It is well known that El Niño refers to anomalous warm episodes during which sea surface temperatures (SSTs) in the central and eastern tropical Pacific are warmer than normal. Due to its significant impacts of weather and climate globally, it is of great importance to monitor and forecast the evolution of El Niño events.

In early May 2014, anomalous warming was observed in the equatorial central and eastern Pacific, and the pattern and magnitude of SSTs bore a close resemblance to those in May 1997, with the latter subsequently developing into the strongest El Niño in the 20th century. Therefore, a super El Niño event was expected to occur during subsequent months in 2014. However, the anomalous warming in the equatorial central-eastern Pacific decayed in summer 2014, analogous with its 1990 counterpart, the aborted El Niño, while rather different from the situation in summer 1997, when a mature El Niño had already occurred.

Dr. LI Jianying and Prof. MAO Jiangyu et al. conducted a comprehensive study on the physical processes, comparing the dominant factors responsible for the strength of El Niño events in terms of the preceding instability of the coupled ocean–atmosphere system and westerly wind bursts (WWBs) in 2014 with those in 1997 and 1990, separately.

A schematic diagram showing the key physical processes responsible for a strong El Niño event (Image plotted by IAP)

Although the unstable ocean–atmosphere system formed over the tropical Pacific in the preceding winter of 2014, the strength of the preceding instability was relatively weak. Weak oceanic eastward-propagating downwelling Kelvin waves were forced by the weak WWBs over the equatorial western Pacific in March 2014, as in February 1990. The consequent positive upper-oceanic heat content anomalies in the spring of 2014 induced only weak positive SSTAs in the central-eastern Pacific – unfavorable for the subsequent generation of summertime WWB sequences.

Because the equatorial western Pacific was not cooled in spring 2014 as in 1997, the equatorial central and eastern Pacific was unable to be warmed sufficiently. Positive Bjerknes feedback failed to establish in early summer 2014, anomalous warming in the equatorial central-eastern Pacific was thus suspended in summer. As a consequence, the strength of the 2014 El Niño event was weaker than expected.