The El Niño events of 2015 and 2016, heralded by similar early signs, had very divergent amplitudes. Their modelling highlights the importance of unpredictable high-frequency winds. The ability to predict the phenomenon is therefore severely limited.
Chance sometimes makes light of science. It appears that its influence belied predictions of the occurrence of a major El Niño event in 2015, which ultimately occurred in 2016… “On that occasion, we rediscovered the importance of stochastic events, which shape the magnitude of the climate-related phenomenon”, says oceanographer-climatologist Matthieu Lengaigne. He is the co-author of a recent publication illustrating this surprising discovery (1). The El Niño phenomenon corresponds with an episodic anomaly affecting the climate of the Pacific and surrounding regions. Ordinarily, a large warm water mass remains concentrated in the western part of the ocean and the Indonesian region. It is kept in this position by trade winds, i.e. steady easterly winds found in the tropics.
Drought in the west, floods in the east
This balance is disrupted in years when El Niño events occur. Trade winds ease and warm water reaches the centre of the ocean, even its eastern edge. A number of consequences ensue, hardly suited to anthropic activities, including an eastward shift of seasonal rainfall, causing droughts in Asia and Oceania and leaving South America drenched in torrential rain. Similarly, the arrival of this warm water mass on the coasts disrupts the usual rise of nutrients from the depths to the surface. This impoverishment of the environment affects the abundance of marine communities and places a heavy burden on the fishing activity, which is crucial in coastal countries. Scientists analysed and modelled the mechanisms involved in the occurrence of El Niño events. They learned to recognise early warning signs one year in advance.
Expected historical anomaly
“All warning lights were flashing for the imminence of a major phenomenon in 2015, explains the specialist. There was every indication that an El Niño event of exceptional magnitude would occur, similar to those of 1982 and 1997”. Of course, scientists alerted the international community, as everyone remembered the devastating impact of historical El Niño events, which led to droughts, crop failures and forest fires in the western basin, as well as floods and a decline in fish stocks in the east.
However, contrary to scientific forecasts, 2015 was not marked by a large-scale phenomenon, far from it... Smarting from this snub, the experts hardly dared raise the possibility of a strong El Niño event the following year, in 2016, despite the clarity of early signs similar to those of 2015. And as if to taunt them, the phenomenon took historic proportions in 2016?One of the strongest events recorded. “Obviously, our research failed to cover all factors which contributed to the magnitude of the anomaly”, admits the researcher.
Scientists resumed their research and highlighted the role of high-frequency winds ?Blowing for brief periods compared to the regularity of the trade winds blowing from the Indian Ocean to the Pacific. Pushing the warm water mass to the centre and east of the basin, they are likely to reinforce the effects of a nascent El Niño event and, where relevant, turn it into a major event. They did not blow in 2015, which explains why the phenomenon remained moderate. In 2016 however, they enhanced its impact. To confirm this assumption, researchers modelled prevailing conditions at the beginning of each period, and added the action of westerly winds of various intensities. The result is clear: these winds are actually capable, depending on their strength, to minimise or enhance the magnitude of the El Niño anomaly. “More importantly, the impact of these winds, relating to weather and climate conditions, on the Indian and Pacific oceans is absolutely unpredictable, says the specialist. Chance should be factored in when predicting El Niño events”.
1. Puy M. , Vialard J., Lengaigne M., Guilyardi E., DiNezio P. N., Voldoire A., Balmaseda M., Madec G., Menkes C., Mcphaden M. J., Influence of Westerly Wind Events stochasticity on El Niño amplitude: the case of 2014 vs. 2015, Climate Dynamics, 2017