Preprints
https://doi.org/10.5194/cp-2021-58
https://doi.org/10.5194/cp-2021-58

  03 Jun 2021

03 Jun 2021

Review status: this preprint is currently under review for the journal CP.

Reduced El Niño variability in the mid-Pliocene according to the PlioMIP2 ensemble

Arthur Merlijn Oldeman1, Michiel L. J. Baatsen1, Anna S. von der Heydt1,2, Henk A. Dijkstra1,2, Julia C. Tindall3, Ayako Abe-Ouchi4, Alice R. Booth5, Esther C. Brady6, Wing-Le Chan4, Deepak Chandan7, Mark A. Chandler8, Camille Contoux9, Ran Feng10, Chuncheng Guo11, Alan M. Haywood3, Stephen J. Hunter3, Youichi Kamae12, Qiang Li13, Xiangyu Li14, Gerrit Lohmann15, Daniel J. Lunt16, Kerim H. Nisancioglu11, Bette L. Otto-Bliesner6, W. Richard Peltier7, Gabriel M. Pontes17, Gilles Ramstein9, Linda E. Sohl8, Christian Stepanek15, Ning Tan9,18, Qiong Zhang13, Zhongshi Zhang14, Ilana Wainer17, and Charles J. R. Williams16,19 Arthur Merlijn Oldeman et al.
  • 1Institute for Marine and Atmospheric research Utrecht (IMAU), Department of Physics, Utrecht University, Utrecht, The Netherlands
  • 2Centre for Complex Systems Science, Utrecht University, Utrecht, The Netherlands
  • 3School of Earth and Environment, University of Leeds, Woodhouse Lane, Leeds, West Yorkshire, LS29JT, UK
  • 4Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, 277-8564, Japan
  • 5School of Ocean and Earth Science, University of Southampton, National Oceanography Centre, Southampton, UK
  • 6National Center for Atmospheric Research, (NCAR), Boulder, CO 80305, USA
  • 7Department of Physics, University of Toronto, Toronto, M5S 1A7, Canada
  • 8CCSR/GISS, Columbia University, New York, NY 10025, USA
  • 9Laboratoire des Sciences du Climat et de l’Environnement, LSCE/IPSL, CEA-CNRS-UVSQ Université Paris-Saclay, 91191 Gif-sur-Yvette, France
  • 10Department of Geosciences, College of Liberal Arts and Sciences, University of Connecticut, Storrs, CT 06033, USA
  • 11NORCE Norwegian Research Centre, Bjerknes Centre for Climate Research, 5007 Bergen, Norway
  • 12Faculty of Life and Environmental Sciences, University of Tsukuba, Tsukuba, 305-8572, Japan
  • 13Department of Physical Geography and Bolin Centre for Climate Research, Stockholm University, Stockholm, 10691, Sweden
  • 14Department of Atmospheric Science, School of Environmental studies, China University of Geoscience, Wuhan 430074, China
  • 15Alfred-Wegener-Institut – Helmholtz-Zentrum für Polar and Meeresforschung (AWI), Bremerhaven, 27570, Germany
  • 16School of Geographical Sciences, University of Bristol, Bristol, BS8 1QU, UK
  • 17Oceanographic Institute, University of São Paulo, Brazil
  • 18Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
  • 19NCAS-Climate, Department of Meteorology, University of Reading, Reading, UK

Abstract. The mid-Pliocene warm period (3.264–3.025 Ma) is the most recent geological period during which atmospheric CO2 levels were similar to recent historical values (~400 ppm). Several proxy reconstructions for the mid-Pliocene show highly reduced zonal sea surface temperature (SST) gradients in the tropical Pacific Ocean, indicating an El Niño-like mean state. However, past modelling studies do not show these highly reduced gradients. Efforts to understand mid-Pliocene climate dynamics have led to the Pliocene Model Intercomparison Project (PlioMIP). Results from the first phase (PlioMIP1) showed clear El Niño variability (albeit significantly reduced) and did not show the greatly reduced time-mean zonal SST gradient suggested by some of the proxies.

In this work, we study El Niño-Southern Oscillation (ENSO) variability in the PlioMIP2 ensemble, which consists of additional global coupled climate models and updated boundary conditions compared to PlioMIP1. We quantify ENSO amplitude, period, spatial structure and flavour, as well as the tropical Pacific annual mean state in mid-Pliocene and pre-industrial simulations. Results show a reduced ENSO amplitude in the model-ensemble mean (−24 %) with respect to the pre-industrial, with 15 out of 17 individual models showing such a reduction. Furthermore, the spectral power of this variability considerably decreases in the 3–4 year band. The spatial structure of the dominant empirical orthogonal function shows no particular change in the patterns of tropical Pacific variability in the model-ensemble mean, compared to the pre-industrial. Although the time-mean zonal SST gradient in the equatorial Pacific decreases for 14 out of 17 models (0.2 °C reduction in the ensemble mean), there does not seem to be a correlation with the decrease in ENSO amplitude. The models showing the most ‘El Niño-like’ mean state changes show a similar ENSO amplitude as in the pre-industrial reference, while models showing more ‘La Niña-like’ mean state changes generally show a large reduction in ENSO variability. The PlioMIP2 results show a reasonable agreement both with time-mean proxies indicating a reduced zonal SST gradient, as well as reconstructions indicating a reduced, or similar, ENSO variability.

Arthur Merlijn Oldeman et al.

Status: open (until 29 Jul 2021)

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Arthur Merlijn Oldeman et al.

Arthur Merlijn Oldeman et al.

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Short summary
In this work, we have studied the behaviour of El Niño events in the mid-Pliocene, a period of around three million years ago, using a collection of seventeen climate models. It is an interesting period to study, as it saw similar atmospheric carbon dioxide levels as the present-day. We find that the El Niño events were less strong in the mid-Pliocene simulations, when compared to pre-industrial climate. Our results could help to interpret El Niño behaviour in future climate projections.