Decadal variability and forcing mechanisms of the tropical Indian Ocean wind-driven circulation
Abstract
The surface zonal current (U) anomalies in the equatorial Indian Ocean (IO) reveals a significant peak at a period of 10�15�years, underscoring the predominance of decadal variability. On decadal timescales, U anomalies exhibit a pronounced north�south dipole pattern, characterized by a strengthening (weakening) trend on the equator (north of the equator). The positive U trend is attributed to the anomalous westerlies on the equator, which induce downwelling Kelvin waves and deepen the thermocline depth over the south-eastern IO region causes the eastward flow over there. The leading two principal components (PCs) of the detrended decadal U anomalies explain 31% and 17% of its total variance, respectively. PC1 regressed upon the surface circulation, depicts a meridional tripole structure, with a weak westward current at the equator flanked by strong eastward flows on either side of the equator. The strong westerlies and cyclonic circulation anomalies in the south of the equator trigger these anomalous current patterns. In contrast, PC2 is dominated by a strong westward equatorial flow, driven by notable easterly wind anomalies. The regressions of PCs on the global decadal sea surface temperature and wind anomalies reveal that the Interdecadal Pacific Oscillation is a major forcing for the first mode on the other hand, the second mode of current variability is largely controlled by the IO equatorial wind forcing. These findings highlight the significant role of the Indo-Pacific climate drivers in shaping decadal circulation patterns of the IO.
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