About the Indian Ocean Dipole

The Indian Ocean Dipole (IOD) is a coupled ocean and atmosphere phenomenon in the equatorial Indian Ocean that affects the climate of Australia and other countries that surround the Indian Ocean basin (Saji et al. 1999).

The IOD is commonly measured by an index that is the difference between sea surface temperature (SST) anomalies in the western (50°E to 70°E and 10°S to 10°N) and eastern (90°E to 110°E and 10°S to 0°S) equatorial Indian Ocean. The index is called the Dipole Mode Index (DMI). The map below shows the east and west poles of the IOD for November 1997; a positive IOD year.

A positive IOD period is characterised by cooler than normal water in the tropical eastern Indian Ocean and warmer than normal water in the tropical western Indian Ocean (see map below for an example of a typical positive IOD SST pattern). A positive IOD SST pattern has been shown to be associated with a decrease in rainfall over parts of central and southern Australia. For more information about Australian rainfall patterns during positive IOD years click here.

Conversely, a negative IOD period is characterised by warmer than normal water in the tropical eastern Indian Ocean and cooler than normal water in the tropical western Indian Ocean. A negative IOD SST pattern has been shown to be associated with an increase in rainfall over parts of southern Australia. For more information about Australian rainfall patterns during negative IOD years click here.

Map of Indian Ocean
Figure 1: Map shows departures from average ocean surface temperatures in November 1997 at the height of the 1997 positive IOD event. The east and west poles of the IOD are marked with black boxes.

References

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