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Long-term variations found in the North Pacific are reviewed with special reference to decadal scale variation.
An EOF analysis for SST fields shows existence of two distinctive patterns: one is that with the El Niño/Southern Oscillation (ENSO) time scale with period of 3-4 years and the other is that with interdecadal (DC) time scale. The leading mode of DC time scale is a meridional dipole pattern: north-south oscillation of low latitude and mid latitude oceans (central North Pacific). This pattern is very robust and is confirmed by the composite analysis. Time coefficient of this mode showed abrupt change like a step function around 1976 and continued to late 1980s. It was found that atmospheric general circulation pattern over the North Pacific also varies coherently with SST fields described above.
Analysis for the upper thermal condition also shows the existence of DC time scale variation coherent with SST field. Time-longitude matrix of 0-400 m vertically averaged temperature anomalies along the 35-45°N latitude band, and that of SST. It is seen that heat content anomaly changed its sign from positive before 1976 to negative after that corresponding to that of SST anomalies.
Spin-up/spin-down of the North Pacific subtropical gyre in recent decades can be also detected. Temporal change of heat content difference between the north (39°N) and south (31°N) regions of the Kuroshio, which clearly shows the increasing trend of heat content difference from 1960s to 1980s. That is, this means the North Pacific subtropical gyre has been gradually spun-up during the analyzed period. This change can be explained by that of wind stress field over the North Pacific. Corresponding to this change, thicknesses of the wintertime mixed layer and 15-19°C layer (thickness of Subtropical Mode Water) coherently vary.
What is the physical mechanism for the interdecadal scale variation found in the North Pacific? Our working hypothesis in future research is one in which subduction process plays an essential role in the mechanism: "Suduction Oscillation".
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