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Generation of annual period Rossby waves in the South Atlantic Ocean by the wind stress curl Reason, Christopher James Charles


A theoretical study is presented of generation of first mode annual period baroclinic Rossby waves by the observed wind stress curl in the South Atlantic and South West Indian Oceans. The forcing wind field for the area 15 S to 51 S, 45 W to 41 E was obtained from an harmonic analysis at the annual period of the monthly mean wind stress curl values derived from Hellerman and Rosenstein's 1983 data. The annual harmonic of the wind stress curl was then used to drive a linear, reduced gravity model of the South Atlantic and South West Indian Oceans bounded by the latitudes 15 S and 51 S and by longitudes 46 W and 50 E. Boundary geometries and oceans are represented to 1.0 and 0.2 degree accuracy, respectively, by a finite difference grid in spherical polar co-ordinates. Successive over-relaxation and a leap frog time differencing scheme are used to solve the two dimensional Rossby wave equation which includes relative vorticity and a Rossby radius that varies with latitude. By assuming quasi-geostrophy, the steady state model response is restricted to first mode baroclinic Rossby waves. In the model South Atlantic Ocean, the response consists of long Rossby waves which generally propagate south westwards across the ocean and which exhibit refraction of wave energy towards the equator as in the theory of Schopf et al (1981). Short Rossby waves with eastward energy propagation are generated in the small area of the Indian Ocean included in the model domain. However, since the model grid has a resolution of approximately 22 km these waves may not be sufficiently accurately represented in the study. Medium to long waves generated to the south east of Africa may reflect their energy off this landmass into the Indian Ocean. Slowness curve theory and wavenumber computations along wave rays in the South Atlantic are applied to match the model wave trains with probable sources. It is found that wind stress curl maxima off the Namibian coast near 25 S,10 E, near the Agulhas Plateau at 38 S,25 E and in the South Atlantic Ocean interior near 38 S,10 W are the most efficient wave generators. The results of the study are generally consistent with earlier theoretical studies of the North Pacific by White and Saur (1981) and by Cummins et al (1986) and of the North Atlantic by Krauss and Wuebber (1982).

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