The Pacific Decadal Oscillation (PDO) is a climatic event which cover vast areas of the Pacific Ocean over periods of 20 to 30 years.
The PDO has positive and negative phases. The climate impacts experienced during a PDO event can go hand-in-hand with impacts from El Nino or La Nina. If both phenomena are in the same phase, their associated impacts can be amplified. In the opposite phase, the associated impacts on global climate may be reduced.
Positive phases of the PDO tend to be associated with periods of more rapid global warming whilst cold PDO events have been linked to severe droughts spanning many years in south western USA, as well as increased rainfall over eastern Australia.
It is thought that negative phases could be linked to times of slower warming. This is because cold phases of the PDO tend to increase mixing of colder, deep ocean waters with warmer surface waters. This temporarily reduces the rate of global warming caused by increasing greenhouse gas emissions. Positive phases have the opposite effect.
The cause of changes in the PDO has yet to be identified and it may even be due to a combination of factors including long-lasting fingerprints of El Nino and La Nina events in the tropical Pacific Ocean; changes in atmospheric pressure the northern Pacific; the impact of industrial pollution; and natural variability.
This important phenomenon continues to be a subject of ongoing research.