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Learning From an Extreme El Niño

Agus Santosoon, EOS

A recent paper in Reviews of Geophysics describes new scientific insights on the El Niño-Southern Oscillation from analysis of the 2015-2016 extreme El Niño.

The El Niño-Southern Oscillation (ENSO) cycle is a periodic change in the ocean-atmosphere system of the tropical Pacific Ocean but which has knock-on effects on weather around the world. In an article recently published in Reviews in GeophysicsSantoso et al. [2017] examined the 2015-2016 El Niño event which was particularly extreme. The editor asked one of the authors to explain the characteristics of that El Niño and what scientists learned from it.

Why was the El Niño event of 2015-2016 particularly significant?

The 2015-2016 event was a particularly strong El Niño, comparable to the extreme 1997-1998 El Niño which was dubbed “the climate event of the 20th century”. It developed rapidly and had spectacular climate impacts worldwide. It also followed on the heels of new research that found the frequency of extreme El Niños will likely double in the 21st century if greenhouse gas emissions continue unabated.

Another reason why the event was significant is that we previously had observed only two extreme El Niños with any sort of detail in our short instrumental record: the 1997-1998 and the 1982-1983 events. These two El Niños shared similar key characteristics with each other. Thus, observing another extreme event was very valuable for testing the current state of our understanding about extreme El Niños. Nature did not cooperate completely though, since it turned out that the 2015-2016 El Niño had some important difference from its extreme predecessors. It was a monster in its own special way.

Sea level anomalies in November 1997 (left) and November 2015 (right) highlight the comparable strength between the two extreme El Niño events with apparent differences in pattern. Credit: AVISO/CNES/CLS 2015

How were the characteristics of that El Niño different from ones observed previously?

Tropical Pacific sea surface temperature (SST) anomalies (in other words, SST deviations from normal) are the most commonly used metric to compare El Niño events.

For the 2015-2016 El Niño, the SST anomaly peaked toward the central equatorial Pacific, whereas the 1982-1983 and 1997-1998 events peaked closer to the South American coast. Consequently, the 2015-2016 El Niño exhibited anomalously higher rainfall in the central Pacific than in the eastern Pacific, which is opposite to the 1982-1983 and 1997-1998 patterns.

Rainfall anomalies for the average of 1982/83 and 1997/98 El Niño (left), 2015/16 El Niño (center), and an average weak El Niño event (right). Credit: Santoso et al., 2017, Figure 15abc

In fact, SSTs and rainfall in the central Pacific broke records in 2015-2016. Also, even though the 2015-2016 eastern Pacific rainfall was not as high as during the 1982-1983 and 1997-1998 events, it was still substantially higher than in other garden-variety El Niño events. Another interesting feature is that the 2015-2016 event did not show a clear west to east migration of SST anomalies along the equator, which is a unique characteristic of the previous extreme El Niños.

What new insights did the 2015-2016 events give into the nature of extreme El Niño?

We generally classify ENSO events into Eastern-Pacific and Central-Pacific types (based on the peak region of the SST anomalies). According to this classification, the 1982-1983 and 1997-1998 events were strongly of the Eastern Pacific type. All La Niñas and moderate El Niños tend to be classified as Central Pacific events. The 2015-2016 El Niño was somewhat a mixture of the two. It was similar to the strong 1972-1973 El Niño but more intense in many ways.

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