The cold upwelled waters also increase the east-west SST gradient, again reinforcing the strong easterly trade winds across the Pacific. This east-west SST gradient induces strong easterly winds, which force water to the west and cause greater upwelling of colder water in the east. Images from the Australian Bureau of Meteorology.īecause low-level winds move toward warm surface waters in the tropics, the difference in sea-surface temperatures (SSTs) between the cooler eastern Pacific and warmer western Pacific reinforces the easterly trad e winds during neutral conditions. The illustrations show how the coupled ocean-atmosphere system typically behaves during each event. Neutral, El Niño and La Niña conditions in the equatorial Pacific. As the water accumulates and stagnates, it continues to warm. When the surface water arrives in the western Pacific, land surfaces restrict its further movement and so the water “piles up”, making sea level approximately half a meter higher around Indonesia than it is off the coast of Ecuador. Because the upwelled equatorial and co astal waters originate deep below the surface, they are cold and rich with nutrients. During neutral conditions, waters in the eastern Pacific are upwelled along the equator and the coast of South America. Cold water from below moves up to replace the shifted surface water in a process referred to as upwelling. The sun heats water at the surface of the ocean, but the trade winds push surface water by friction from east to west in the Pacific. Air naturally moves from areas of high pressure to low pressure, so this difference in pressure moves the equatorial air, known as the trade winds, from the coast of South America toward the western Pacific Ocean. See Figure 2 (top) for a graphic illustrating neutral conditions.ĭuring neutral years (and on average) atmospheric pressure is low in the warmer western tropical Pacific (referenced at Darwin, Australia), and relatively higher in the cooler central/eastern tropical Pacific (referenced at Tahiti). The main indicators of these changes are pressure and temperature. As the conditions of the ocean change, the atmosphere responds, and vice versa. The equatorial Pacific climate acts as a “coupled system” because the state of the ocean and atmosphere depend on each other. In order to understand the ENSO cycle, we must first understand what the Pacific is like in its neutral state. ![]() ![]() Several years of neutral (or normal) conditions can persist between La Niña and El Niño events. The Southern Oscillation part of the term ENSO refers to the atmospheric component: the shifting of atmospheric pressure between the central/eastern Pacific and the western Pacific. ![]() They named the phenomenon El Niño (the boy child) because the effects were often seen most prominently during the Christmas season. Before 20 th century scientists studied ENSO, Peruvian fisherman observed the warmer ocean waters off of the South American coast and the impact the temperature had on their fisheries. Figures made in the IRI ENSO Maproom.Įl Niño refers to the ocean component of ENSO. Figure 1: Sea-surface temperature anomalies during a strong El Niño event (top) and La Niña event (bottom).
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