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Interannual variability in the tropical western Indian Ocean

African Journal of Marine Science is co-published by NISC (Pty) Ltd and Taylor & Francis The tropical Indian Ocean experiences strong, seasonally reversing winds. Generally, strong south-westerly and north-easterly winds blow in the austral winter (June– September) and summer (December–March) from and to the tropical western Indian Ocean respectively. Thus, austral winter and summer are correspondingly termed the South-West Monsoon and the North-East Monsoon. The transitions occur in April–May (spring) and October– November (autumn) with weak surface winds that are dominated by a westerly component along the equator (Schott et al. 2009). The seasonally reversing winds in the tropical Indian Ocean influence the sea surface temperature (SST) and the upper ocean circulation. Weak winds in the transition periods correspond with significant SST warming, and these winds drive the eastward-flowing equatorial currents, termed ‘Wyrtki jets’. These jets are a combined response of direct forcing by semi-annual zonal equatorial winds (Wyrtki 1973), wave reflection (e.g. O’Brien and Hurlburt 1974) and basin resonance (Jensen 1993; Han et al. 1999). The strong winds during the South-West Monsoon lead to significant cooling in the tropical western Indian Ocean. The winds strengthen the East African Coastal Current (EACC; ~2 m s–1), which is supplied by the North-East Madagascar Current (NEMC) to the south, and the EACC supplies the northward-flowing Somali Current (SC) to the north (Newell 1957; Figure 1a). During the North-East Monsoon, the north-easterly winds force the SC to flow southwards and also weaken the EACC (~0.2 m s–1). This results in a confluence of the EACC and SC to form the eastward-flowing South Equatorial CounterCurrent (SECC) (Newell 1957; Figure 1b). The reversing monsoon winds vary each year, resulting in considerable interannual variability (Webster et al. 1999). Such variability has been associated with the El Niño–Southern Oscillation (ENSO) (Reason et al. 2000; Annamalai and Murtugudde 2004; Schott et al. 2009) and the Indian Ocean Dipole (IOD) (Behera et al. 2000; Yamagata et al. 2004). Given that the tropical Indian Ocean forms the largest warm pool on Earth, which shapes both regional and global climate variability (Schott et al. 2009), any interannual change in its SST can lead to large-scale ocean–atmosphere variability. Thus, it is essential to gain a better understanding of SST variability in the tropical Indian Ocean. The influences of the IOD and ENSO on the tropical Indian Ocean are phase-locked annually. An IOD event begins in late austral autumn/early winter, peaks during September–November, and most of the SST anomalies disappear by January of the following year (Vinayachandran et al. 2009). The SST anomalies in the tropical Indian Ocean basin develop slowly during an El Niño event, peaking in the Interannual variability in the tropical western Indian Ocean