BOX 4.1
Pacific Ocean Climate Patterns

ENSO. The El Niño-Southern Oscillation is a quasi-periodic climate pattern that occurs across the tropical Pacific Ocean about every 2 to 7 years. It is characterized by variations in the sea-surface temperature of the tropical eastern Pacific Ocean. In the warm El Niño phase, warm ocean temperatures in the tropical eastern Pacific are accompanied by high air surface pressures in the tropical western Pacific (Figure). In the cool La Niña phase, the pattern is reversed. The reversal in surface air pressure between the eastern and western tropical Pacific is known as the Southern Oscillation.

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FIGURE (Top) Sea-surface temperature anomalies (shading) and sea-level pressure (contours) associated with the warm phase of ENSO (i.e., El Niño) for the 1900–1992 period. Positive contours are dashed and negative contours are solid. (Bottom) Multivariate ENSO index for 1950–2009. The index is based on variables observed over the tropical Pacific, including sea-level pressure, surface wind, sea surface temperature, surface air temperature, and cloudiness. Positive (red) index values indicate El Niño events and negative (blue) values indicate La Niña events. SOURCE: Figure and details on how the index is computed are given in <http://www.esrl.noaa.gov/psd/enso/mei/>.

PDO. The Pacific Decadal Oscillation is often described as a long-lived (i.e., decadal) El Niño-like pattern of Pacific climate variability. Like ENSO, the PDO has warm and cool phases, as defined by patterns of ocean temperatures in the northeast and tropical Pacific Ocean (Figure).

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FIGURE The Pacific Decadal Oscillation. (Top) Typical winter patterns of sea surface temperature (colors), sea-level pressure (contours), and surface wind stress (arrows) during positive (warm) and negative (cool) phases of PDO. Temperature anomalies are in degrees Celsius. (Bottom) History of the PDO index (the principal component of monthly sea surface temperature anomalies in the North Pacific Ocean poleward of 20°N) from 1900 to 2010. SOURCE: Figure obtained with permission granted by Nate Mantua at the University of Washington’s Joint Institute for the Study of Atmosphere and Ocean.



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