ENSO

ENSO, or the El Niño-Southern Oscillation, is a climate pattern arising from coupled interactions between the tropical Pacific Ocean and the atmosphere. It is the dominant source of year-to-year climate variability on a global scale. ENSO has three phases: El Niño (warming in the central and eastern tropical Pacific), La Niña (cooling in those regions), and an intervening period of neutral conditions known as ENSO-neutral. The cycle is irregular, typically occurring every two to seven years and lasting several months to a few years.

El Niño is characterized by weaker trade winds, warmer sea surface temperatures in the Niño 3.4 region, and a shift in convection toward the central or eastern Pacific. La Niña involves stronger trade winds and cooler central and eastern tropical Pacific waters. These SST anomalies influence global weather patterns through atmospheric teleconnections, altering rainfall and temperature across continents. Common impacts include increased rainfall in the southern United States and Peru during El Niño, drought in Australia and Southeast Asia, and altered monsoon dynamics in Africa and Asia.

Monitoring and forecasting rely on indices such as the Oceanic Niño Index (ONI), a three-month running mean of SST anomalies in the Niño 3.4 region, and the Southern Oscillation Index (SOI), which measures atmospheric pressure differences between Tahiti and Darwin. ENSO-neutral conditions occur when neither El Niño nor La Niña conditions are present.

Longer-term trends related to climate change remain a topic of ongoing research, with questions about potential changes in the frequency, intensity, and regional impacts of ENSO events. Overall, ENSO remains a key natural factor in global climate variability and forecast science.