Physical Mechanisms Responsible for the Transition from a Warm to a Cold State of the El Niño–Southern Oscillation

Abstract

Abstract Based on the switch of a significant sea surface temperature anomaly (SSTA) over the central equatorial Pacific (the Nino-3.4 region) from ≥0.5°C to ⩽−0.5°C, three types of transitions from the warm (El Nino) to the cold (La Nina) phase of the El Nino–Southern Oscillation can be identified. They are the spring occurrence (SP) type, in which the SSTA first falls below −0.5°C in April or May after the termination of an El Nino event; the summer occurrence (SU) type, in which the SSTA does not reach this threshold until July or later; and the nonoccurrence (NON) type, in which the SSTA never reaches the threshold. Of the 12 El Nino episodes that occurred during the period of 1951–97, the number in each type is 3, 4, and 5, respectively. No significant difference in the SSTA composites can be found among the three types prior to the termination of the El Nino; however, the subsurface ocean temperatures have very different structures and temporal evolutions. Over the eastern equatorial Pacific, the th...