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<div class=Section1>
<p class=MsoNormal><i>Nature </i><b>515</b>, 550–553<o:p></o:p></p>
<p class=MsoNormal>doi:10.1038/nature13963<o:p></o:p></p>
<p class=MsoNormal>Published online 26 November 2014<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal><b><span style='font-size:18.0pt;font-family:"Arial","sans-serif"'>Evolution
and forcing mechanisms of El Niño over the past 21,000 years<o:p></o:p></span></b></p>
<p class=MsoNormal><a
href="http://www.nature.com/nature/journal/v515/n7528/full/nature13963.html#auth-1"><span
class=fn><b><span style='color:#5C7996;text-decoration:none'>Zhengyu Liu</span></b></span></a>1
2<span class=comma><b>,</b></span> <a
href="http://www.nature.com/nature/journal/v515/n7528/full/nature13963.html#auth-2"><span
class=fn><b><span style='color:#5C7996;text-decoration:none'>Zhengyao Lu</span></b></span></a>3<span
class=comma><b>,</b></span> <a
href="http://www.nature.com/nature/journal/v515/n7528/full/nature13963.html#auth-3"><span
class=fn><b><span style='color:#5C7996;text-decoration:none'>Xinyu Wen</span></b></span></a>3<span
class=comma><b>,</b></span> <a
href="http://www.nature.com/nature/journal/v515/n7528/full/nature13963.html#auth-4"><span
class=fn><b><span style='color:#5C7996;text-decoration:none'>B. L.
Otto-Bliesner</span></b></span></a>4<span class=comma><b>,</b></span> <a
href="http://www.nature.com/nature/journal/v515/n7528/full/nature13963.html#auth-5"><span
class=fn><b><span style='color:#5C7996;text-decoration:none'>A. Timmermann</span></b></span></a>5
&<span class=apple-converted-space><b> </b></span><a
href="http://www.nature.com/nature/journal/v515/n7528/full/nature13963.html#auth-6"><span
class=fn><b><span style='color:#5C7996;text-decoration:none'>K. M. Cobb</span></b></span></a>6<o:p></o:p></p>
<p class=MsoNormal>1Department of Atmospheric and Oceanic Sciences and Nelson
Center for Climatic Research, University of Wisconsin-Madison, Madison,
Wisconsin 53706, USA<o:p></o:p></p>
<p class=MsoNormal>2Laboratory of Climate, Ocean and Atmosphere Studies, School
of Physics, Peking University, Beijing, 100871, China 3Climate and Global
Dynamics Division, National Center for Atmospheric Research, Boulder, Colorado
80307-3000, USA 4International Pacific Research Center and Department of
Oceanography, School of Ocean and Earth Science and Technology, University of
Hawaii at Manoa, Honolulu, Hawaii 96822, USA 5 School of Earth and Atmospheric
Sciences, Georgia 6 Institute of Technology, Atlanta, Georgia 30332, USA<o:p></o:p></p>
<p class=MsoNormal><span style='font-family:"Arial","sans-serif"'><o:p> </o:p></span></p>
<p class=MsoNormal style='margin-left:0in;text-indent:-.25in;mso-list:l3 level1 lfo1'><![if !supportLists]><span
style='font-size:10.0pt;font-family:Symbol'><span style='mso-list:Ignore'>·<span
style='font:7.0pt "Times New Roman"'>
</span></span></span><![endif]><b><span style='font-family:"Arial","sans-serif"'><o:p> </o:p></span></b></p>
<p style='mso-margin-top-alt:0in;margin-right:0in;margin-bottom:15.0pt;
margin-left:0in'><span style='font-family:"Arial","sans-serif"'>The El Niño
Southern Oscillation (ENSO) is Earth’s dominant source of interannual
climate variability, but its response to global warming remains highly uncertain</span><sup><span
style='font-size:9.0pt;font-family:"Arial","sans-serif"'><a
href="http://www.nature.com/nature/journal/v515/n7528/full/nature13963.html#ref1"
title="Collins, M. et al. The impact of global warming on the tropical Pacific ocean and El Nino. Nature Geosci. 3, 391-397 (2010)"
id=ref-link-15><span style='color:#5C7996;text-decoration:none'>1</span></a></span></sup><span
style='font-family:"Arial","sans-serif"'>. To improve our understanding of
ENSO’s sensitivity to external climate forcing, it is paramount to
determine its past behaviour by using palaeoclimate data and model simulations.
Palaeoclimate records show that ENSO has varied considerably since the Last
Glacial Maximum (21,000 years ago)</span><sup><span style='font-size:9.0pt;
font-family:"Arial","sans-serif"'><a
href="http://www.nature.com/nature/journal/v515/n7528/full/nature13963.html#ref2"
title="Moy, C. et al. Variability of El Nino/Southern Oscillation activity at millennial timescales during the Holocene epoch. Nature 420, 162-166 (2002)"
id=ref-link-16><span style='color:#5C7996;text-decoration:none'>2</span></a>,<span
class=apple-converted-space> </span><a
href="http://www.nature.com/nature/journal/v515/n7528/full/nature13963.html#ref3"
title="Tudhope, A. et al. Variability in the El Nino-Southern Oscillation through a glacial-interglacial cycle. Science 291, 1511-1517 (2001)"
id=ref-link-17><span style='color:#5C7996;text-decoration:none'>3</span></a>,<span
class=apple-converted-space> </span><a
href="http://www.nature.com/nature/journal/v515/n7528/full/nature13963.html#ref4"
title="Riedinger, M. et al. A [sim]6100 14C yr record of El Nino activity from the Galapagos islands. J. Paleolimnol. 27, 1-7 (2002)"
id=ref-link-18><span style='color:#5C7996;text-decoration:none'>4</span></a>,<span
class=apple-converted-space> </span><a
href="http://www.nature.com/nature/journal/v515/n7528/full/nature13963.html#ref5"
title="Koutavas, A. & Joanides, S. El Nino-Southern Oscillation extrema in the Holocene and Last Glacial Maximum. Paleoceanography 27 PA4208 http://dx.doi.org/10.1029/2012PA002378 (2012)"
id=ref-link-19><span style='color:#5C7996;text-decoration:none'>5</span></a>,<span
class=apple-converted-space> </span><a
href="http://www.nature.com/nature/journal/v515/n7528/full/nature13963.html#ref6"
title="Cobb, K. et al. Highly variable El Nino-Southern Oscillation throughout the Holocene. Science 339, 67-70 (2013)"
id=ref-link-20><span style='color:#5C7996;text-decoration:none'>6</span></a>,<span
class=apple-converted-space> </span><a
href="http://www.nature.com/nature/journal/v515/n7528/full/nature13963.html#ref7"
title="Conroy, J. et al. Holocene changes in eastern tropical Pacific climate inferred from a Galapagos lake sediment record. Quat. Sci. Rev. 27, 1166-1180 (2008)"
id=ref-link-21><span style='color:#5C7996;text-decoration:none'>7</span></a>,<span
class=apple-converted-space> </span><a
href="http://www.nature.com/nature/journal/v515/n7528/full/nature13963.html#ref8"
title="Rein, B. et al. El Nino variability off Peru during the last 20,000 years. Paleoceanography 20 PA4003 http://dx.doi.org/10.1029/2004PA001099 (2005)"
id=ref-link-22><span style='color:#5C7996;text-decoration:none'>8</span></a>,<span
class=apple-converted-space> </span><a
href="http://www.nature.com/nature/journal/v515/n7528/full/nature13963.html#ref9"
title="Sadekov, A. et al. Paleoclimate reconstructions reveal a strong link between El Nino-Southern Oscillation and tropical Pacific mean state. Nature Commun. 4 2692 http://dx.doi.org/10.1038/ncomms3692 (2013)"
id=ref-link-23><span style='color:#5C7996;text-decoration:none'>9</span></a></span></sup><span
style='font-family:"Arial","sans-serif"'>, and some data sets suggest a gradual
intensification of ENSO over the past ~6,000 years</span><sup><span
style='font-size:9.0pt;font-family:"Arial","sans-serif"'><a
href="http://www.nature.com/nature/journal/v515/n7528/full/nature13963.html#ref2"
title="Moy, C. et al. Variability of El Nino/Southern Oscillation activity at millennial timescales during the Holocene epoch. Nature 420, 162-166 (2002)"
id=ref-link-24><span style='color:#5C7996;text-decoration:none'>2</span></a>,<span
class=apple-converted-space> </span><a
href="http://www.nature.com/nature/journal/v515/n7528/full/nature13963.html#ref5"
title="Koutavas, A. & Joanides, S. El Nino-Southern Oscillation extrema in the Holocene and Last Glacial Maximum. Paleoceanography 27 PA4208 http://dx.doi.org/10.1029/2012PA002378 (2012)"
id=ref-link-25><span style='color:#5C7996;text-decoration:none'>5</span></a>,<span
class=apple-converted-space> </span><a
href="http://www.nature.com/nature/journal/v515/n7528/full/nature13963.html#ref7"
title="Conroy, J. et al. Holocene changes in eastern tropical Pacific climate inferred from a Galapagos lake sediment record. Quat. Sci. Rev. 27, 1166-1180 (2008)"
id=ref-link-26><span style='color:#5C7996;text-decoration:none'>7</span></a>,<span
class=apple-converted-space> </span><a
href="http://www.nature.com/nature/journal/v515/n7528/full/nature13963.html#ref8"
title="Rein, B. et al. El Nino variability off Peru during the last 20,000 years. Paleoceanography 20 PA4003 http://dx.doi.org/10.1029/2004PA001099 (2005)"
id=ref-link-27><span style='color:#5C7996;text-decoration:none'>8</span></a></span></sup><span
style='font-family:"Arial","sans-serif"'>. Previous attempts to simulate the
transient evolution of ENSO have relied on simplified models</span><sup><span
style='font-size:9.0pt;font-family:"Arial","sans-serif"'><a
href="http://www.nature.com/nature/journal/v515/n7528/full/nature13963.html#ref10"
title="Clement, A., Seager, R. & Cane, M. Suppression of El Nino during the Mid-Holocene by changes in the Earth/'s orbit. Paleoceanography 15, 731-737 (2000)"
id=ref-link-28><span style='color:#5C7996;text-decoration:none'>10</span></a></span></sup><span
class=apple-converted-space><span style='font-family:"Arial","sans-serif"'> </span></span><span
style='font-family:"Arial","sans-serif"'>or snapshot</span><sup><span
style='font-size:9.0pt;font-family:"Arial","sans-serif"'><a
href="http://www.nature.com/nature/journal/v515/n7528/full/nature13963.html#ref11"
title="Roberts, W. An Investigation into the Causes for the Reduction in the Variability of the El Nino-Southern Oscillation in the Early Holocene in a Global Climate Model. PhD thesis, Univ. Washington. (2007)"
id=ref-link-29><span style='color:#5C7996;text-decoration:none'>11</span></a>,<span
class=apple-converted-space> </span><a
href="http://www.nature.com/nature/journal/v515/n7528/full/nature13963.html#ref12"
title="Liu, Z., Kutzbach, J. & Wu, L. Modeling climatic shift of El Nino variability in the Holocene. Geophys. Res. Lett. 27, 2265-2268 (2000)"
id=ref-link-30><span style='color:#5C7996;text-decoration:none'>12</span></a>,<span
class=apple-converted-space> </span><a
href="http://www.nature.com/nature/journal/v515/n7528/full/nature13963.html#ref13"
title="Otto-Bliesner, B. et al. Modeling El Nino and its tropical teleconnections during the glacial-interglacial cycle. Geophys. Res. Lett. 30 10.1029/2003GL08553 (2003)"
id=ref-link-31><span style='color:#5C7996;text-decoration:none'>13</span></a></span></sup><span
class=apple-converted-space><span style='font-family:"Arial","sans-serif"'> </span></span><span
style='font-family:"Arial","sans-serif"'>experiments. Here we analyse a series
of transient Coupled General Circulation Model simulations forced by changes in
greenhouse gasses, orbital forcing, the meltwater discharge and the ice-sheet
history throughout the past 21,000 years. Consistent with most palaeo-ENSO
reconstructions, our model simulates an orbitally induced strengthening of ENSO
during the Holocene epoch, which is caused by increasing positive
ocean–atmosphere feedbacks. During the early deglaciation, ENSO
characteristics change drastically in response to meltwater discharges and the
resulting changes in the Atlantic Meridional Overturning Circulation and
equatorial annual cycle. Increasing deglacial atmospheric CO</span><sub><span
style='font-size:9.0pt;font-family:"Arial","sans-serif"'>2</span></sub><span
class=apple-converted-space><span style='font-family:"Arial","sans-serif"'> </span></span><span
style='font-family:"Arial","sans-serif"'>concentrations tend to weaken ENSO,
whereas retreating glacial ice sheets intensify ENSO. The complex evolution of
forcings and ENSO feedbacks and the uncertainties in the reconstruction further
highlight the challenge and opportunity for constraining future ENSO responses.<o:p></o:p></span></p>
<p class=MsoNormal><o:p> </o:p></p>
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