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On December 10, 2009, the National Oceanic and Atmospheric Administration (NOAA) reported that El Niño conditions spotted earlier in 2009 had strengthened. From October to November 2009, sea surface temperature anomalies increased across the eastern and central equatorial Pacific Ocean. Predictions varied, however, regarding when El Niño would reach its peak strength.

These color-coded images show El Niño conditions in November (top) and October (bottom) 2009. The images are based on data from the Advanced Microwave Scanning Radiometer for EOS (AMSR-E) on NASA’s Aqua satellite. White areas show where sea surface temperatures were near normal, orange-red areas show unusually warm waters, and blue areas show unusually cold waters. The animation shows daily changes in October and November 2009. In the later image, sea surface temperatures across the equatorial Pacific have increased. What was a broad band of orange in October with two streaks of red in the eastern Pacific has become an intense band of red. Increased sea surface temperatures also appear in the higher latitudes—east of Japan, around Australia, and in the South Pacific midway between South America and New Zealand.

Under normal (non-El Niño) conditions, waters in the eastern tropical Pacific Ocean (near South America) are kept cool by regular westward-blowing trade winds. As the winds blow surface waters toward the western Pacific, a deep pool of warm water builds around Indonesia. Meanwhile, in the eastern Pacific, cooler waters well up from deep in the ocean to replace the water being pushed westward. During El Niño, the easterly trade winds weaken. The warm pool of water in the western Pacific gradually sloshes eastward, warming the central Pacific. Without the trade winds to drive upwelling in the eastern Pacific, the surface waters become warmer than normal.

El Niño is only one facet of climate pattern that involves both the ocean and the atmosphere. The atmospheric component is called the Southern Oscillation, which is a strengthening or weakening of the difference in air pressure between the western and the eastern Pacific. During a negative phase of the Southern Oscillation (which is the phase linked to El Niño events in the water), air pressure at the surface of the Pacific around Indonesia becomes unusually high, while the pressure at the surface over the eastern Pacific becomes unusually low. It is the combination of ocean and atmosphere disruptions that produces such strong impacts on global climate patterns. The NOAA Climate Prediction Center stated that this El Niño-Southern Oscillation event would likely continue strengthening or remain moderately strong over the next few months, exerting a substantial influence on global weather and climate during that time.

NASA Earth Observatory image by Kevin Ward, based on data provided by the NASA Earth Observations (NEO) Project. Caption by Michon Scott and Rebecca Lindsey.