The Relationship between Contiguous El Nino and La Nina Revealed by Self-Organizing Maps

This study introduces a new methodology for identifying El Nino and La Nina events. Sea surface temperature (SST) anomaly patterns for El Nino and La Nina onset, peak, and end phases are classified by self-organizing maps (SOM) analysis. Both onset and end phases for El Nino and La Nina exhibit eastern Pacific (EP) and central Pacific (CP) types. The SST anomaly patterns in peak phase can be classified into EP, EP-like, and CP types for El Nino, and EP, mixed (MIX), and CP types for La Nina.The general type of each El Nino or La Nina event is then defined according to the SST type for each of the three phases. There is no robust connection between the general types of the contiguous El Nino and La Nina except that the MIX La Nina rarely induces a subsequent CP El Nino. However, there are strong relationships between the end-phase type of El Nino and the onset-phase type of the subsequent La Nina. The EP-end-type El Nino favors transition to the CP-onset-type La Nina, while the CP-end-type El Nino favors transition to the EP-onset-type La Nina. On the other hand, the CP-end-type La Nina favors transition to EP-onset-type El Nino. Furthermore, an El Nino that occurs after the decay of La Nina favors initiating as an EP-onset type. These relationships are driven by different atmosphere-ocean dynamics, such as coupled air-sea feedback, thermocline feedback, slow SST mode, and Bjerknes feedbacks.