Indian Ocean Dipole

Mar 23, 2011; 12:32 PM ET

OCEANIC INDEX WEIGHS UPON WEATHER FROM SOUTH ASIA TO AUSTRALIA

I have cranked out a lot of words in this column, yet I have had little to say about the oceanic indices.

Foremost of these is the well-known ENSO (El Nino and its sibling, La Nina). Others deal with the Atlantic, Pacific, Arctic and even Antarctic waters.

There is also the Indian Ocean Dipole (IOD), which is a measure of the east-west heat distribution across this ocean. The state of temperature distribution across the Indian Ocean has heavy bearing upon patterns of rainfall across the ocean basin and its borderlands, which happen to be dominated by monsoons (seasonal winds and resulting rainfall patterns).

I am only beginning to acquaint myself with this (and other) oceanic indices. There seem to be a number of websites to draw upon, such as the Australian Bureau of Meteorology (BOM).

The BOM site (drawing from a work by Saji et al. 1999), calls the IOD “a coupled ocean and atmosphere phenomenon in the equatorial Indian Ocean that affects climate of Australia and other countries” of the ocean basin.

The actual index, the Dipole Mode Index (DMI), is the difference between sea surface temperatures of specific swatches of the western and eastern equatorial Indian Ocean.

An example of a positive Indian Ocean Dipole in 1997 (Australian BOM).

The IOD is positive when the western Indian Ocean is warmer than usual, concurrent with below-normal temperatures in the eastern Indian Ocean.

Conversely, the IOD is negative when the western Indian Ocean has below-normal temperatures concurrent with an eastern Indian Ocean that is warmer than normal.

IOD AND THE SOUTH WEST MONSOON

The BOM details some of the weather/climate impact upon Australia’s weather in light of the IOD. If the IOD is important for weather and climate in this Southern continent, how much more so for that of South Asia?

The all-important South West Monsoon of South Asia is driven, at least in part, by heat energy entering the atmosphere from the Indian Ocean. Therefore, it should follow that the behavior of the South West Monsoon, including distribution and amount of rainfall, would hinge upon the temperature state of the Indian Ocean (the IOD).

Basically, my understanding is that a positive IOD promotes a well-developed South West Monsoon. Conversely, the negative IOD tends to hinder the SW Monsoon.

I understand that the state of the IOD is one key input to the SW Monsoon rainfall outlook crafted yearly by the India Meteorology Department (IMD).