Pacific Decadal Oscillation (PDO) index
The Pacific Decadal Oscillation (PDO) index value for 2014 (annual mean) was +1.1. Negative values have generally been observed since around 2000 on a decadal timescale; this was the first positive annual mean value in eight years.
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 2014 (annual mean) was +1.1.
Focusing on the decadal variability of the PDO index, its values varied from positive to negative during the 1940s and from negative to positive during the late 1970s, and were generally positive until the 1990s (the positive value corresponds to the pattern in which SSTs in the central part of the North Pacific are lower than their normals).
Although negative values have generally been observed since around 2000 on a decadal timescale, the SST anomaly pattern was modified by atmospheric forcing in 2014 and the annual mean PDO index value turned positive for the first time in eight years. The monthly PDO index has been positive since February 2014, in contrast to the situation of the last decade. As recent studies have suggested that PDO is a key factor in the global climate system (including Japan's climate and the 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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