The Pacific Decadal Oscillation (PDO) may have moved into a warming phase during the last year, according to the National Oceanic and Atmospheric Administration (NOAA) of the US.
A graph showing NOAA’s Pacific Decadal Oscillation Index. Red bars correspond to a positive index and blue bars to a negative index. Courtesy: NOAA
The PDO is a long term pattern of heat exchange between the ocean and the atmosphere that is centred over the mid-latitudes of the Pacific basin and is linked to a cycle in sea surface temperature (SST) variations.
The PDO plays an important role in the overall transfer of heat between the world’s ocean and the atmosphere. NOAA suggests that the Pacific may have moved to a phase where the Pacific Ocean surrenders more heat to the atmosphere than it absorbs.
Extreme phases of the PDO are classified as being either warm or cool, as defined by SST anomalies in the north east and tropical Pacific Ocean. The PDO had been in a negative or cooling phase for a number of years prior to 2014 but several factors suggest this may have changed:
- The monthly PDO Index maintained by NOAA has been consistently positive for the year to end-June 2015 (see table right);
- Waters in the tropical Pacific, off the west coast of North America, and north eastern Pacific waters off Alaska, have all been warmer than normal;
- Waters east of Japan have been colder than normal;
- Sea surface temperatures (SSTs) in the North Pacific Basin hit record highs during 2014.
The PDO tends to remain in a predominantly warm or predominantly cool phase for many years but can be subject to relatively short and repeated excursions into the opposite phase during these periods. This means that it will be some time before data can confirm whether the current positive PDO Index indicates a short term warming excursion during a continuing cold phase or the beginning of a longer term sustained warm phase.
During a warm phase, the west Pacific becomes cooler and part of the eastern ocean warms; during a cool phase, the opposite pattern occurs. When SSTs are anomalously cool in the interior north Pacific and warm along the Pacific coast of North America, and when sea level pressures are below average over the north Pacific, the PDO Index has a positive value. When the climate anomaly patterns are reversed, with warm SST anomalies in the interior and cool SST anomalies along the North American coast, or above average sea level pressures over the north Pacific, the PDO Index has a negative value.
NOAA reports in its recently published State Of The Climate In 2014 document that the SST anomaly pattern in the north Pacific during 2014 “resembled the positive phase of the PDO”. NOAA oceanographer Greg Johnson told a NOAA press conference last week that he believes that the PDO may have shifted: “2014 was the first year in that shift and it is possible that it may slip back into a cold phase but we won’t know for five years,” he said.
Some climate scientists believe that these changes in the Pacific Ocean may presage an end to the so called pause – or hiatus – in global warming that has been apparent in some surface and atmospheric temperature records since the late 1990s.