365 Days of Climate Awareness 114 – The Pacific Decadal Oscillation

 

The Pacific Decadal Oscillation (PDO) is a linked ocean-atmosphere alternation of warm and cool sea surface temperatures in the northern and eastern Pacific, on the scale of several years to over a decade. There are several drivers, which include the El Nino (ENSO) oscillation, random atmospheric variation and, according to recent research, anthropogenic global warming. As with the North Atlantic Oscillation, this is an area of active research with no well-defined consensus, but the trend is toward considering it a natural variation, and not a rhythmic oscillation.

(Top) PDO modes, positive (left) and negative (right). (Bottom) Positive (El Niño) and negative (La Niña) phases of ENSO. Arrows represent wind vectors (speed and direction).

That is not to say it is entirely a mirage: positive and negative events are correlated with other phenomena. But with such irregular period and magnitude, the PDO can’t well be called a cycle. As Michael Mann theorizes, the PDO is more likely manifesting the planet’s warming trend.

PDO Index, 1900-2020, with rolling average.

Its positive phase involves cold sea surface temperatures (SSTs) in the north midlatitudes from south of Alaska west to Japan, while the equatorial region of the eastern Pacific is warm. Its negative phase is the opposite: warm in the northern midlatitudes, cool in the south equatorial region.

Land temperature effects of a positive PDO event.

Linkages between the PDO and ENSO can have considerable lag times, of up to three years, as the connections involve slow oceanic Rossby waves and mixing processes down to mid-depths of several hundred meters. The magnitudes of PDO events can vary on the scale of years to several decades.

Land precipitation effects of a positive PDO event.

The Pacific Decadal Oscillation is associated with variations in weather across the Pacific. A positive cycle is correlated with warmer than average weather throughout western North America, Alaska, western Canada and Australia, and cooler weather in the Gulf region, eastern Canada and eastern Siberia, and a less active than usual Indian monsoon. Precipitation is above average along the west coast of North America, and below average in Siberia, Canada, the Gulf region and Australia.

The negative phase comes with the opposite: cooler weather than normal across the west coast of North America, Alaska, western Canada and Australia, warmer weather in the Gulf region, eastern Canada, and Siberia, and a more active Indian monsoon. Precipitation is below average along the west coast of North America, and above average in Siberia, Canada, the Gulf region and Australia.

Tomorrow: the North Pacific Gyre Oscillation.

Be brave, and be well.