Abstract
A two-layer planetary geostrophic model is adopted to study the breaking of planetary waves in the presence of Ekman pumping and the associated-mean flow. The governing equation for the interface is a quasi-linear equation, which is solved analytically by the method of characteristics. The waves are forced by annual or interannual upwelling or downwelling along the eastern boundary of a subtropical gyre. It is found that the time and position at which breaking occurs is mainly determined by the speed and depth of the eastern boundary perturbation, while the intensity of a breaking front is mainly determined by the amplitude of the perturbation. The breaking of a planetary wave is affected significantly by Ekman pumping and the associated mean flow, particularly for annual and interannual forcing. Downward Ekman pumping, as in a subtropical gyre, suppresses breaking in downwelling waves caused by eastern boundary upwelling, but enhances breaking in upwelling waves caused by eastern boundary downwelling. In the presence of steady downward Ekman pumping, downwelling breaking will not occur except for interfaces near the surface. The structure and intensity of a breaking front is also discussed.
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