Journal of Oceanography, Vol. 62 (No. 2), pp. 227-234, 2006

Impact of Quasi-decadal Variability in the Tropical Pacific on ENSO Modulations

Takuya Hasegawa¹* and Kimio Hanawa²

¹Oceanographic Research Department, Meteorological Research Institute, Nagamine, Tsukuba 305-0052, Japan
²Department of Geophysics, Tohoku University, Aoba-ku, Sendai 980-8578, Japan

(Received 27 June 2005; in revised form 8 December 2005; accepted 9 December 2005)

Abstract: The impact of quasi-decadal (QD: 8 to 18 years) variability in the tropical Pacific on ENSO events is investigated. It is found that there is a significant difference in the behavior of ENSO events between the phases of positive and negative anomalies of the QD Niño-3.4 index. During the period of negative QD-scale Niño-3.4 index, ENSO events, especially La Niña events, occur more frequently, and larger amplitudes of thermal anomalies related to El Niño events appear over the central to eastern equatorial Pacific. Furthermore, propagations of upper ocean heat content anomaly and a phase relationship between upper ocean heat content and Niño-3 index in the equatorial Pacific, which have been pointed out by previous studies, are clearly detected during the period of negative QD Niño-3.4 index.