<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML><HEAD>
<META content="text/html; charset=iso-8859-1" http-equiv=Content-Type>
<META name=GENERATOR content="MSHTML 8.00.6001.18702">
<STYLE></STYLE>
</HEAD>
<BODY bgColor=#ffffff><FONT size=2 face=Arial>
<DIV class=article-heading><FONT size=3><STRONG>Nature </STRONG></FONT></DIV>
<DIV class=article-heading>Published online 24 July 2013 </DIV>
<DIV class=article-heading><A
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12363.html?WT.ec_id=NATURE-20130725">http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12363.html?WT.ec_id=NATURE-20130725</A></DIV>
<DIV class=article-heading> </DIV>
<DIV class=article-heading><FONT size=4>Seasonal sea surface cooling in the
equatorial Pacific cold tongue controlled by ocean mixing</FONT></DIV>
<DIV class="vcard c1"><A class=name
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12363.html?WT.ec_id=NATURE-20130725#auth-1"
jQuery1720391876109902351="60"><SPAN class=fn>James N. Moum</SPAN></A><SPAN
class=comma>,</SPAN><SUP><A
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12363.html#a1">1</A></SUP>
<A class=name
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12363.html?WT.ec_id=NATURE-20130725#auth-2"
jQuery1720391876109902351="61"><SPAN class=fn>Alexander Perlin</SPAN></A><SPAN
class=comma>,</SPAN><SUP><A
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12363.html#a1">1</A>,
<A
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12363.html#a3">3</A></SUP>
<A class=name
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12363.html?WT.ec_id=NATURE-20130725#auth-3"
jQuery1720391876109902351="62"><SPAN class=fn>Jonathan D. Nash</SPAN></A><SUP><A
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12363.html#a1">1</A></SUP>
<SPAN>& </SPAN><A class=name
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12363.html?WT.ec_id=NATURE-20130725#auth-4"
jQuery1720391876109902351="63"><SPAN class=fn>Michael J.
McPhaden</SPAN></A><SUP><A
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12363.html#a2">2</A></SUP>
</DIV>
<DIV class="vcard c1">1 College of Earth, Ocean and Atmospheric Sciences,
Oregon State University, Corvallis, Oregon 97331, USA</DIV>
<DIV class="vcard c1">2 NOAA/Pacific Marine Environmental Laboratory, Seattle,
Washington 98115, USA</DIV>
<DIV>3 Deceased.</DIV>
<DIV><STRONG></STRONG> </DIV>
<DIV><STRONG>Summary</STRONG></DIV>
<DIV class=journal-title>Sea surface temperature (SST) is a critical control on
the atmosphere<SUP><A id=ref-link-5
title="Xie, S.-P. Satellite observations of cool ocean-atmosphere interaction. Bull. Am. Meteorol. Soc. 85, 195-208 (2004)"
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12363.html#ref1">1</A></SUP>,
and numerical models of atmosphere–ocean circulation emphasize its accurate
prediction. Yet many models demonstrate large, systematic biases in simulated
SST in the equatorial ‘cold tongues’ (expansive regions of net heat uptake from
the atmosphere) of the Atlantic<SUP><A id=ref-link-6
title="Richter, I. & Xie, S.-P. On the origin of equatorial Atlantic biases in coupled general circulation models. Clim. Dyn. 31, 587-598 (2008)"
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12363.html#ref2">2</A></SUP>
and Pacific<SUP><A id=ref-link-7
title="Wittenberg, A. T., Rosati, A., Lau, N. C. & Ploshay, J. J. GFDL/'s CM2 global coupled climate models. Part III: Tropical Pacific climate and ENSO. J. Clim. 19, 698-722 (2006)"
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12363.html#ref3">3</A></SUP>
oceans, particularly with regard to a central but little-understood feature of
tropical oceans: a strong seasonal cycle. The biases may be related to the
inability of models to constrain turbulent mixing realistically<SUP><A
id=ref-link-8
title="Jouanno, J., Marin, F., Du Penhoat, Y., Sheinbaum, J. & Molines, J.-M. Seasonal heat balance in the upper 100 m of the equatorial Atlantic ocean. J. Geophys. Res. 116, C09003 (2011)"
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12363.html#ref4">4</A></SUP>,
given that turbulent mixing, combined with seasonal variations in atmospheric
heating, determines SST. In temperate oceans, the seasonal SST cycle is clearly
related to varying solar heating<SUP><A id=ref-link-9
title="Xie, S. On the genesis of the equatorial annual cycle. J. Clim. 7, 2008-2013 (1994)"
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12363.html#ref5">5</A></SUP>;
in the tropics, however, SSTs vary seasonally in the absence of similar
variations in solar inputs<SUP><A id=ref-link-10
title="Praveen Kumar, B., Vialard, J., Lengaigne, M., Murty, V. S. N. & McPhaden, M. J. TropFlux: air-sea fluxes for the global tropical oceans[mdash]description and evaluation. Clim. Dyn. 38, 1521-1543 (2012)"
href="http://www.nature.com/nature/journal/vaop/ncurrent/full/nature12363.html#ref6">6</A></SUP>.
Turbulent mixing has long been a likely explanation, but firm, long-term
observational evidence has been absent. Here we show the existence of a
distinctive seasonal cycle of subsurface cooling via mixing in the equatorial
Pacific cold tongue, using multi-year measurements of turbulence in the ocean.
In boreal spring, SST rises by 2<SPAN class=mb><SPAN
class=mb> </SPAN></SPAN>kelvin when heating of the upper ocean by the atmosphere
exceeds cooling by mixing from below. In boreal summer, SST decreases because
cooling from below exceeds heating from above. When the effects of lateral
advection are considered, the magnitude of summer cooling via mixing (4 kelvin
per month) is equivalent to that required to counter the heating terms. These
results provide quantitative assessment of how mixing varies on timescales
longer than a few weeks, clearly showing its controlling influence on seasonal
cooling of SST in a critical oceanic regime.</DIV></FONT></BODY></HTML>