Indian Ocean circulation and climate variability

Abstract

[1] In recent years, the Indian Ocean (IO) has been discovered to have a much larger impact on climate variability than previously thought. This paper reviews climate phenomena and processes in which the IO is, or appears to be, actively involved. We begin with an update of the IO mean circulation and monsoon system. It is followed by reviews of ocean/atmosphere phenomenon at intraseasonal, interannual, and longer time scales. Much of our review addresses the two important types of interannual variability in the IO, El Niño -Southern Oscillation (ENSO) and the recently identified Indian Ocean Dipole (IOD). IOD events are often triggered by ENSO but can also occur independently, subject to eastern tropical preconditioning. Over the past decades, IO sea surface temperatures and heat content have been increasing, and model studies suggest significant roles of decadal trends in both the Walker circulation and the Southern Annular Mode. Prediction of IO climate variability is still at the experimental stage, with varied success. Essential requirements for better predictions are improved models and enhanced observations.

North Atlantic forcing of tropical Indian Ocean climate

“…2). During summer, about 6 Sv of surface water flows southwards with the cross-equatorial meridional Ekman transport (MET), involving the export of low-salinity waters from the Bay of Bengal [35][36][37] . During boreal winter, the direction of currents in the tropical eastern Indian Ocean is reversed (Extended Data Fig.…”

“…2). Westwards flowing surface waters join the northeast monsoon current south of Sri Lanka and are affected by the northwards-directed MET 35,37 . Despite the seasonality in the currents, it seems that SST and SSS off northwestern Sumatra are not considerably affected by seasonally reversing surface current direction in the eastern Indian Ocean 37 Fig.…”

Coherent Tropical Indo-Pacific Interannual Climate Variability

“…For SDIex in April, this correlation reaches values of 0.83, while in August-October the correlation to Niño-3.4 at short lags is still up to 0.5. Mature El Niños are known to be associated with anticyclonic winds in the southeastern tropical IO, causing downwelling Rossby waves that arrive in the southwestern IO several months later, lower the thermocline, and hence increase SST (Xie et al 2002;Schott et al 2009). …”

“…Since much research has been done on the influence of ENSO on the SD (Xie et al 2002; section 4.1 of Schott et al 2009), we will focus here on influences of the SD onto ENSO. As we have seen in section 4a, for all significant MSSA modes during and after a La Niña or cool SD event, zonal winds converge over Indonesia, while later on, westerly winds propagate from the IO to the PO, hinting at an atmospheric bridge mechanism (section 5a).…”

The Effect of the Australian-Maritime Continents on the Indian Ocean Dipole Mode in an Idealized Coupled General Circulation Model

“…The climate of the tropical Indian Ocean shows variability on various timescales (Schott et al 2009), but one of the dominant features is the seasonal monsoon variability, which follows a distinct annual cycle over the northern Indian Ocean accompanied by southwesterly (northeasterly) winds in the boreal summer (winter). On interannual timescales, the Indian Ocean Dipole (IOD) mode, during which sea surface temperature (SST) is cooler (warmer) than the seasonal climatology in the eastern (western) equatorial Indian Ocean, is an important phenomenon (Saji et al 1999;Webster et al 1999;Ueda and Matsumoto 2000).…”