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Earth System Dynamics An interactive open-access journal of the European Geosciences Union
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 18 Apr 2019

Research article | 18 Apr 2019

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This discussion paper is a preprint. A revision of the manuscript is under review for the journal Earth System Dynamics (ESD).

Influence of position and strength of westerlies and trades on Agulhas leakage and South Benguela Upwelling

Nele Tim1,2, Eduardo Zorita1, Kay-Christian Emeis2, Franziska U. Schwarzkopf3, Arne Biastoch3,4, and Birgit Hünicke1 Nele Tim et al.
  • 1Helmholtz-Zentrum Geesthacht, Institute of Coastal Research, Geesthacht, Germany
  • 2Universität Hamburg, Institute for Geology, Hamburg, Germany
  • 3GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
  • 4Kiel University, Christian-Albrechts-Platz 4, 24118 Kiel, Germany

Abstract. The westerlies and trade winds over the South Atlantic and Indian Ocean are important drivers of the regional oceanography around Southern Africa, including features such as the Agulhas current, the Agulhas leakage and the Benguela upwelling. The Agulhas leakage is the transport of warm and saline water from the Indian Ocean into the South Atlantic. The leakage is stronger during intensified westerlies and probably also when the wind systems are shifted poleward. Here we analyzed the wind stress of different observational and modelled atmospheric data sets (covering the last two millennia, the recent decades and the 21st century) with regard to the intensity and position of the south-easterly trades and the westerlies. The analysis reveals that variations of both wind systems go hand in hand. A poleward shift and intensification of westerlies and trades took place during the recent decades. Furthermore, the upwelling in South Benguela slightly intensified and the characteristics of the water masses fed into the upwelling region changed with a poleward shift of the trades. Projections for strength and position of the westerlies in the 21st century depend on assumed CO2 emissions. In the strongest emission scenario a further southward displacement will occur, whereas a northward shift is modelled in the weakest emission scenario, possibly due to the dominating driving effect of ozone recovery. Thus, the Agulhas leakage has intensified during the last decades and is projected to increase if greenhouse gas emission are not reduced. This will have a small impact on Benguela upwelling strength, but will have consequences for water mass characteristics in the upwelling region. An increased contribution of Agulhas water to the upwelling feed water masses will import more preformed nutrients and oxygen into the upwelling region.

Nele Tim et al.
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Nele Tim et al.
Nele Tim et al.
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Short summary
Our study reveals that the latitudinal position and the intensity of southern hemisphere trades and westerlies is correlated. In the last decades westerlies have shifted poleward and have intensified. Furthermore, the latitudinal shifts and intensity of the trades and westerlies impacts the sea surface temperatures around Southern Africa and in the Benguela upwelling region. The future development of the wind stress depends on the strength of greenhouse gas forcing.
Our study reveals that the latitudinal position and the intensity of southern hemisphere trades...