Pacific Decadal Oscillation (PDO) index
The Pacific Decadal Oscillation (PDO) index value for 2015 (annual mean) was +1.6. Negative values were generally observed from around 2000 to the early 2010s, and positive values have been recorded since 2014.
Time-series of the PDO index
The red line represents annual mean values for the PDO index,
the blue line represents five-year running mean values,
and the gray bars represent monthly valuies.
The Pacific Decadal Oscillation (PDO) is an SST anomaly pattern of Pacific climate variability that shifts phases on an inter-decadal time scale usually covering more than 10 years. The PDO index is defined as the projections of monthly mean SST anomalies onto their first EOF vectors in the North Pacific (north of 20ºN). The EOF vectors are derived for the period from 1901 to 2000, and climatology is defined as monthly mean for the same period. Globally averaged monthly mean SST anomalies are subtracted from each monthly mean SST anomaly before calculation of the first EOF vector in order to eliminate the effects of global warming.
When the PDO index is positive (negative), SSTs in the central part of the North Pacific are likely to be lower (higher) than their normals (i.e., typical SST anomalies in the positive phase of the PDO). In addition, when the index is positive (negative), sea level pressures (SLPs) values in the high latitudes of the North Pacific are likely to be lower (higher) than their normals. This indicates that the Aleutian Low is stronger (weaker) than its normal in winter and spring (typical SLP anomalies in the positive phase of the PDO).
The Pacific Decadal Oscillation (PDO) index value for 2015 (annual mean) was +1.6.
Focusing on the decadal variability of the PDO index, values were generally positive (corresponding to a pattern in which SSTs in the central part of the North Pacific are lower than their normals) from the late 1920s to the early 1940s and from the late 1970s to around 2000, and generally negative from the late 1940s to the mid-1970s and from around 2000 to the early 2010s. The SST anomaly pattern has been modified by atmospheric forcing since around 2014, and the annual mean PDO index value was positive in 2014 and 2015. Monthly PDO index has been positive since February 2014, which shows different condition from the last decade. With suggestions from recent studies that PDO is thought to be one of the key factors to the global climate system including Japan's climate and acceleration / hiatus of global warming, continuous monitoring of its variability is needed.
Tokyo Climate Center, Climate Prediction Division, 1-3-4 Otemachi, Chiyoda-ku, Tokyo, Japan.
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