El Nino-Southern Oscillation

El Nino-Southern Oscillation (ENSO)

The El Nino-Southern Oscillation is a coupled ocean-atmosphere phenomenon centered in the tropical Pacific Ocean. It is the dominant mode of interannual climate variability on Earth, with a quasi-periodic cycle of 2-7 years.

Components

ENSO consists of two coupled components:

1. El Nino / La Nina (oceanic): Anomalous warming (El Nino) or cooling (La Nina) of sea surface temperatures in the central and eastern equatorial Pacific. The canonical El Nino pattern involves SST anomalies exceeding +0.5 C in the Nino 3.4 region (5 N - 5 S, 170 W - 120 W).

2. Southern Oscillation (atmospheric): A seesaw in sea-level pressure between the western Pacific (Darwin, Australia) and the eastern Pacific (Tahiti). The Southern Oscillation Index (SOI) quantifies this pressure difference.

During El Nino, the Walker circulation weakens, trade winds slacken, and the warm pool extends eastward. During La Nina, trade winds strengthen and the cold tongue intensifies.

ENSO and the Galapagos

The Galapagos Archipelago sits at the eastern edge of the equatorial Pacific and is profoundly affected by ENSO:

• El Nino: SSTs around the islands can rise by 3-5 C. The marine inversion weakens or disappears, suppressing the Garua season. Convective rainfall increases dramatically, causing flooding and vegetation blooms in normally arid lowlands. The Pacific Equatorial Undercurrent weakens, reducing nutrient upwelling.

• La Nina: SSTs drop below normal as upwelling intensifies. The Garua season strengthens, with more persistent fog and stratocumulus. Lowland drought conditions intensify.

Schneider et al. (2025, Geophys. Res. Lett.) demonstrated that local SST variability, modulated by ENSO, directly controls the occurrence of heavy rainfall events in the archipelago. The DARWIN project climate analysis (Schmidt et al., 2025, Int. J. Climatol.) captured these ENSO-driven precipitation modes through Dynamical downscaling at meso-scale resolution.

Teleconnections

ENSO drives global climate anomalies through atmospheric teleconnections:

• Drought in Australia and Southeast Asia during El Nino
• Enhanced rainfall in western South America
• Weakened Indian monsoon
• Modified jet stream patterns affecting North American and European winter weather
• Altered tropical cyclone activity in the Pacific and Atlantic basins

Monitoring

ENSO is monitored through:

• Nino region SST indices (Nino 1+2, 3, 3.4, 4)
• Southern Oscillation Index (SOI)
• Equatorial subsurface temperature from the TAO/TRITON buoy array
• Outgoing longwave radiation (OLR) as a proxy for tropical convection
• Reanalysis products that assimilate these observations

See also: Pacific Equatorial Undercurrent, Galapagos Cold Pool, Sea surface temperature, GReNI, El Nino and the Galapagos