Diversity of La Niña Onset

The mechanisms of La Niña onset diversity remain unclear. Here we identified three La Niña onset types using the K-means cluster analysis of equatorial SSTA evolutions from the preceding summer to the developing winter. The first onset type is characterized by a transition from a neutral year to La Niña (N2L). The second type is a transition from a central Pacific (CP) El Niño to La Niña (CE2L). The third type is a transition from a super El Niño to La Niña (SE2L). A key preceding signal for N2L is the warming in the tropical North Atlantic (TNA). During the autumn prior to N2L onset, positive SST and precipitation anomalies occurred in the TNA, and they induced anomalous easterlies in the equatorial western Pacific, which further triggered upwelling oceanic Kelvin waves, shallower equatorial thermocline and anomalous westward zonal currents, initiating a cooling at the equator through the zonal advective feedback. The onset of CE2L was caused by preceding anomalous easterlies in the equatorial eastern Pacific (EP), a direct response to the central Pacific heating associated with CP El Niño. The anomalous easterlies strengthened local surface latent heat flux and anomalous upwelling, leading to a cooling in EP. The SE2L onset was primarily attributed to a substantially shoaling of ocean thermocline associated with the discharge of the preceding super El Niño.