NPGO Mode of the Upper Sea Temperature Anomalies in the North Pacific during Winter

In this paper, the empirical orthogonal function (EOF) method is used to analyze the upper sea temperature anomalies in the North Pacific during winter with higher resolution Simple Ocean Data Assimilation (SODA) data. The spatial pattern of the second EOF mode is similar to the classical North Pacific Gyre Oscillation (NPGO) mode, as is the time series. Therefore, the second EOF mode is hereafter referred to as the NPGO mode. Results indicate that the NPGO exists not only at the sea surface, but also at the entire upper ocean. The spatial structure of the NPGO mode is characterized by a dipole-like structure in the meridional direction with maximum anomalous centers located near 170°W above a depth of 100 m. The anomalously warm band between 25°N and 30°N extends from 120°E to the Mideast Pacific, while the northern area is anomalously cool. In addition, obvious temperature anomalies exist from the sea surface to the upper sea level at the eastern area of Honshu Island. The formation of this spatial feature has a close relationship with the North Pacific Oscillation (NPO) mode and the wind stress anomalies in the mid-latitude westerly zone, which is the direct reason for the NPGO modes. The time series of the NPGO mode shows obvious decadal variation of approximately 13 years. The five-year moving average of the time series shows that its amplitude has become larger since the middle 1970s, and the climate transition of 1976/1977 and 1988/1989 show peak values of this series. Finally, the NPGO index of the sea temperature anomalies in the upper North Pacific during winter is defined, which can reflect the decadal variation of NPGO mode more accurately.