What causes the Pacific decadal oscillation?

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What causes the Pacific decadal oscillation in which a large part of the pacific ocean gets hotter than normal - this effect superimposes itself on El Nino.Is the pacific decadal oscillation linked to El Nino - does one cause the other?
 

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What causes the Pacific decadal oscillation in which a large part of the pacific ocean gets hotter than normal - this effect superimposes itself on El Nino.Is the pacific decadal oscillation linked to El Nino - does one cause the other?
At this point the cause is unknown. The terms tends to be used for the North Pacific with the El Nino Southern Oscillation being used for the South Pacific. The PDO is a longer (possibly 20 years) cycle than the ENSO (6 to 18 months) but the ENSO can have periods in which either El Nino (warmer water) or la Nina (cooler water). Both cycles were noticed by fishermen before oceanographers or climatologists took notice.

The warm cycle of the PDO may tend to favor El Nino events with the cool cycle favoring la Nina events. There is a good correlation between these cycles and air temperatures.
http://icecap.us/images/uploads/THE_GREAT_PACIFIC_CLIMATE_SHIFT_II.pdf

the period since the late 70's has been dominated by El Nino events and warmer temperatures. the previous period with la Nina events was cooler. Some climatologists speculate that the current la Nina may indicate the beginning of a period of cooler temperatures.
 

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