Indian Ocean Dipole

Definition

An ocean-atmosphere oscillation in the tropical Indian Ocean, sometimes called the "Indian Nino," defined by the difference in sea-surface temperature between its western (Arabian Sea) and eastern (near Indonesia) parts.

Key points

• A positive IOD has warmer-than-normal water in the western Indian Ocean and cooler water in the east (near Sumatra); a negative IOD is the reverse.
• A positive IOD generally strengthens the Indian south-west monsoon and can offset the drying effect of an El Nino; a negative IOD tends to weaken the monsoon.
• It is one of the three big drivers of Indian monsoon variability, alongside the El Nino-Southern Oscillation (ENSO) in the Pacific and local factors.
• It was identified by Indian and other scientists in the late 1990s; its phases are tracked through the Dipole Mode Index.
• Because a positive IOD can counter an El Nino, the two together better explain a given year's monsoon than either alone.

Why it matters for CAPF

The positive (warm west) versus negative (warm east) definition, the link to monsoon strength, and the interplay with El Nino are climatology and current-affairs friendly facts; verify the latest year's IOD phase.

Common confusion

Positive IOD (warm western Indian Ocean, helps the monsoon) versus negative IOD (warm east, weakens it); the IOD is in the Indian Ocean, while El Nino and La Nina are in the Pacific; a positive IOD can offset an El Nino.

Parent note

oceanography