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<article language="en">
	<journal>
		<journal_title>Nonlinear Processes  in Geophysics</journal_title>
		<journal_url>www.nonlin-processes-geophys.net</journal_url>
		<issn>1023-5809</issn>
		<eissn>1607-7946</eissn>
		<volume_number>15</volume_number>
		<issue_number>3</issue_number>
		<publication_year>2008</publication_year>
	</journal>
	<doi>10.5194/npg-15-417-2008</doi>
	<article_url>http://www.nonlin-processes-geophys.net/15/417/2008/</article_url>
	<abstract_html>http://www.nonlin-processes-geophys.net/15/417/2008/npg-15-417-2008.html</abstract_html>
	<fulltext_pdf>http://www.nonlin-processes-geophys.net/15/417/2008/npg-15-417-2008.pdf</fulltext_pdf>
	<start_page>417</start_page>
	<end_page>433</end_page>
	<publication_date>2008-05-28</publication_date>
	<article_title content_type="html">A delay differential model of ENSO variability: parametric instability and the distribution of extremes</article_title>
	<authors>
		<author numeration="1" affiliations="1,4">
			<name>M. Ghil</name>
			<email>ghil@atmos.ucla.edu</email>
		</author>
		<author numeration="2" affiliations="2">
			<name>I. Zaliapin</name>
		</author>
		<author numeration="3" affiliations="3">
			<name>S. Thompson</name>
		</author>
	</authors>
	<affiliations>
		<affiliation numeration="1" content_type="html">Dépt. Terre-Atmosphère-Océan and Laboratoire de Météorologie Dynamique, Ecole Normale Supérieure, Paris, France</affiliation>
		<affiliation numeration="2" content_type="html">Dept. of Mathematics and Statistics, University of Nevada, Reno, NV, USA</affiliation>
		<affiliation numeration="3" content_type="html">Dept. of Mathematics and Statistics, University of Radford, VA, USA</affiliation>
		<affiliation numeration="4" content_type="html">Dept. of Atmospheric and Oceanic Sciences and Institute of Geophysics and Planetary Physics, University of California Los Angeles, CA, USA</affiliation>
	</affiliations>
	<abstract content_type="html">We consider a delay differential equation (DDE) model for El-Niño Southern
Oscillation (ENSO) variability.
The model combines two key mechanisms that participate in ENSO dynamics:
delayed negative feedback and seasonal forcing.
We perform stability analyses of the model in the three-dimensional space of its
physically relevant parameters.
Our results illustrate the role of these three parameters:
strength of seasonal forcing &lt;i&gt;b&lt;/i&gt;, atmosphere-ocean coupling &lt;i&gt;κ&lt;/i&gt;,
and propagation period &lt;i&gt;τ&lt;/i&gt; of oceanic waves across the Tropical Pacific.
Two regimes of variability, stable and unstable, are separated by a sharp neutral
curve in the (&lt;i&gt;b&lt;/i&gt;, &lt;i&gt;τ&lt;/i&gt;) plane at constant &lt;i&gt;κ&lt;/i&gt;.
The detailed structure of the neutral curve becomes very irregular and
possibly fractal, while individual trajectories within the unstable region
become highly complex and possibly chaotic, as the atmosphere-ocean coupling
&lt;i&gt;κ&lt;/i&gt; increases.
In the unstable regime, spontaneous transitions occur in the mean &quot;temperature&quot;
(i.e., thermocline depth), period, and extreme annual values,
for purely periodic, seasonal forcing.
The model reproduces the Devil&apos;s bleachers characterizing other ENSO models,
such as nonlinear, coupled systems of partial differential equations;
some of the features of this behavior have been documented in general circulation
models, as well as in observations.
We expect, therefore, similar behavior in much more detailed and realistic models,
where it is harder to describe its causes as completely.</abstract>
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