ENSO

The internationally used acronym for the El Niño - Southern Oscillation climate pattern (El Niño - Southern Oscillation, in English). This natural phenomenon is the clearest manifestation of the coupling between the atmosphere and the ocean. It is a large-scale bimodal oscillation, whose negative phase is El Niño and positive phase La Niña. The onset of an ENSO episode is marked by significant surface pressure anomalies between Tahiti, in French Polynesia, and Darwin, in northern Australia, as well as sea surface temperatures in the central and eastern equatorial Pacific band. In Climatology, the so-called Southern Oscillation Index (known as SOI) is used to determine when a given phase of ENSO begins. That index is given by the standardized difference of the atmospheric pressures measured at the aforementioned places. Under normal conditions - with a neutral ENSO -, pressure is lower in Darwin than in Tahiti, which maintains an east-to-west air flow (trade wind regime) that transports large quantities of water vapor to the eastern part of the Pacific basin, giving rise to the characteristic rainy weather of northern Australia and the neighboring Papua New Guinea and Indonesia. Under El Niño conditions, the pressure difference on either side of the basin weakens, the trade winds do likewise, upward flows of humid air are reduced, and ocean currents are altered, with surface sea water warming in the aforementioned band. All of this notably modifies wind patterns and precipitation regimes throughout the Pacific region. While Australia experiences droughts, on the other side of the basin, on the west coast of the equatorial band of South America, torrential rains occur that cause flooding. With the La Niña phenomenon (the positive or cold phase of ENSO), precisely the opposite occurs. In both phases, the impact of ENSO is not confined to the Pacific basin, with many more regions of the world where its effects are felt.