IOD Area MAP from BOM -Australia
Indian Ocean Dipole
Sustained changes in the difference between sea surface temperatures of the tropical western and eastern Indian Ocean are known as the Indian Ocean Dipole or IOD. The IOD is one of the drivers of Indian Monsoon. The IOD has three phases: neutral, positive and negative.
Neutral IOD phase
Water from the Pacific flows over to East Indian Ocean ( between the islands of Indonesia). Air rises above this area and falls over the western half of the Indian Ocean basin, blowing westerly winds along the equator. The SST difference between +0.4ºC to -0.4ºC is considered as Neutral IOD phase.
Temperatures are close to normal across the tropical Indian Ocean, and hence the neutral IOD does not affect the Indian Southwest Monsoon.
Positive IOD phase
Westerly winds weaken along the equator allowing warm water to shift towards Africa. Changes in the winds also allow cool water to rise up from the deep ocean in the east. This sets up a temperature difference across the tropical Indian Ocean with cooler than normal water in the east and warmer than normal water in the west. The SST difference of +0.4ºC or higher is considered as Positive IOD phase.
Generally this means there is more moisture than normal in the atmosphere over West Indian Ocean & Arabian Sea. This changes the path of weather systems coming towards India, often resulting in more rainfall during Southwest Monsoon. Positive IOD also is thought to mitigate the adverse effects of El Nino over India.
Negative IOD phase
Westerly winds intensify along the equator, allowing warmer waters to concentrate near Equatorial East Indian Ocean. This sets up a temperature difference across the tropical Indian Ocean, with warmer than normal water in the east and cooler than normal water in the west. The SST difference of -0.4ºC or lower is considered as Negative IOD phase.
A negative IOD typically adversely affects the Indian Southwest Monsoon rainfall resulting in below-average rainfall over India.