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The sensitivity of the Seychelles–Chagos thermocline ridge to large-scale wind anomalies

Juliet C. Hermes and Chris J. C. Reason Hermes, J. C., and Reason, C. J. C. 2009. The sensitivity of the Seychelles–Chagos thermocline ridge to large-scale wind anomalies. – ICES Journal of Marine Science, 66: 000–000.


The Seychelles–Chagos thermocline ridge (SCTR) in the southwest tropical Indian Ocean is important for regional climate, the Madden–Julian Oscillation, as well as upper-ocean nutrients and related phytoplankton and zooplankton densities. Subsurface variability in this region has been proved to influence the overlying sea surface temperatures, which in turn can influence eastern African rainfall. There is evidence that austral summers with a deeper (shallower) SCTR tend to have more (less) tropical cyclone (TC) days in the Southwest Indian Ocean. The importance of this relationship was underlined during the 2006/2007 austral summer, when areas of Madagascar and central Mozambique experienced devastating floods, because of ten named tropical storms, including several intense TCs, effecting on these areas. At the same time, the SCTR during this season was anomalously deep, partly because of a downwelling Rossby wave that propagated across the South Indian Ocean during the previous austral winter/spring. In this paper, a regional ocean model is used to investigate the effect of remote forcing on this region and to study the sensitivity of the SCTR to changes in the large scale winds over the South Indian Ocean, with a particular focus on the events of the 2006/2007 austral summer.

Keywords: Rossby waves, Seychelles–Chagos thermocline ridge, southwest tropical Indian Ocean, upwelling.
Received 15 August 2008; accepted 18 February 2009.
J. C. Hermes: South African Environmental Observation Network, Private Bag X2, Roggebaai 8012, South Africa.
Chris J. C. Reason: Department of Oceanography, University of Cape Town, Rondebosch, Cape Town, South Africa.
Correspondence to J. C. Hermes: tel: +27 21 402 3547; fax: +27 21 402 3674; e-mail: juliet@saeon.ac.za.

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