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Interdecadal Pacific Oscillation (IPO)

The IPO is characterized by sea surface temperature fluctuations and sea level pressure changes in the north and south Pacific Ocean that occur on a 15-30 year cycle. During positive phases, ocean temperatures are warmer in the tropical Pacific, but cooler than average in the north Pacific. During negative phases of the IPO, Pacific Ocean temperatures are cooler than average in the tropical regions and warmer than average in the northern regions.  The IPO is similar to the Pacific Decadal Oscillation (PDO) in terms of sea temperature changes, but affects ocean waters south of 20o N Latitude, which the PDO does not.


During the 20th century, there were positive IPO phases in 1925-1946 and 1978-1998. A negative phase occurred from 1947-1976. What makes the IPO interesting is that it can help to modulate ENSO (El Nino-Southern Oscillation) effects for Australia and New Zealand. A positive phase enhances the prevailing conditions, which leads to increased precipitation in southern New Zealand and drier conditions in northern New Zealand. A negative phase weakens prevailing conditions and has opposite effects to those just described.

Because the IPO fluctuates less frequently than other climate indices, less is understood about it. In particular, its exact relationship to global climate change is poorly understood. Of interest is the fact that El Nino events increased between 1978 and 1998, a period during which the IPO was in a positive phase. There is much speculation as to whether this was due to the IPO or to global warming in general. There is also some speculation that the IPO may be linked to ENSO events and to sea surface temperature changes in general. In other words, water temperature changes may cause the IPO and not the other way around.

Bottom Line

The IPO is one of the least well understood of all climate indices. Its relationship to other indices, such as ENSO and PDO are still unclear. Given its long cycle periods, it may be some time before the IPO is understood to the same level of detail as other indices.