Physical mechanisms responsible for the transition from a warm to a cold state of the El Nino-Southern Oscillation

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 thermocline depth is the smallest in the SP events in the spring following the El Nino event. The decrease in the mixed layer depth also propagates eastward in both types of cold events but with different speeds. When and if a La Nina event will occur appears to depend on the timing of the enhancement of the central and eastern Pacific trades off the equator. A strengthening of the Pacific subtropical highs in both the Northern and Southern Hemispheres is apparently responsible for such an enhancement. Once the strengthening of the trades occurs, the SST and near-equatorial zonal wind anomalies will follow to initiate the onset of the La Nina. In the SP type, the subtropical highs in both hemispheres in the eastern and central Pacific strengthen starting at around October of the El Nino year, which then enhances the northeast and southeast trades off the equatorial Pacific east of the date line. Due to Ekman forcing, the enhanced easterlies will cause surface water to drift poleward, which then reduces the depth of the thermocline. This upwelling sets up Rossby waves that propagate westward. By the following January, the negative anomalies in mixed-layer depth have reached the western boundary of the Pacific. They are then reflected and propagate eastward as a slow, coupled air-sea mode, which reduces the thermocline depth in the equatorial region. This results in a cooling of the ocean, which then induces equatorial easterly anomalies. The eastward-propagating wave reaches the central equatorial Pacific by spring so that the SSTA over the Nino-3.4 falls below -0.5°C, and hence the onset of the SP-type La Nina. In the SU type, the subtropical high in the South Pacific does not strengthen until spring of the year following the El Nino. The above process is therefore delayed so that the onset does not occur until July. For the NON type, the subtropical highs never strengthened, and so no switch in the zonal wind anomalies, and hence no La Nina, takes place.