El Niño-Southern Oscillation
Introduction
The El Niño-Southern Oscillation (ENSO) is a recurring climate pattern involving changes in the temperature of waters in the central and eastern tropical Pacific Ocean. On periods ranging from about three to seven years, the surface waters across a large swath of the tropical Pacific Ocean warm or cool by anywhere from 1°C to 3°C, compared to normal. This oscillation between warmer than average (El Niño) and cooler than average (La Niña) conditions has wide-ranging impacts on global weather and climate patterns.
El Niño
El Niño is a phase of the ENSO, characterized by a warm anomaly of sea surface temperatures that extends across the central and eastern tropical Pacific Ocean. This warming typically peaks during the Northern Hemisphere winter. The atmospheric response to this warming results in a slackening of the trade winds, and an increase in rainfall over the eastern equatorial Pacific and a decrease over Indonesia and the western Pacific. The low-level surface winds, which normally blow from east to west along the equator, instead weaken or, in some cases, start blowing the other direction from west to east.
La Niña
La Niña is the cool phase of the ENSO, characterized by a cool anomaly of sea surface temperatures across the eastern and central tropical Pacific Ocean. During a La Niña year, winter temperatures are warmer than normal in the Southeast United States and cooler than normal in the Northwest. La Niña causes increased rainfall across the western equatorial Pacific, Indonesia and the Philippines, as well as in northern South America, and southern Africa. It also causes decreased rainfall in the central and eastern equatorial Pacific.
ENSO Neutral
ENSO Neutral is the phase between El Niño and La Niña, when sea surface temperatures across the equatorial Pacific Ocean are near average. The atmospheric conditions, such as wind and rainfall patterns, are also near average during this phase.
Impacts of ENSO
The ENSO has significant impacts on weather and climate patterns across the globe. During El Niño events, there is an increase in rainfall across the eastern equatorial Pacific, leading to flooding in Peru and Ecuador, and a decrease in rainfall over Indonesia and Australia, leading to drought conditions. In the United States, El Niño events are associated with warmer than average winters in the northern tier of states and cooler than average winters in the southern tier of states.
During La Niña events, there is an increase in rainfall across the western equatorial Pacific, Indonesia and the Philippines, and a decrease in rainfall in the central and eastern equatorial Pacific. In the United States, La Niña events are associated with cooler than average winters in the northern tier of states and warmer than average winters in the southern tier of states.
Measurement and Prediction
The ENSO is measured by the Oceanic Niño Index (ONI), which is based on sea surface temperature anomalies in the Niño 3.4 region of the Pacific Ocean. The ONI is one of several indices used to monitor the ENSO, along with the Southern Oscillation Index (SOI), and the Multivariate ENSO Index (MEI).
Prediction of the ENSO is a major focus of climate modeling research. Accurate prediction of the ENSO can provide valuable information for agricultural planning, water management, and disaster mitigation.