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indian ocean dipole

The Indian Ocean Dipole (IOD) is a coupled ocean-atmosphere climate mode centered in the tropical Indian Ocean. Similar to ENSO, the state of the IOD affects convection over Indonesia and thus regional precipitation. However, there is still scientific debate about whether the IOD actually exists, and if so, whether it is independent of ENSO or not.

The Dipole Mode Index is defined as the SST anomaly difference between the eastern (eastern Africa coast) and the western tropical Indian Ocean (western Sumatra coast). The IOD in its positive (or "anomalous") mode is characterized by cooling of surface waters in the southeastern equatorial Indian Ocean and warming in the western equatorial Indian Ocean. The changes in SST during IOD events are found to be associated with changes in the surface wind field of the central equatorial Indian Ocean. In fact, the winds reverse from teh usual westerly to an easterly direction during positive IOD events (Saji et al., 1999; Behera and Yamagata, 2001). Furthermore, the atmospheric convection that is normally situated over the eastern Indian Ocean warm pool shifts to the west. This configuration results in heavy rain in eastern Africa and leaves the Indonesian region with little rain, resulting in droughts and forest fires. Fossil corals from off Sumatra recorded IOD events for several times in the in the Holocene (Abram et al., 2003).


References:

Behera, S.K. and T. Yamagata (2001), Impact of the Indian Ocean Dipole on the Southern Oscillation, Journal of Climate (submitted).

Saji, N. H., B. N. Goswami, P. N. Vinayachandran and T. Yamagata (1999), A dipole mode in the tropical Indian Ocean, Nature, 401, 360-363.


Fig.1.: Sequence of events over the 1997-98 IOD anomaly. Top: Easterly winds resulting from the anomalous SST gradient along the equator with cooler SST in the eastern and warmer SST in the western Indian Ocean. Middle: Formation of an Ekman ridge in the central Indian Ocean and the forcing of westward-propagating downwelling equatorial Rossby waves. Bottom: Return to normal conditions by cooling of the western Indian Ocean through enhanced mixing and coastal Ekman transports from stronger than average monsoon winds and through circulation changes associated with the weakening of the 1997/98 El Nino. Figure after Webster et al. (1999).