Why does the CP El Niño Less Frequently Change into La Niña than the EP El Niño?

It is less likely for the Central-Pacific (CP) El Niño (38%) than the Eastern-Pacific (EP) El Niño (75%) to change into La Niña after its decay since 1958. We explore the cause of this contrasting feature by comparing the transitional composites of both the EP and CP El Niño to the non-transitional composites. The differences in the transition between the two types of El Niño stem from the facts (1) that they invoke different patterns of the surface wind anomalies on the Pacific for their transitions and (2) that the corresponding transition wind pattern can be induced by the EP El Niño but impede by the CP El Niño.

By comparing the transitional and non-transitional groups of El Niño, we find the reversal of western Pacific wind (WPw) from surface westerlies anomalies to easterlies since November is required for the EP El Niño to change into La Niña. On the other hand, the southward shift of central Pacific wind (CPw) since December is required for the CP El Niño to change into La Niña. The reversal induces ocean waves propagating eastward to trigger the La Niña phase, whereas the shift intensifies local surface heat flux cooling to start the La Niña phase. The different transition wind patterns reflect the thermocline dynamics control over the EP El Niño and the mixed-layer dynamics control over the CP El Niño.

The WPw reversal is found to be caused by the intensification of the western Pacific subtropical high or the warming of the Indian Ocean sea surface temperatures, both of which can be induced by a developing EP El Niño event. The WPw reversal required for the transition can often be produced by the EP El Niño itself. Therefore, the EP El Niño is likely to change into La Nina. The CPw shift required for the transition of the CP El Niño is found to be impeded by the atmospheric stochastic forcing and El Niño itself. Therefore, the CP El Niño is not likely to change into La Niña.