Cross-equatorial winds control El Niño diversity and change

Abstract

Over the past two decades, El Niño events have weakened on average and their sea surface temperature (SST) anomalies shifted westward towards the central Pacific. Moreover, the intertropical convergence zone (ITCZ), which typically migrates southward from its northerly position during El Niño events, has not crossed the Equator since 1998. The causes of these changes remain under debate1,2,3,4,5. Here, using in situ, satellite and atmospheric reanalysis data, we show they can be related to a multidecadal strengthening of cross-equatorial winds in the eastern Pacific. This gradual strengthening of meridional winds is unlikely to be caused by El Niño/Southern Oscillation (ENSO) changes, and contains signals forced both locally and from outside the tropical Pacific, probably from the tropical North Atlantic. Coupled model simulations in which the observed cross-equatorial wind strengthening is superimposed successfully reproduce the key features of the recent changes in tropical climate. In particular, the tropical mean state experiences a ‘La Niña-like’ change, the ENSO amplitude weakens by about 20%, the centre of the SST anomalies shifts westward and the ITCZ now rarely crosses the Equator. Thus, cross-equatorial winds are found to modulate tropical Pacific mean state and variability, with implications for quantifying projected changes in ENSO under anthropogenic warming.

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Fig. 1: Observed multidecadal changes in tropical Pacific mean state and ENSO.
Fig. 2: Observed trends and variations in cross-equatorial winds.
Fig. 3: Coupled model response to the superimposed cross-equatorial wind anomaly.
Fig. 4: Cross-equatorial winds and their projected changes (RCP8.5 (2066–2095) minus historical (1971–2000)) in CMIP5 models.

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Acknowledgements

This research was supported by grants to A.V.F. from the NSF (AGS- 0163807) and NASA (NNX17AH21G). S.H. was supported by a NASA Earth and Space Sciences Graduate Fellowship, and a Scripps Institutional Postdoctoral Fellowship. We also acknowledge computational support from the Yale University Faculty of Arts and Sciences High Performance Computing facility and from the NSF/NCAR Yellowstone Supercomputing Center.

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S.H. and A.V.F. contributed equally to designing the research and writing the manuscript. S.H. performed the data analysis and numerical simulations and, together with A.V.F., interpreted the results.

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Correspondence to Shineng Hu.

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Hu, S., Fedorov, A.V. Cross-equatorial winds control El Niño diversity and change. Nature Clim Change 8, 798–802 (2018). https://doi.org/10.1038/s41558-018-0248-0

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