[Answered]What is the Indian Ocean Dipole? What are its characteristic features? Discuss its impact on Indian monsoon.

Indian Ocean Dipole (IOD) is an atmosphere-ocean coupled phenomenon in the Indian Ocean, characterised by a difference in sea-surface temperatures. IOD is the difference between the temperature of eastern (Bay of Bengal) and the western Indian Ocean (Arabian Sea). Indian monsoon depends upon not only El Nino La Nina but also IOD and other such ocean phenomena.

Characteristics of IOD:

1. Temperature difference: IOD occurs because of temperature difference. This temperature difference results into pressure difference which results in flowing of winds between eastern and western parts of Indian Ocean.
2. Development: IOD develops in the equatorial region of Indian Ocean from April to May, peaking in October.
3. ThreePhases: The IOD has three phases such as Neutral, Positive and Negative IOD.

a.Neutral Phase of IOD: During this 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 equato

b.Positive Phase of IOD: During this phase the Westerly winds weaken along the equator, allowing warm water to move to Africa. Changes in the winds also allow a rise of cool water from the deep ocean in the east. This creates a temperature difference across the tropical Indian Ocean with cooler than normal water in the east and warmer than normal water in the west. This event has been found to be beneficial for the monsoon.

c.Negative Phase of IOD: During this phase westerly winds intensify along the equator, allowing the concentration of warmer waters near Australia. This creates a temperature difference across the tropical Indian Ocean, with warmer than normal water in the east and cooler than normal water in the west. This event obstructs the progression of monsoon over India.

How does it impact the southwest monsoon?

1. Increased rainfall due to positive IOD: There is no established correlation between Indian summer monsoon rainfall and IOD. But, studies have shown that a positive IOD year sees more than normal rainfall over central India. It was demonstrated that a positive IOD index often negated the effect of El Nino Southern Oscillation (ENSO), resulting in increased Monsoon rains in several ENSO years.
2. Droughts Due to negative IOD: A negative IOD, on the other hand, complements El NiNo leading to severe drought.
3. Cyclones: At the same time, positive IOD results in more cyclones than usual in Arabian Sea. Negative IOD results in stronger than usual cyclogenesis (Formation of Tropical Cyclones) in the Bay of Bengal. Cyclogenesis in Arabian Sea is suppressed during this time.

Thus, an IOD can either aggravate or weaken the impact of El Nino on Indian monsoon. If there is a positive IOD, it can bring good rains to India despite an El Nino year. For example, positive IOD had facilitated normal or excess rainfall over India in 1983, 1994 and 1997 despite an El Nino in those years. Similarly, during years such as 1992, a negative IOD and El Nino had cooperatively produced deficient rainfall.