Giant Battle Between Oceans Is On, Monsoon Seems Vulnerable

Hyderabad: A gargantuan ongoing tug of war between two of the largest oceans, the Pacific and the Indian, could well determine the fate of the monsoon season this year.

At one end are predictions of formation of a strong ‘Little Boy’ El Nino, the monsoon spoilsport that visits the Pacific Ocean once every few years and first recorded in the 1600s, and at the other, the Indian Ocean Dipole (IOD), first taken note of in 1999 and is a result of an interplay of sea surface temperature differences between the Eastern Indian Ocean in the Bay of Bengal, or the Western Indian Ocean, better known as the Arabian Sea.

“At present, the IOD conditions are neutral. And a close watch is being kept on how things will shape up,” according to Dr. Akasapu Dharma Raju, a senior scientist at the India Meteorological Department, Hyderabad.

El Nino, the warming of a large swathe of water in the central Pacific Ocean, results in changes in wind patterns and drags moisture away from the Indian landmass. On the other hand, the IOD, which results in Arabian Sea waters becoming warmer than the sea in the Bay of Bengal, helps push moisture towards India, acting as a counterbalancing force against the El Nino influence.

Once referred to as the ‘new kid on the block’ by the National Oceanic and Atmospheric Administration of the US owing to the relatively recent understanding of the phenomenon, the IOD, when in its positive phase, alters temperature, wind and rainfall patters in the region.

“A negative IOD is typically associated with El Nino years, while a positive phase is associated with the La Nina years when the conditions are right for good monsoon seasons. But this year, at present, the IOD is in a neutral phase, and it is quite possible that it will evolve into a positive phase. If this occurs, then it will help in the monsoon overcoming some of the negative impacts of El Nino,” Dr Dharma Raju said.

Explaining how the IOD works, Dr Dharma Raju explained that all it takes is a difference of .5 to 2 degrees Celsius in Indian Ocean’s temperatures on the two sides of the Indian peninsula. The waters in the Bay are warmer during negative IOD years, but the Arabian Sea gets warmer than the Bay during positive IOD years. What this means is because the Arabian sea gets warmer, there is more moisture in the air, and as the landmass is much hotter in the summer months, these water laden winds get drawn over the land resulting in decent to even good monsoons. This is like the Indian Ocean having its own little La Nina going on during positive IOD years,” he said.

While the IMD as well as other weather forecasters, have said the long-term outlook for the monsoon appears to be one of concern, the IOD phenomenon could just be the silver lining on the clouds. The ocean temperatures in the Bay and Arabian Sea are monitored by the Indian National Centre for Ocean Information Services (Incois), based in Hyderabad, and data is streamed continuously and a close watch is maintained on the IOD. “Even ships passing through the Bay and the Arabian Sea come in handy as they too add to the data gathering. We will likely have a better grasp of how the IOD is shaping up in the weeks to come, Dr Dharma Raju said.

What is Indian Ocean Dipole

Difference in sea surface temperatures between the Eastern Indian Ocean in Bay of Bengal, and Western Indian Ocean (Arabian Sea)

An IOD event usually starts around May or June, intensifies in the following months and peaks between August and October and then rapidly decays

Also known as the Indian Niño, IOD is an irregular sea surface temperature oscillation in which the western Indian Ocean alternately becomes warmer and colder than the eastern part of the ocean

IOD now in neutral phase - Water flows from the Pacific between Indonesia’s islands, keeping seas warm to the northwest of Australia. Air rises above this area and falls across the western half of the Indian Ocean basin, blowing westerly winds along the equator.

When IOD is positive, Westerly winds weaken along the Equator, allowing warm water to move to Africa. Changing winds result in rise of cool water from the deep ocean in the east resulting in a temperature difference leading to warmer-than-normal water in the west.