The El NiΓ±o-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 1Β°C to 3Β°C, compared to normal.
This oscillating warming and cooling pattern, referred to as the ENSO cycle, directly affects rainfall distribution in the tropics and can have a strong influence on weather across the United States and other parts of the world. El NiΓ±o and La NiΓ±a are the extreme phases of the ENSO cycle; between these two phases is a third phase called ENSO-neutral.
El NiΓ±o: A warming of the ocean surface, or above-average sea surface temperatures (SST), in the central and eastern tropical Pacific Ocean. Over Indonesia, rainfall tends to become reduced while rainfall increases over the central and eastern tropical Pacific Ocean. The low-level surface winds, which normally blow from east to west along the equator ("easterly winds"), instead weaken or, in some cases, start blowing the other direction (from west to east or "westerly winds"). In general, the warmer the ocean temperature anomalies, the stronger the El NiΓ±o (and vice-versa).
La NiΓ±a: A cooling of the ocean surface, or below-average sea surface temperatures (SST), in the central and eastern tropical Pacific Ocean. Over Indonesia, rainfall tends to increase while rainfall decreases over the central and eastern tropical Pacific Ocean. The normal easterly winds along the equator become even stronger. In general, the cooler the ocean temperature anomalies, the stronger the La NiΓ±a (and vice-versa).
Neutral: Neither El NiΓ±o or La NiΓ±a. Often tropical Pacific SSTs are generally close to average. However, there are some instances when the ocean can look like it is in an El NiΓ±o or La NiΓ±a state, but the atmosphere is not playing along (or vice versa).
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