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<DIV>Nature </DIV>
<DIV>Volume: 504<SPAN>, </SPAN>Pages: 126–130</DIV>
<DIV class=published>doi:10.1038/nature12683</DIV>
<DIV class="received first">Published online 17 November 2013 </DIV>
<DIV><A
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html?WT.ec_id=NATURE-20131205">http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html?WT.ec_id=NATURE-20131205</A></DIV>
<H1 class=article-heading>Late-twentieth-century emergence of the El Niño
propagation asymmetry and future projections</H1>
<DIV class="vcard c1"><A class=name
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html?WT.ec_id=NATURE-20131205#auth-1"><SPAN
class=fn>Agus Santoso</SPAN></A><SUP><A
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#a1">1</A></SUP>
<A class=name
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html?WT.ec_id=NATURE-20131205#auth-2"><SPAN
class=fn>Shayne McGregor</SPAN></A><SUP><A
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#a1">1</A></SUP>
<A class=name
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html?WT.ec_id=NATURE-20131205#auth-3"><SPAN
class=fn>Fei-Fei Jin</SPAN></A><SUP><A
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#a2">2</A></SUP>
<A class=name
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html?WT.ec_id=NATURE-20131205#auth-4"><SPAN
class=fn>Wenju Cai</SPAN></A><SUP><A
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#a3">3</A></SUP>
<A class=name
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html?WT.ec_id=NATURE-20131205#auth-5"><SPAN
class=fn>Matthew H. England</SPAN></A><SUP><A
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#a1">1</A></SUP>
<A class=name
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html?WT.ec_id=NATURE-20131205#auth-6"><SPAN
class=fn>Soon-Il An</SPAN></A><SUP><A
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#a4">4</A></SUP>
<A class=name
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html?WT.ec_id=NATURE-20131205#auth-7"><SPAN
class=fn>Michael J. McPhaden</SPAN></A><SUP><A
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#a5">5</A></SUP>
<A class=name
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html?WT.ec_id=NATURE-20131205#auth-8"><SPAN
class=fn>Eric Guilyardi</SPAN></A><SUP><A
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#a6">6</A>,
<A
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#a7">7</A></SUP>
</TIME></DIV><ASIDE>
<DIV>1 Australian Research Council (ARC) Centre of Excellence for Climate System
Science and Climate Change Research Centre, Level 4 Mathews Building, The
University of New South Wales, Sydney 2052, Australia 2 Department of
Meteorology, School of Ocean and Earth Science and Technology (SOEST),
University of Hawaii at Manoa, 2525 Correa Road, Honolulu, Hawaii 96822, USA 3
Commonwealth Scientific and Industrial Research Organisation (CSIRO), Marine and
Atmospheric Research, 107-121 Station Street, Aspendale, Victoria 3195,
Australia 4 Department of Atmospheric Sciences, Yonsei University, 50 Yonsei-ro,
Seodaemun-Gu, Seoul 120-749, South Korea 5 National Oceanic and Atmospheric
Administration (NOAA)/Pacific Marine Environmental Laboratory, Seattle,
Washington 98115, USA 6 Laboratoire d’Océanographie et du Climat:
Expérimentation et Approches Numériques/Institut Pierre Simon Laplace
(IPSL)/Centre national de la recherché scientifique (CNRS), tour 45-55, étage 4,
pièce 406, Université Pierre et Marie Curie, 4 Jussieu, 75252 Paris Cedex 05,
France 7 National Centre for Atmospheric Science (NCAS)—Climate, Department of
Meteorology, University of Reading, Earley Gate, Reading RG6 6BB, UK</DIV>
<DIV> </DIV></ASIDE></HEADER><SECTION>
<DIV class="content ">
<P class=first-paragraph-abs>SUMMARY</P>
<P class=first-paragraph-abs>The El Niño/Southern Oscillation (ENSO) is the
Earth’s most prominent source of interannual climate variability, exerting
profound worldwide effects<SUP><A id=ref-link-2
title="McPhaden, M. J., Zebiak, S. E. & Glantz, M. H. ENSO as an integrating concept in Earth science. Science 314, 1740-1745 (2006)"
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#ref1">1</A>,
<A id=ref-link-3
title="Lehodey, P., Bertignac, M., Hampton, J., Lewis, A. & Picaut, J. El Nino/Southern Oscillation and tuna in the western Pacific. Nature 389, 715-718 (1997)"
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#ref2">2</A>,
<A id=ref-link-4
title="Bove, M. C., O'Brien, J. J., Eisner, J. B., Landsea, C. W. & Niu, X. Effect of El Nino on U.S. landfalling hurricanes, revisited. Bull. Am. Meteorol. Soc. 79, 2477-2482 (1998)"
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#ref3">3</A>,
<A id=ref-link-5
title="Wilhite, D. A., Wood, D. A. & Meyer, S. J. in Climate Crisis (eds Glantz, M., Katz, R. & Krenz, M.) 75-78 (UNEP, 1987)"
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#ref4">4</A>,
<A id=ref-link-6
title="Changnon, S. A. Impacts of 1997[mdash]98 El Nino generated weather in the United States. Bull. Am. Meteorol. Soc. 80, 1819-1827 (1999)"
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#ref5">5</A>,
<A id=ref-link-7
title="Liu, Z. & Alexander, M. Atmospheric bridge, oceanic tunnel, and global climatic teleconnections. Rev. Geophys. 45, RG2005, http://dx.doi.org/10.1029/2005RG000172 (2007)"
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#ref6">6</A>,
<A id=ref-link-8
title="Wallace, J. M. et al. On the structure and evolution of ENSO-related climate variability in the tropical Pacific: lessons from TOGA. J. Geophys. Res. 103, 14241-14259 (1998)"
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#ref7">7</A></SUP>.
Despite decades of research, its behaviour continues to challenge scientists. In
the eastern equatorial Pacific Ocean, the anomalously cool sea surface
temperatures (SSTs) found during La Niña events and the warm waters of modest El
Niño events both propagate westwards, as in the seasonal cycle<SUP><A
id=ref-link-9
title="Wallace, J. M. et al. On the structure and evolution of ENSO-related climate variability in the tropical Pacific: lessons from TOGA. J. Geophys. Res. 103, 14241-14259 (1998)"
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#ref7">7</A></SUP>.
In contrast, SST anomalies propagate eastwards during extreme El Niño events,
prominently in the post-1976 period<SUP><A id=ref-link-10
title="Wallace, J. M. et al. On the structure and evolution of ENSO-related climate variability in the tropical Pacific: lessons from TOGA. J. Geophys. Res. 103, 14241-14259 (1998)"
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#ref7">7</A>,
<A id=ref-link-11
title="Wang, B. & An, S.-I. A mechanism for decadal changes of ENSO behaviour: roles of background wind changes. Clim. Dyn. 18, 475-486 (2002)"
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#ref8">8</A>,
<A id=ref-link-12
title="An, S.-I. & Jin, F.-F. Nonlinearity and asymmetry of ENSO. J. Clim. 17, 2399-2412 (2004)"
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#ref9">9</A>,
<A id=ref-link-13
title="McPhaden, M. J. & Zhang, X. Asymmetry in zonal phase propagation of ENSO sea surface temperature anomalies. Geophys. Res. Lett. 36 L13703 http://dx.doi.org/10.1029/2009GL038774 (2009)"
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#ref10">10</A></SUP>,
spurring unusual weather events worldwide with costly consequences<SUP><A
id=ref-link-14
title="Bove, M. C., O'Brien, J. J., Eisner, J. B., Landsea, C. W. & Niu, X. Effect of El Nino on U.S. landfalling hurricanes, revisited. Bull. Am. Meteorol. Soc. 79, 2477-2482 (1998)"
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#ref3">3</A>,
<A id=ref-link-15
title="Wilhite, D. A., Wood, D. A. & Meyer, S. J. in Climate Crisis (eds Glantz, M., Katz, R. & Krenz, M.) 75-78 (UNEP, 1987)"
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#ref4">4</A>,
<A id=ref-link-16
title="Changnon, S. A. Impacts of 1997[mdash]98 El Nino generated weather in the United States. Bull. Am. Meteorol. Soc. 80, 1819-1827 (1999)"
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#ref5">5</A>,
<A id=ref-link-17
title="Liu, Z. & Alexander, M. Atmospheric bridge, oceanic tunnel, and global climatic teleconnections. Rev. Geophys. 45, RG2005, http://dx.doi.org/10.1029/2005RG000172 (2007)"
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#ref6">6</A>,
<A id=ref-link-18
title="Cai, W. et al. More extreme swings of the South Pacific convergence zone due to greenhouse warming. Nature 488, 365-369 (2012)"
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#ref11">11</A></SUP>.
The cause of this propagation asymmetry is currently unknown<SUP><A
id=ref-link-19
title="McPhaden, M. J. & Zhang, X. Asymmetry in zonal phase propagation of ENSO sea surface temperature anomalies. Geophys. Res. Lett. 36 L13703 http://dx.doi.org/10.1029/2009GL038774 (2009)"
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#ref10">10</A></SUP>.
Here we trace the cause of the asymmetry to the variations in upper ocean
currents in the equatorial Pacific, whereby the westward-flowing currents are
enhanced during La Niña events but reversed during extreme El Niño events. Our
results highlight that propagation asymmetry is favoured when the westward mean
equatorial currents weaken, as is projected to be the case under global
warming<SUP><A id=ref-link-20
title="Vecchi, G. A. et al. Weakening of tropical Pacific atmospheric circulation due to anthropogenic forcing. Nature 441, 73-76 (2006)"
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#ref12">12</A>,
<A id=ref-link-21
title="DiNezio, P. et al. Climate response of the equatorial Pacific to global warming. J. Clim. 22, 4873-4892 (2009)"
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#ref13">13</A>,
<A id=ref-link-22
title="Sen Gupta, A., Ganachaud, A., McGregor, S., Brown, J. N. & Muir, L. Drivers of the projected changes to the Pacific Ocean equatorial circulation. Geophys. Res. Lett. 39 L09605 http://dx.doi.org/10.1029/2012GL051447 (2012)"
href="http://www.nature.com/nature/journal/v504/n7478/full/nature12683.html#ref14">14</A></SUP>.
By analysing past and future climate simulations of an ensemble of models with
more realistic propagation, we find a doubling in the occurrences of El Niño
events that feature prominent eastward propagation characteristics in a warmer
world. Our analysis thus suggests that more frequent emergence of propagation
asymmetry will be an indication of the Earth’s warming
climate.</P></DIV></FONT></BODY></HTML>