Tropical ocean surface winds

As a graduate student, I was initially attracted to the simple models of the tropical atmosphere (the “Gill” Model), and which had then been popularly used for modeling the coupled tropical ocean-atmosphere system. However, it wasn’t clear at the time if these models were accurately representative of tropical ocean surface winds, the latter being important to the coupling.

I came across an intriguing paper that showed through a momentum budget analysis (and modeling friction to be proportional to the wind speed) that the inferred friction coefficients in the meridional direction was 2-3 times larger than in the zonal direction. In Chiang and Zebiak 2000 we derived a simple model of the tropical boundary layer winds to show that the reason for this was that strong zonal winds above the boundary layer mixed down, reducing the retarding influence of surface momentum fluxes. This was the first paper that I produced from conception through publication, so I have a special fondness for this paper.

At the time, there were two competing paradigms for interpreting the underlying driver of anomalous tropical surface winds: one view argued that the underlying sea surface temperature pattern set up pressure gradients in the boundary layer; while another view argued that the winds were a consequence of elevated diabatic heating in the free troposphere. In Chiang et al. 2001 we developed a model incorporating those two influences to test their relative roles in driving anomlous tropical ocean surface winds; we found that elevated heating was responsible for zonal wind anomalies, and sea surface temperature gradients were responsible for the meridional winds.

Chiang, J.C.H. and Zebiak, S.E., 2000. Surface wind over tropical oceans: Diagnosis of the momentum balance, and modeling the linear friction coefficient. Journal of Climate, 13(10), pp.1733-1747.

Chiang, J.C.H., Zebiak, S.E. and Cane, M.A., 2001. Relative roles of elevated heating and surface temperature gradients in driving anomalous surface winds over tropical oceans. Journal of the atmospheric sciences, 58(11), pp.1371-1394.