Why is 2014 El Niño not a super El Niño?
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Tuesday, 6 January 2015: 5:30 PM
122BC (Phoenix Convention Center - West and North Buildings)
Li-Ciao Hong, Research Center for Environmental Change, Taipei, Taiwan; and H. Lin and F. F. Jin
Signs similar to the condition before the onset of 1997 El Niño including the build-up of large positive upper-ocean heat content anomalies along the equatorial Pacific and strong westerly wind bursts over the western tropical Pacific emerge early in 2014, suggesting that there may be a super El Niño in the end of the year. Nevertheless, analysis of three past super El Niño events indicates that several other key preconditions are necessary to trigger a super El Niño. In all of known super El Niño cases, unique features like a “super-ma La Niña”, a weak La Nina yet driving a strong Walker cell, and a simultaneous emergence of anomalous low pressure systems over Hawaii and Philippine Sea facilitate favorable tropical Pacific air-sea conditions for the onset of a super El Niño. Furthermore, these preconditions set up an anomalous Southern Hemispheric transverse cell when a super El Niño onset. According to Hong et al. (2014), this anomalous transverse cell, a distinct feature during super El Niño's developing phase, serves as a booster to further amplify their growth. Thus, these events grow into super El Niños.
In this study, the precursors of a super El Niño and the Southern Hemisphere booster are examined and analyzed for the case of 2014. The results show that the super-ma La Niña did not occur in 2013 and two North Pacific low systems stayed still and did not migrate to the north of central equatorial Pacific. As a result, anomalous low-level westerlies are relative weak over the central Pacific and enhanced convection is symmetric to the equator when it onset, failing to establish a southern Hemisphere booster to further strengthen El Niño event in 2014 summer. It turns out that the growth of this event halted and started to weaken after June; the Niño-3.4 SST anomaly index even dropped to 0 °C in the first week of August. A probability prediction model is also utilized to show the probability of the occurrence of a super El Niño in 2014 is low.