What is an El Nino?
El Nino is a disruption of the ocean-atmosphere system in the tropical Pacific having important consequences for weather around the globe. Among these consequences are increased rainfall across the southern tier of the U.S. and in Peru, which has caused destructive flooding, and drought in the west Pacific, sometimes associated with devastating brush fires in Australia. Observations of conditions in the tropical Pacific are considered essential for predicting short-term (a few months to one year) climate variations. To provide necessary data, NOAA operates a network of buoys that measure temperature, currents, and winds in the equatorial band. These buoys daily transmit data that are available in real time to researchers and forecasters around the world.
In normal, non-El Nino conditions (top panel of schematic diagram), the trade winds blow towards the west across the tropical Pacific. These winds pile up warm surface water in the west Pacific, making sea surface temperature about 8 degrees Celsius higher than the east off South America. This cold water is nutrient rich, supporting high levels of primary productivity, diverse marine ecosystems, and major fisheries.
During El Nino (bottom panel of the schematic diagram), the trade winds relax in the central and western Pacific leading to a deepening of the layer of warm surface water in the eastern Pacific, and an elevation of this layer in the west. This leads to a reduction in the upwelling and cuts off the supply of nutrient rich deep water to the upper ocean layers. The result is a rise in sea surface temperature and a drastic decline in primary productivity, the latter of which adversely affects higher trophic levels of the food chain, including commercial fisheries in this region. Rainfall follows the warm water eastward, with associated flooding in Peru and drought in Indonesia and Australia. The eastward displacement of the atmospheric heat source overlaying the warmest water results in large changes in the global atmospheric circulation, which in turn force weather changes in regions far removed from the tropical Pacific.
El Nino can be seen in measurements of the sea surface temperature, such as those shown above, which were made from the Tropical Atmosphere-Ocean Array of moored buoys. In January 1991, the sea surface temperatures and the winds were near normal, with warm water in the Western Pacific Ocean (in red on the top panel of January 1991 plot), and cool water, called the "cold tongue" in the Eastern Pacific Ocean (in green on the top panel of the January 1991 plot). The winds in the Western Pacific are very weak (see the arrows pointing in the direction the wind is blowing towards), and the winds in the Eastern Pacific are blowing towards the west (towards Indonesia). The bottom panel of the January 1991 plot shows anomalies, the way the sea surface temperature and wind differs from a normal January. In this plot, the anomalies are very small (yellow/green), indicating a normal January.
January 1992 was the peak of an El Nino year. In January 1992, the warm water (red in the top panel of the January 1992 plot) has spread from the western Pacific Ocean towards the east (in the direction of South America), the "cold tongue" (green color in the top panel of the January 1992 plot) has weakened, and the winds in the western Pacific, usually weak, are blowing strongly towards the east, pushing the warm water eastward. The anomalies show clearly that the water in the center of the Pacific Ocean is much warmer (red) than in a normal January.
An animation of the 1991 El Nino is on the PMEL server at:
El Nino can be seen in Sea Surface Temperature in the Equatorial Pacific Ocean