El Niño nuance: A distinct difference between 1997/98 and 2015/16 extreme seasons

In the climate science community, the 2015/16 El Niño is considered the first extreme El Niño of the 21st century. Climatologists have put forth great effort studying this anomalous season. This event still sparks research interests because of the large geographic region of warm sea surface temperature (SST) anomalies along the equator, but also because several major characteristics are distinct from the 1997/98 extreme El Niño. The most definitive difference concerns warm SST anomalies in the far eastern Pacific (FEP). During the 2015/16 El Niño, warm SST anomalies in the FEP were relatively modest. However, in 1997/98, the SST anomalies in the FEP were  twice the magnitude of those in the 2015/16 El Niño.

SST anomalies in the FEP not only have impacts on the marine ecosystem in the whole eastern Pacific, but they also affect the socioeconomic status of the countries along South America’s west coast. The Pacific coastal climate of Peru and Ecuador, under normal/neutral ENSO conditions, is cool and dry due to intense upwelling of cold water, which leads to nutrient rich ocean water for fish and many marine species. During El Niño seasons, which produce strong warm SST anomalies along the coast, marine food chains and biodiversity are affected significantly. Furthermore, warm SST anomalies tend to increase convection in the region, triggering heavy rainfall and causing flooding. During the 1997/98 El Niño, the devastating rainfall events in Northern Peru caused economic losses to exceed an estimated $3.5 million.

Citing the significant global influences of SST anomalies in the FEP, Prof. Jing-Jia Luo and his team, a group of researchers in the Institute for Climate and Application Research (ICAR) of Nanjing University of Information Science and Technology (NUIST), investigated the underlying mechanisms behind the dramatic difference in FEP SST anomalies between the two extreme El Niño events. Advances in Atmospheric Sciences recently published their findings.

The study suggests that the modestly warm FEP SST anomalies during the 2015/16 El Niño are closely related to the strong southeasterly wind anomalies that existed at the same time in the southeastern Pacific. These irregular wind patterns resulted from warm SST anomalies in the northeastern Pacific and the concurrent cold SST anomalies in the southeastern Pacific. Researchers also believe that decadal variability in ENSO phases may have also partially contributed to the strong southeasterly wind anomalies in the southeastern Pacific. Thus, ocean circulation and surface evaporation changes restrained the rapid growth of warm SST anomalies in the FEP.