine surface dataset confirms these results. Furthermore, it is shown that to first approximation the friction coefficients are independent of longitude and season in the tropical band between ∼20°S and ∼20°N. Poleward of 20°N and 20°S, the coefficients are no longer solely a function of latitude. To explain these empirical results, a simple analytical model of the friction coefficient is formulated based on the simplest K-theory mixed-layer parameterization, assuming constant viscosity. The model does a good job of reproducing the observed zonal friction coefficient, but does poorly for meridional friction. The poor result is thought to be from model sensitivity to the specified planetary boundary layer (PBL) thickness. By reversing the calculation, using observed meridional friction coefficients, and assuming no meridional winds at PBL top, model PBL heights were derived that compared favorably with zonally averaged inversion heights for June–August over the tropical Atlantic.

This model suggests that both coefficients increase away from the equator because of the decrease in PBL thickness. Furthermore, the zonal friction coefficient is smaller than the meridional coefficient because strong zonal winds at the top of the boundary layer mixes down, reducing the retarding influence of surface zonal momentum fluxes. The results also suggest that the boundary layer top winds and height are important components in modeling surface winds over the tropical oceans.

Corresponding author address: Mr. John C. Chiang, Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964.

Email: jchiang@ldeo.columbia.edu

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