Mentors Comment:

The questions comes from the topic Salient Features of World’s Physical Geography. Its straight, to the point and nothing hidden in the question.

The first part of the question will address the concept and characteristics of IOD.

Then jump to the next requirement of the question where you have to discuss the role of IOD in Indian monsoon.

Here discuss the role of + IOD on normal rainfall, – IOD on drought like situation. Dont miss the relationship between IOD and El-Nino and how it affect each other.

One of the best way to score in these types of questions is to draw a diagram to show the phenomenon. It will be relatable to the examiner and you will score easy marks!

Model Answer:

Indian Ocean Dipole (IOD) is an atmosphere-ocean coupled phenomenon in the tropical Indian Ocean (like the El Nino is in the tropical Pacific), characterised by a difference in sea-surface temperatures.

Characteristics of IOD:

• IOD is the difference between the temperature of eastern (Bay of Bengal) and the western Indian Ocean (Arabian Sea).
• This temperature difference results into pressure difference which results in flowing of winds between eastern and western parts of Indian Ocean.
• IOD develops in the equatorial region of Indian Ocean from April to May peaking in October.
• A ‘positive IOD’ — or simply ‘IOD’ — is associated with cooler than normal sea-surface temperatures in the eastern equatorial Indian Ocean and warmer than normal sea-surface temperatures in the western tropical Indian Ocean.
• The opposite phenomenon is called a ‘negative IOD’, and is characterised by warmer than normal SSTs in the eastern equatorial Indian Ocean and cooler than normal SSTs in the western tropical Indian Ocean

How does it impact the southwest monsoon?

• Although El Nino Southern Oscillation [ENSO] was statistically effective in explaining several past droughts in India, in the recent decades the ENSO-Monsoon relationship seemed to weaken in the Indian subcontinent.
• For e.g. the 1997, strong ENSO failed to cause drought in India.
• It was later discovered that just like ENSO was an event in the Pacific Ocean, a similar seesaw ocean-atmosphere system in the Indian Ocean was also at play which was IOD.
• However, 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 ENSO, resulting in increased Monsoon rains in several ENSO years like the 1983, 1994 and 1997.
• Further, it was shown that the two poles of the IOD – the eastern pole (around Indonesia) and the western pole (off the African coast) were independently and cumulatively affecting the quantity of rains for the Monsoon in the Indian subcontinent.
• The indicated connection is between below-normal SST in the eastern Indian Ocean and above-normal rain over central India, and vice versa.
• A negative IOD, on the other hands, complements El NiNo leading to severe drought.
• 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 Bay of Bengal. Cyclogenesis in Arabian Sea is suppressed during this time.
• But there is some anomaly in the phenomenon of IOD and Monsoon.

Conclusion:

The cumulative effect of these factors along with ITCZ, absence of  Sub Tropical Westerly Jet (STWJ) is that a low pressure is created over the Indian subcontinent while a high pressure is formed in the Indian Ocean. This pushes the moist, rain bearing monsoon winds from the Arabian sea towards the subcontinent. The blockage of these winds by Western Ghats causes rains in the southern region while that by Himalayas leads to downpours over North India.