Indian Ocean Dipole

Notes

The Indian Ocean Dipole (IOD), also called the Indian Nino, is an irregular oscillation of sea-surface temperatures between the western and eastern tropical Indian Ocean. It was identified in 1999 by N.H. Saji and colleagues.

Measurement: the Dipole Mode Index (DMI) is the difference between the SST anomaly in the western (50-70°E, 10°S-10°N) and eastern (90-110°E, 10°S-equator) tropical Indian Ocean.

Phases

Positive IOD: the western Indian Ocean (near Africa) is warmer, and the eastern (near Indonesia) is cooler; easterly wind anomalies along the equator. Effects: more rainfall in India (a good monsoon, especially in the late season) and East Africa (floods); drought in Indonesia and Australia; bushfires in Australia.
Negative IOD: the reverse; the eastern ocean is warmer. Effects: weaker monsoon rainfall in India and wetter conditions in Australia.
Neutral.

Timing: it develops in May-June, peaks in September-October, and decays in winter.

Interaction with ENSO

A positive IOD often coincides with El Nino and can counter El Nino's drying effect on the Indian monsoon (1997, 2019).
A negative IOD with El Nino can worsen a monsoon deficit.

Related: the Equatorial Indian Ocean Oscillation (EQUINOO) is the atmospheric counterpart. The Madden-Julian Oscillation (MJO) is a 30-60 day intraseasonal oscillation affecting the active and break phases of the monsoon. India's IITM (Pune) and IMD monitor these.

Key Points

IOD: SST difference between the west and east Indian Ocean; discovered 1999 (Saji).
Positive IOD: warm west; good Indian monsoon.
Negative IOD: weak monsoon.
Peaks in September-October.
A positive IOD can offset El Nino.

MCQs

Q1. In a positive Indian Ocean Dipole, the:

(a) Eastern Indian Ocean is warmer
(b) Western Indian Ocean is warmer
(c) Pacific is cooler
(d) Arabian Sea is frozen
Ans: (b).

Q2. A positive IOD typically results in what for the Indian monsoon?

(a) Good rainfall
(b) Drought
(c) Delayed monsoon always
(d) No change
Ans:
(a).

Q3. The IOD was first identified in:

(a) 1950
(b) 1975
(c) 1999
(d) 2010
Ans: (c).