Modes of Interannual and Interdecadal Variability of Pacific SST

The multichannel singular spectrum analysis has been used to characterize the spatio–temporal structures of interdecadal and interannual variability of SST over the Pacific Ocean from 20°S to 58°N. Using the Comprehensive Ocean–Atmosphere Data Set from 1950 to 1993, three modes with distinctive spatio-temporal structures were found. They are an interdecadal mode, a quasi-quadrennial (QQ) oscillation with a period of 51 months, and a quasi-biennial (QB) oscillation with a period of 26 months. The interdecadal mode is a standing mode with opposite signs of SST anomalies in the North Pacific and in the tropical Pacific. The amplitude of this mode is larger in the central North Pacific than in the tropical Pacific. This mode contributes 11.4% to the total variance. It is associated with cooling in the central North Pacific and warming in the equatorial Pacific since around 1976–77. The QQ oscillation exhibits propagation of SST anomalies northeastward from the Philippine Sea and then eastward along 40°N, but behaves more like a standing wave over the tropical Pacific. It explains nearly 20% of the total variance. The QB oscillation is localized in the Tropics and is characterized by the westward propagation of SST anomalies near the equator. This mode accounts for 7.4% of the total variance. Since the interdecadal mode is apparently independent of QB and QQ oscillations, it may play an important role in configuring the state of the tropical SST anomalies, which in turn affects the strength of the El Niño–Southern Oscillation phenomenon. It seems likely that the higher phase of the interdecadal mode since 1976–77 has raised the background SST state, on which the superposition of the QQ and QB oscillations produced the strongest warm event on record in 1982–83, as well as more frequent warm events since 1976.