The linkage between two ENSO types/modes and the interdecadal changes of ENSO around the year 2000

Abstract

This study focuses on the interdecadal changes in ENSO properties emerging around the year 2000. Compared to 1980–1999, after 2000, the ENSO amplitude weakened, the occurrence of the central Pacific (CP) El Niño increased, and the eastern Pacific (EP) El Niño became suppressed. Meanwhile, the dominant period of ENSO shortened from quasi-quadrennial (QQ) to quasi-biennial (QB). The authors show that these changes in ENSO properties are evidently consistent with the change in the stability of the ENSO mode through connecting the two ENSO types with the two coupled ENSO modes, i.e. the QQ and QB modes. It is suggested that the relative activity or stability of the two ENSO modes changed after the year 2000. The intensity of both the QQ and QB mode weakened. The QQ mode, which is linked to EP ENSO and was significantly strong during 1980–1999, became much weaker after 2000 in terms of the EP type almost disappearing. Compared with the weakness of the QQ mode, the QB mode, as manifested by the CP type, remained active and became dominant in the tropical Pacific after 2000. Analysis shows that the changes in mean states in the tropical Pacific were likely responsible for the interdecadal ENSO changes around the year 2000.