Late-twentieth-century emergence of the El Niño propagation asymmetry and future projections

• Published in: Nature (London) 2013-12, Vol.504 (7478), p.126-130
• Main Authors: Santoso, Agus, McGregor, Shayne, Jin, Fei-Fei, Cai, Wenju, England, Matthew H., An, Soon-Il, McPhaden, Michael J., Guilyardi, Eric
• Format: Article
• Language: English
• Subjects: 704/106/694/2786; 704/106/829/2737; Asymmetry; Budgets; Climate change; Climate models; Climate variability; Computer Simulation; Cooling; Earth, ocean, space; El Nino; El Nino-Southern Oscillation - history; Environmental aspects; Exact sciences and technology; External geophysics; Forecasts and trends; Geophysics; Global temperature changes; Global Warming; Heat; History, 20th Century; Humanities and Social Sciences; La Nina; letter; multidisciplinary; Ocean currents; Pacific Ocean; Physics; Physics of the oceans; Precipitation; Propagation; Science; Sea surface temperature; Sea-air exchange processes; Seasons; Southern Oscillation; Upper ocean; Water Movements; Weather
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature12683

The El Niño propagation asymmetry (in which sea surface temperature anomalies propagate eastwards during an extreme El Niño event) is shown to be caused by the variations in upper ocean currents in the equatorial Pacific Ocean; increased occurrences of the propagation asymmetry may be a manifestation of global greenhouse warming. Equatorial Pacific current and El Niño/La Niña asymmetry The extreme El Niño climate events of 1982/83 and 1997/98 were associated with eastward-propagating warm anomalies and led to highly unusual weather events and widespread environmental disruption. By contrast, during moderate El Niños unusually warm waters persist in the eastern equatorial Pacific and La Niña upwellings of cool, deep waters are more evident, both features tending to propagate westward. A new analysis now shows that this propagation asymmetry can be linked to variations in upper ocean currents, and that the asymmetry is stronger in a warming world as the equatorial currents and the trade winds are expected to weaken. In scenarios of future warming, the eastward-propagating El Niños double in frequency. The El Niño/Southern Oscillation (ENSO) is the Earth’s most prominent source of interannual climate variability, exerting profound worldwide effects 1 , 2 , 3 , 4 , 5 , 6 , 7 . Despite decades of research, its behaviour continues to challenge scientists. In the eastern equatorial Pacific Ocean, the anomalously cool sea surface temperatures (SSTs) found during La Niña events and the warm waters of modest El Niño events both propagate westwards, as in the seasonal cycle 7 . In contrast, SST anomalies propagate eastwards during extreme El Niño events, prominently in the post-1976 period 7 , 8 , 9 , 10 , spurring unusual weather events worldwide with costly consequences 3 , 4 , 5 , 6 , 11 . The cause of this propagation asymmetry is currently unknown 10 . Here we trace the cause of the asymmetry to the variations in upper ocean currents in the equatorial Pacific, whereby the westward-flowing currents are enhanced during La Niña events but reversed during extreme El Niño events. Our results highlight that propagation asymmetry is favoured when the westward mean equatorial currents weaken, as is projected to be the case under global warming 12 , 13 , 14 . By analysing past and future climate simulations of an ensemble of models with more realistic propagation, we find a doubling in the occurrences of El Niño events that feature prominent eastward propagation characte