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Why Were Sea Surface Temperatures so Different in the Eastern Equatorial Atlantic in June 2005 and 2006?

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  • 1 Université de Toulouse, UPS (OMP–PCA), LEGOS, Toulouse, France
  • | 2 IRD, LEGOS, Toulouse, France
  • | 3 Centre National de Recherches Météorologiques/GAME (Météo-France, CNRS) Toulouse, France
  • | 4 Université de Toulouse, UPS (OMP–PCA), LEGOS, Toulouse, France
  • | 5 IRD, LEGOS, Cotonou, Benin
  • | 6 IRD—Unité de Service 191, Centre IRD de Bretagne, Brest, France
  • | 7 Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida
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Abstract

A comparison of June 2005 and June 2006 sea surface temperatures in the eastern equatorial Atlantic exhibits large variability in the properties of the equatorial cold tongue, with far colder temperatures in 2005 than in 2006. This difference is found to result mainly from a time shift in the development of the cold tongue between the two years. Easterlies were observed to be stronger in the western tropical Atlantic in April–May 2005 than in April–May 2006, and these winds favorably preconditioned oceanic subsurface conditions in the eastern Atlantic. However, it is also shown that a stronger than usual intraseasonal intensification of the southeastern trades was responsible for the rapid and early intense cooling of the sea surface temperatures in mid-May 2005 over a broad region extending from 20°W to the African coast and from 6°S to the equator. This particular event underscores the ability of local intraseasonal wind stress variability in the Gulf of Guinea to initiate the cold tongue season and thus to dramatically impact the SST in the eastern equatorial Atlantic. Such intraseasonal wind intensifications are of potential importance for year-to-year variability in the onset of the African monsoon.

Corresponding author address: Frédéric Marin, IRD, LEGOS, 14 Ave. Edouard Belin, Toulouse F-31400, France. Email: frederic.marin@ird.fr

Abstract

A comparison of June 2005 and June 2006 sea surface temperatures in the eastern equatorial Atlantic exhibits large variability in the properties of the equatorial cold tongue, with far colder temperatures in 2005 than in 2006. This difference is found to result mainly from a time shift in the development of the cold tongue between the two years. Easterlies were observed to be stronger in the western tropical Atlantic in April–May 2005 than in April–May 2006, and these winds favorably preconditioned oceanic subsurface conditions in the eastern Atlantic. However, it is also shown that a stronger than usual intraseasonal intensification of the southeastern trades was responsible for the rapid and early intense cooling of the sea surface temperatures in mid-May 2005 over a broad region extending from 20°W to the African coast and from 6°S to the equator. This particular event underscores the ability of local intraseasonal wind stress variability in the Gulf of Guinea to initiate the cold tongue season and thus to dramatically impact the SST in the eastern equatorial Atlantic. Such intraseasonal wind intensifications are of potential importance for year-to-year variability in the onset of the African monsoon.

Corresponding author address: Frédéric Marin, IRD, LEGOS, 14 Ave. Edouard Belin, Toulouse F-31400, France. Email: frederic.marin@ird.fr

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