The PDO is another cyclic pattern in the oceans that causes climate variability. This pattern affects the Pacific Ocean on a cycle that lasts 20 to 30 years. The PDO is most visible in the North Pacific/North American area (north of 20° N latitude). During a warm (positive) phase, the western Pacific becomes cool and the eastern part warms. During a cool (negative) phase, the western Pacific is warm and the eastern cool.
One consequence of the PDO is major changes in northeast Pacific marine ecosystems. When the phenomenon is in a warm cycle, coastal fisheries are fertile and productive in Alaska, but less productive off the west and northwest coast of the contiguous United States. Cold cycles are the opposite. Scientists have characterized the PDO by changes in (1) sea surface temperature, (2) sea level pressure, and (3) wind patterns.
Scientists have not been able to figure out yet what causes the PDO. The 20-30 year phases seem fairly consistent: cool phases existed from 1890 to 1925 and 1945 to 1977. Warm phases existed from 1925 to 1946 and 1977 to 1998. Some scientists, such as Benjamin Giese of the College of Geosciences at Texas A&M, have suggested that long-term changes in the Pacific Ocean temperatures may be an important clue to understanding global warming. Because of the abrupt changes that occur on a 20-30 year cycle in the PDO, they propose that maybe global warming is as much a result of some natural cyclic variation as human activity.
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