Indian Ocean Dipole (IOD)



UPSC 2017

QUES –  With reference to ‘Indian Ocean Dipole (IOD)’ sometimes mentioned in the news while forecasting Indian monsoon, which of the following statements is/are correct?
1. IOD phenomenon is characterized by a difference in sea surface temperature between tropical Western Indian Ocean and tropical Eastern Pacific Ocean.
2. An IOD phenomenon can influence an El Nino’s impact on the monsoon.
Select the correct answer using the code given below:
(a) 1 only
(b) 2 only
(c) Both 1 and 2
(d) Neither 1 nor 2

Answer (b)

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. 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.

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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.

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Impact of IOD on southwest monsoon

A positive IOD leads to greater monsoon rainfall and more active (above normal rainfall) monsoon days while negative IOD leads to less rainfall and more monsoon break days (no rainfall).

Types of IOD

There are three types of IOD: normal IOD, early IOD and prolonged IOD. An early IOD, which peaks in the mid-monsoon months (July and August), plays a significant role in enhancing monsoon rainfall even though its intensity is medium compared to other IODs. The normal IOD and prolonged IOD peak in September, October, November and hence play a lesser role in the monsoon rainfall.

Why early IOD enhances monsoon rainfall?

During an early IOD, the combined effect of Arabian sea evaporation and stronger cross equatorial flow — winds blowing from the southern tropics to the north across the equator — play an important role in enhancing the monsoon activity over the Indian subcontinent. Also, there are fewer breaks in monsoon conditions during early IOD events.

Combined effect of El Nino and positive IOD

The effect of El Nino in general is to decrease the monsoon rainfall over the Indian subcontinent. The IOD on the other hand increases the monsoon activity over India.

During weak El Nino years (1994 and 2006) there were no break events in the peak monsoon months of July and August (early IOD), whereas in the case of moderate (1987), strong (1972) and very strong El Nino years (1982, 1997) more break events were observed during the mid-monsoon months

Impact of IOD on different parts of the world

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