On average, El Niño events have weakened and the centre of maximum sea surface temperature anomalies has shifted to the west over the past two decades. New research suggests that the strengthening of cross-equatorial winds in the eastern Pacific can cause these changes.
El Niño events have dramatic impacts on global weather and climate. In 1997/1998, for example, the strongest observed El Niño event resulted in 22,000 fatalities and US$36 billion in economic losses worldwide, including catastrophic flooding in Latin America and California, and widespread drought in Southeast Asia. Such impacts are dependent on the diversity of the spatial structure and amplitude of El Niño, which in recent years have seen a weakening and westward shift of maximum sea surface temperature (SST) anomalies (Fig. 1). However, the physical processes responsible for this change in El Niño diversity have been widely debated. Writing in Nature Climate Change, Shineng Hu and Alexey Fedorov now report that such changes can be linked to a strengthening of cross-equatorial winds in the eastern Pacific, forced both locally and remotely1.
El Niño — the leading mode of global climate variability — describes an anomalous pattern of SST in the tropical Pacific, occurring every 2–7 years through coupled atmosphere–ocean processes. Typically, El Niño events are characterized by anomalous warming in the eastern tropical Pacific Ocean. In recent decades, however, central Pacific El Niño events (in which SST anomalies are located in the central Pacific) have become increasingly frequent2, affecting atmospheric teleconnections and their subsequent global temperature and precipitation patterns3,4.
As a result, the climate community has given considerable attention to understanding central Pacific El Niño in terms of its mechanisms, influence on weather and climate variability, prediction in climate models, future changes under global warming and (perhaps most importantly) their apparent increasing frequency (Fig. 1b,c). Two hypotheses have so far been proposed to explain this phenomenon: (1) a natural decadal-scale La Niña-like mean state change5 (Fig. 1a) that favours the zonal advective feedback processes that drive central Pacific El Niño events, and/or (2) anthropogenic processes6 that cause the shallowing of the equatorial thermocline in the central tropical Pacific, leading to central Pacific El Niño.
Using numerous observational datasets and coupled model simulations, Hu and Fedorov1 offer a new hypothesis that emphasizes the strengthening of southerly winds extending from the southeastern to the northeastern subtropical Pacific (that is, cross-equatorial winds) in driving the observed change in El Niño diversity. First they show that the intertropical convergence zone (ITCZ), which typically migrates from its northerly position to the Southern Hemisphere during El Niño events, has not crossed the Equator since 1998 — even during the extreme El Niño event of 2015. As the position of the ITCZ is mainly determined by the intensity of cross-equatorial winds, this absence of crossings hints at their strengthening, which acts to maintain cool SSTs in the eastern tropical Pacific through enhanced upwelling and increased latent heat flux to the atmosphere. Consequently, the rise of anomalously warm SSTs is hindered in the eastern tropical Pacific, pushing the centre of maximum anomalous SST westwards. This hypothesis is tested by conducting idealized model experiments in which observed cross-equatorial wind anomalies are superimposed, revealing an increasing ratio of central Pacific El Niño occurrences, as well as the reduction of El Niño/Southern Oscillation variability to the east of the dateline.
This result raises the important question of what causes the strengthening of the cross-equatorial winds in the eastern tropical Pacific. Hu and Fedorov suggest that this can be linked to local processes associated with the meridional SST gradient in the eastern tropical Pacific and the warming of the tropical North Atlantic through atmospheric teleconnections3,7. For example, warming of the tropical North Atlantic induces both zonal and meridional wind changes over the tropical Pacific by perturbing the local Walker circulation. This indicates that understanding the enhancement of coupled Pacific–Atlantic processes8 — which may occur due to natural variability, anthropogenic forcing, or their combined influences — is important to explain observed and projected changes to El Niño diversity.
In highlighting the role of cross-equatorial winds in forcing a westward shift of El Niño SST anomalies, it becomes apparent that further study is needed to determine the relationships between the position of the ITCZ and El Niño diversity, both for the present and for future climates. It is known that the meridional movement of the ITCZ is a complex process, largely influenced by the thermal contrast between hemispheres9. Unfortunately, projections of future changes in cross-equatorial winds remain highly uncertain in the Coupled Model Intercomparison Project (CMIP) phase 5 climate models due to the double-ITCZ problem10: a long-standing tropical model bias in which excessive precipitation is produced off both sides of the Equator, thus forming two bands of convergence. As a consequence, CMIP5 models underestimate cross-equatorial winds in the eastern Pacific, resulting in incorrect simulation of the mean state of the tropical Pacific, and thus El Niño diversity. Targeting work to minimize such biases will therefore improve projections of future El Niño.
Despite the short observational datasets and model biases, the hypothesis advanced by Hu and Fedorov to explain observed changes in El Niño diversity outlines the importance of understanding interactions between ocean basins and the hemispheres to correctly understand how El Niño may change under anthropogenic forcing.
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Yeh, SW. Winds shift El Niño flavour. Nature Clim Change 8, 766–767 (2018). https://doi.org/10.1038/s41558-018-0261-3