Sea Surface Temperature in the Subtropical Pacific Boosted the 2015 El Niño and Hindered the 2016 La Niña

Abstract

After the quick decaying of the 2015 super El Niño, the predicted La Niña unexpectedly failed to materialize to the anticipated standard in 2016. Diagnostic analyses, as well as numerical experiments, showed that this ENSO evolution of the 2015 super El Niño and the hindered 2016 La Niña may be essentially caused by sea surface temperature anomalies (SSTAs) in the subtropical Pacific. The self-sustaining SSTAs in the subtropical Pacific tend to weaken the trade winds during boreal spring–summer, leading to anomalous westerlies along the equatorial region over a period of more than one season. Such long-lasting wind anomalies provide an essential requirement for ENSO formation, particularly before a positive Bjerknes feedback is thoroughly built up between the oceanic and atmospheric states. Besides the 2015 super El Niño and the hindered La Niña in 2016, there were several other El Niño and La Niña events that cannot be explained only by the oceanic heat content in the equatorial Pacific. However, the questions related to those eccentric El Niño and La Niña events can be well explained by suitable SSTAs in the subtropical Pacific. Thus, the leading SSTAs in the subtropical Pacific can be treated as an independent indicator for ENSO prediction, on the basis of the oceanic heat content inherent in the equatorial region. Because ENSO events have become more uncertain under the background of global warming and the Pacific decadal oscillation during recent decades, thorough investigation of the role of the subtropical Pacific in ENSO formation is urgently needed.

Southeastern Pacific Error Leads to Failed El Niño Forecasts

“…The cold SSTAs in the SEP region contributed to the anomalous easterly winds in the eastern equatorial Pacific, which suppressed the equatorial SST warming. Meanwhile, following the wind-evaporation-SST feedback, the cold SSTAs in the SEP could sustain themselves and penetrate into the equatorial Pacific (Min et al, 2015(Min et al, , 2017Su et al, 2018). The SSTAs in the SEP region can favor SSTAs development in the equatorial eastern Pacific and may lead to different flavors of El Niño (Min et al, 2017;Zhang et al, 2014).…”

Monitoring the pendulum between El Niño and La Niña events

“…Since the ENSO turned out to be an El Niño event after the summer of 2006 (figures S9 and S10), one may speculate that the present 2017/18 La Niña status could continue to decay during the following summer and could perhaps develop into a warming phase of the ENSO pendulum. However, several key factors can influence the ENSO formation, such as westerly wind bursts , Lian et al 2017 and subtropical Pacific SSTAs (Su et al 2018) in the late spring and early summer seasons. Hence, the final evolution of the present 2017/18 La Niña status in the future presents many uncertainties.…”

“…Perhaps the most interesting application of the ENSO pendulum is associated with the evolution of the 2014-2015 El Niño (figures 4(e)-(f); figures S11-S13). The development of an anticipated strong El Niño event was hindered in the boreal summer of 2014 (Menkes et al 2014, Min et al 2015, Hu and Fedorov 2016, Su et al 2018. The SSTAs in the Niño-3 region warmed up to above 0.5 • C, the SSTAs in the Niño-3.4 were approximately 0.5 • C after May 2014, and the ENSO pendulum turned anticlockwise from its former phase of C7-C10 in May 2014 (figure 4(e); figure S11).…”

“…After March 2015, the SSTA warmed quickly in both the Niño-3 and Niño-3.4 regions (Lian et al 2017, Paek et al 2017, Su et al 2018. The ENSO pendulum began to turn anticlockwise in March 2015 to become a CP-type C10 and then a Cluster C11 in May 2015.…”

Placing the AD 2014–2016 ‘protracted’ El Niño episode into a long-term context

“…A number of papers have examined various aspects of the nature, characteristics and dynamical processes thought to underlie the 2014–2016 El Niño. Some, such as Menkes et al (2014), Min et al (2015), Hartmann (2015) and Maeda et al (2016), have focused on various potential causes of its development in 2014, others such as Lian et al (2017), Santoso et al (2017), Hu and Fedorov (2017) and Su et al (2018) have focused on its strong 2015–2016 signature, while Zhu et al (2016), L’Heureux et al (2017), Ineson et al (2018) and Santoso et al (2019) examined how well it was predicted. Peng et al (2018) examined this El Niño in terms of its influence on the Pacific and North American boreal winter of 2014–2015 and suggested that it was indicative of the operation of the North Pacific Mode (NPM) rather than ENSO.…”