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Generation of Annual-Period Rossby Waves in the South Atlantic Ocean by the Wind Stress Curl

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  • 1 Department of Oceanography, University of British Columbia, Vancouver, B.C., Canada, V6T 1W5
  • | 2 Climate Research Group, Department of Meteorology, McGill University, Montreal, Quebec, Canada, H3A 2K6
  • | 3 Institute of Ocean Science, Sidney, B.C. Canada, V8L 4B2
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Abstract

The properties of first-mode annual-period baroclinic Rossby waves generated by the observed wind stress curl in a numerical model of the South Atlantic and Southwest Indian oceans are presented. The forcing wind field for the area 15°–51°S, 45°W–41°E was obtained from a harmonic analysis at the annual period of the monthly mean wind stress curl values derived from Hellerman and Rosenstein's data, and was used to drive a linear, reduced-gravity model of the South Atlantic and southwest Indian oceans bounded by 15° and 51°S, 46°W and 50°E.

In the South Atlantic Ocean, the response consists of long Rossby waves, which generally propagte their phase southwestward across the ocean and which exhibit refraction of wave energy towards the equator. Short Rossby waves with eastward energy propagation are generated in the small area of the Indian Ocean included in the model domain. Medium to short waves generated to the southeast of Africa 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. The most efficient wave generators are fund to be the wind stress curl maxima 1) off the Namibian coast near 25°S, 10°E; 2) near the Agulhas Plateau at 38°S, 25°E; and 3) in the South Atlantic Ocean interior near 38°S, 10°E.

Abstract

The properties of first-mode annual-period baroclinic Rossby waves generated by the observed wind stress curl in a numerical model of the South Atlantic and Southwest Indian oceans are presented. The forcing wind field for the area 15°–51°S, 45°W–41°E was obtained from a harmonic analysis at the annual period of the monthly mean wind stress curl values derived from Hellerman and Rosenstein's data, and was used to drive a linear, reduced-gravity model of the South Atlantic and southwest Indian oceans bounded by 15° and 51°S, 46°W and 50°E.

In the South Atlantic Ocean, the response consists of long Rossby waves, which generally propagte their phase southwestward across the ocean and which exhibit refraction of wave energy towards the equator. Short Rossby waves with eastward energy propagation are generated in the small area of the Indian Ocean included in the model domain. Medium to short waves generated to the southeast of Africa 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. The most efficient wave generators are fund to be the wind stress curl maxima 1) off the Namibian coast near 25°S, 10°E; 2) near the Agulhas Plateau at 38°S, 25°E; and 3) in the South Atlantic Ocean interior near 38°S, 10°E.

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