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All flavours of El Niño have similar early subsurface origins

Abstract

The El Niño/Southern Oscillation phenomenon, characterized by anomalous sea surface temperatures and winds in the tropical Pacific, affects climate across the globe1. El Niños occur every 2–7 years, whereas the El Niño/Southern Oscillation itself varies on decadal timescales in frequency and amplitude, with a different spatial pattern of surface anomalies2 each time the tropical Pacific undergoes a regime shift. Recent work has shown that Bjerknes feedback3,4 (coupling of the atmosphere and the ocean through changes in equatorial winds driven by changes in sea surface temperature owing to suppression of equatorial upwelling in the east Pacific) is not necessary5 for the development of an El Niño. Thus it is unclear what remains constant through regimes and is crucial for producing the anomalies recognized as El Niño. Here we show that the subsurface process of discharging warm waters always begins in the boreal summer/autumn of the year before the event (up to 18 months before the peak) independent of regimes, identifying the discharge process as fundamental to the El Niño onset. It is therefore imperative that models capture this process accurately to further our theoretical understanding, improve forecasts and predict how the El Niño/Southern Oscillation may respond to climate change.

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Figure 1: SST anomalies for the El Niño composites calculated from SODA version 2.1.6 show that there is always a small warm anomaly in the west Pacific (130°–160° E, 0°–15° S, shown in red box) as early as June–August (−1).
Figure 2: Subsurface temperature anomalies of the El Niño composites from SODA version 2.1.6 calculated with respect to a long-term climatology show a warm SST anomaly in the west.
Figure 3: Subsurface temperature anomalies (averaged over 5° N–5° S) for the El Niño composites calculated with respect to a separate climatology for each period show that the warm anomaly spreads from the west in June–August (−1) along the thermocline to reach the east in March–May(0) in all three regimes.
Figure 4: Plotting August averages of each year on a scatter diagram shows that all El Niño(−1) years (red crosses) fall into the top-right quadrant, meaning they all have positive values of thermocline-depth anomaly in 10° N–10° S, 150° E–150° W and SST anomaly in 130°–160° E, 0°–15° S.

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Acknowledgements

This work was carried out at the Divecha Centre for Climate Change, Indian Institute of Science, Bangalore—560012, India, enabled by grants from the same. The authors would like to thank J. Beauchamp for providing the SODA, Global Ocean Data Assimilation System and Global High-Resolution SST data. N.R. would like to thank S. R. Parampil for assistance with the Ferret software.

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Analysis of the data, production of the figures and writing of the final manuscript was carried out by N.R. The project was supervised and directed by R.M. who interpreted results and also edited the final manuscript.

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Correspondence to Nandini Ramesh.

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The authors declare no competing financial interests.

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Ramesh, N., Murtugudde, R. All flavours of El Niño have similar early subsurface origins. Nature Clim Change 3, 42–46 (2013). https://doi.org/10.1038/nclimate1600

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