The collaboration research between the IAP and PLA University of Science found that onset types of La Ni？a are closely related to end types of last El Ni？o even though there are no robust relations between their peak types. The study was recently published in Journal of Climate.  

Many previous studies pointed out there were two types of El Ni？o, named as eastern Pacific (EP) and central Pacific (CP) types. However, there is no consensus on the classification of historical El Ni？o events, as the definitions and the representative seasons of El Ni？o events are different. As for La Ni？a, some studies even doubt whether they have two types. Moreover, there are debates whether the next La Ni？a would be strong since the current El Ni？o is strong.    

The study probed for a new definition of ENSO events, and identified the types of historical ENSO events in their onset, peak, and end phases by Sea surface temperature (SST) anomaly patterns, using self-organizing maps (SOM) analysis. There results show that both the onset and end patterns of El Ni？o and La Ni？a events can be classified into EP and CP types. The peak patterns of El Ni？o can be classified into EP, EP-like and CP types, while EP and MIX types for the peak patterns of La Ni？a.   

The classification for patterns of SST anomalies for each phase of (top three rows) El Ni？o and (bottom three rows) La Ni？a events. The three phases of the same event are linked by dotted line. The primary peak is marked by circle for the events with multiple peak phases. (Li et al, 2015)

Interestingly, the end-phase type of El Ni？o can strongly influence the onset-phase type of the following La Ni？a. The EP-end-type El Ni？os favor transition to the CP-onset-type La Ni？as, while the CP-end-type El Ni？os favor transition to the EP-onset-type La Ni？as. On the other hand, the end-phase type of La Ni？a can also affect the onset-phase type of the following El Ni？o. For example, the CP-end-type La Ni？as favor transition to EP-onset-type El Ni？os. Furthermore, most of the EP-onset-type El Ni？os closely follow La Ni？as, while the CP-onset-type El Ni？os show more independent. 

However, they pointed out there is no robust connection between the peak-phase types and strength of the contiguous El Ni？o and La Ni？a events. For example, the second strongest El Ni？o (1982/83, EP-peak-type) only induced a weak La Ni？a (1983/84, EP-peak-type), while some weak El Ni？os, such as 1953/54 (CP-peak-type), 1968/70 (CP-peak-type), and 2006/07 (EP-like-peak-type) events, were followed by strong La Ni？as (MIX-peak-type).  

Reference: Xin Li, Chongyin Li, Jian Ling, and Yanke Tan, 2015: The Relationship between Contiguous El Ni？o and La Ni？a Revealed by Self-Organizing Maps. J. Climate, 28, 8118–8134.  

Contact: LING Jian, lingjian@lasg.iap.ac.cn