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<DIV class=article-heading>
<DT>Nature</DT>
<DT class=published>Published online 17 November 2013 </DT>
<DT class=published> </DT></DIV>
<DIV class=article-heading><FONT size=4>Late-twentieth-century emergence of the
El Niño propagation asymmetry and future projections</FONT></DIV>
<DIV class="vcard c1"><A class=name
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12683.html?WT.ec_id=NATURE-20131121#auth-1"
jQuery17200013886807774864395="91"><SPAN class=fn>Agus Santoso</SPAN></A><SPAN
class=comma>,</SPAN><SUP><A
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12683.html#a1">1</A></SUP>
<A class=name
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12683.html?WT.ec_id=NATURE-20131121#auth-2"
jQuery17200013886807774864395="92"><SPAN class=fn>Shayne
McGregor</SPAN></A><SPAN class=comma>,</SPAN><SUP><A
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12683.html#a1">1</A></SUP>
<A class=name
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12683.html?WT.ec_id=NATURE-20131121#auth-3"
jQuery17200013886807774864395="93"><SPAN class=fn>Fei-Fei Jin</SPAN></A><SPAN
class=comma>,</SPAN><SUP><A
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12683.html#a2">2</A></SUP>
<A class=name
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12683.html?WT.ec_id=NATURE-20131121#auth-4"
jQuery17200013886807774864395="94"><SPAN class=fn>Wenju Cai</SPAN></A><SPAN
class=comma>,</SPAN><SUP><A
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12683.html#a3">3</A></SUP>
<A class=name
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12683.html?WT.ec_id=NATURE-20131121#auth-5"
jQuery17200013886807774864395="95"><SPAN class=fn>Matthew H.
England</SPAN></A><SPAN class=comma>,</SPAN><SUP><A
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12683.html#a1">1</A></SUP>
<A class=name
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12683.html?WT.ec_id=NATURE-20131121#auth-6"
jQuery17200013886807774864395="96"><SPAN class=fn>Soon-Il An</SPAN></A><SPAN
class=comma>,</SPAN><SUP><A
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12683.html#a4">4</A></SUP>
<A class=name
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12683.html?WT.ec_id=NATURE-20131121#auth-7"
jQuery17200013886807774864395="97"><SPAN class=fn>Michael J.
McPhaden</SPAN></A><SUP><A
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12683.html#a5">5</A></SUP>
<SPAN>& </SPAN><A class=name
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12683.html?WT.ec_id=NATURE-20131121#auth-8"
jQuery17200013886807774864395="98"><SPAN class=fn>Eric
Guilyardi</SPAN></A><SUP><A
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12683.html#a6">6</A>,
<A
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12683.html#a7">7</A></SUP> </DIV>
<DIV class="vcard c1"><SUP></SUP> </DIV></ASIDE></HEADER><SECTION>
<DIV class="section first no-nav no-title first-no-nav">
<DIV class="content ">
<DIV id=first-paragraph class=first-paragraph-abs>
<P><STRONG>Summary</STRONG></P>
<P>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/vaop/ncurrent/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/vaop/ncurrent/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/vaop/ncurrent/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/vaop/ncurrent/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/vaop/ncurrent/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/vaop/ncurrent/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/vaop/ncurrent/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/vaop/ncurrent/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/vaop/ncurrent/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/vaop/ncurrent/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/vaop/ncurrent/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/vaop/ncurrent/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/vaop/ncurrent/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/vaop/ncurrent/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/vaop/ncurrent/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/vaop/ncurrent/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/vaop/ncurrent/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/vaop/ncurrent/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/vaop/ncurrent/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/vaop/ncurrent/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/vaop/ncurrent/full/nature12683.html#ref14">14</A></SUP>.
By analysing past and future climate simulations of an ensemble of models with
more realistic propagation, <U>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.</U></P></DIV></DIV></DIV></FONT></BODY></HTML>