El Niño and Maritime Continent Rainfall

El Niño is the warm phase of the El Niño-Southern Oscillation, marked by unusually warm surface waters in the central and eastern equatorial Pacific and associated changes in atmospheric circulation. It often suppresses rainfall over the Maritime Continent, the island-and-sea region of Southeast Asia centred on Indonesia, by shifting tropical convection eastward and favouring descending air over the region.

The normal rainfall-producing circulation

Under normal conditions, the tropical western Pacific and adjoining seas form a warm region that supports strong evaporation, moist rising air and frequent convective rainfall.

• The easterly trade winds help maintain warm surface water in the western equatorial Pacific, while colder water rises nearer the eastern Pacific.
• In the Walker circulation, air generally rises over the warm western Pacific and Maritime Continent, flows eastward aloft, descends over the cooler eastern Pacific and returns westward near the surface.
• As moist air rises and cools, water vapour condenses into clouds, producing much of the region's tropical rainfall.

How El Niño suppresses rainfall

During El Niño, ocean warming and atmospheric changes reinforce one another. The usual east-west contrast in Pacific sea-surface temperature weakens, and the main zone of tropical convection shifts towards the central and eastern equatorial Pacific.

• Weaker trade winds allow warm surface water to spread eastward and reduce the normal supply of cold water to the eastern equatorial Pacific surface.
• The Walker circulation weakens and shifts, reducing its normal rising branch over Indonesia and neighbouring areas.
• Anomalous subsidence, or downward air movement, develops over much of the Maritime Continent. Descending air warms through compression, lowering relative humidity and inhibiting deep cloud formation.
• Reduced low-level convergence and moisture supply further weaken convection, so rainfall can decline even though the surrounding tropical seas remain warm.

Why the rainfall response varies

El Niño raises the likelihood of below-normal rainfall, but it does not produce uniform drought across every island or season.

• The rainfall response depends on the strength and location of Pacific warming, the seasonal monsoon, local topography and conditions in the Indian Ocean.
• Rainfall deficits can intensify seasonal dryness, reduce water availability and increase the susceptibility of forests and peatlands to fire; El Niño itself is not an ignition source.