北太平洋冬季上层海温异常的NPGO模态

摘要

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.%　　利用较高分辨率的全球海洋同化分析系统(SODA)资料,对冬季北太平洋上层的海温异常做了整层经验正交函数(EOF)分析,并主要讨论了第二模态的结果。该模态空间结构与经典的北太平洋涡旋振荡(NPGO)一致,且时间系数与NPGO指数也吻合,故北太平洋上层海温异常EOF第二模态可称为NPGO模态。这说明NPGO现象不单纯反映在海表面温度异常上,在上层海温中该现象也存在。在该模态空间场上,水深100 m以上25°N~30°N的副热带处,沿纬圈从120°E向东延伸至中东太平洋均为海温正异常带,其北面则为负异常带,两者构成双带系统；其中在170°W附近分别有正、负异常大值区,中心构成南北偶极子；在本州岛以东海域,从海表直到海洋上层底则有小范围的海温强异常。该模态空间结构的形成与大气NPO模态关系密切,并与中纬度西风大值带上的风应力异常有关,是造成NPGO的直接原因。该模态表现出明显的准13年年代际变化,且对其进行5年滑动平均后发现,从20世纪70年代中期以来,该序列的振幅越来越大,1976/1977年和1988/1989年的两次气候年代际突变均处该序列峰值处。引入了冬季北太平洋上层海温异常的NPGO指数,其能更好反映海洋上层的NPGO现象及其年代际变化。