The impact of Westerly wind bursts and ocean initial state on the development and diversity of El Niño events

Westerly wind bursts (WWBs) that occur in the tropical Pacific near the Dateline are believed to play an important role in the development of El Niño events, even though the direct link is sometimes difficult to establish. Here, following the study of Lengaigne et al. (2004), we conduct numerical simulations in which we reexamine the response of the climate system to an observed wind burst added to a coupled model. Two sets of ensemble experiments are conducted. In the first set, the initial ocean heat content of the system is higher than the model climatology (or recharged), while in the second set it is nearly normal (neutral). For the recharged state, in the absence of WWBs, a moderate central Pacific El Niño (CP) develops in about a year. In contrast, for the neutral state, there develops a weak La Niña. However, when the WWB is imposed, the situation changes dramatically: the recharged state slides into an eastern Pacific El Niño (EP), while the neutral set shifts into a weak CP El Niño instead of previous La Niña conditions. The different response of the system to the exact same perturbations is controlled by the initial state of the ocean and the subsequent ocean-atmosphere interactions involving an interplay between the eastward shift of the warm pool and the warming of the eastern equatorial Pacific. Consequently, the occurrence of different flavors of El Niño, including extreme events, may depend on stochastic atmospheric processes, modulating El Niño properties within a broad continuum.