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<DIV class=formatpublished minmax_bound="true"><FONT size=3>Nature</FONT></DIV>
<DIV class=formatpublished minmax_bound="true">Published online <ABBR
class=published title=2010-09-16T19:38:04Z minmax_bound="true">16 September
2010</ABBR minmax_bound="true"> </DIV>
<DIV class=formatpublished minmax_bound="true">doi:10.1038/news.2010.477 </DIV>
<DIV class=formatpublished minmax_bound="true"> </DIV>
<DIV class=type-of-article minmax_bound="true">As El Niño's cooler sister rolls
round again, <SPAN class=i minmax_bound="true">Nature </SPAN>probes the
environmental pros and cons.</DIV>
<DIV class=type-of-article minmax_bound="true"><FONT size=4>Return of La
Niña</FONT></DIV>
<DIV class=intro minmax_bound="true"><SPAN class=vcard minmax_bound="true"><SPAN
class="author fn" minmax_bound="true">Adam Mann </SPAN></SPAN></DIV>
<DIV><SPAN class=cleardiv minmax_bound="true"><!-- --></SPAN>
<DIV class=entry-content minmax_bound="true" sizset="21" sizcache="45">
<P minmax_bound="true">La Niña, the climatic event in which swathes of the
equatorial east-central Pacific cool, strengthened through August, according to
reports from the US National Oceanic and Atmospheric Administration (NOAA).
Across all latitudes monitored by the NOAA, the ocean surface cooled by 1.3–1.8
ºC. Models predict that La Niña will persist until at least early 2011, and
could, according to the NOAA, cool further during the coming winter. <SPAN
class=i minmax_bound="true">Nature </SPAN>explores its potential effects on the
global environment.</P>
<P minmax_bound="true"><B minmax_bound="true">What is the scientific definition
of La Niña? </B></P>
<P minmax_bound="true">La Niña is a natural 3–6-year cycle, and the cold stretch
of a periodic thermal oscillation in ocean surface temperatures that occurs
throughout the tropical Pacific. Along with the better-known warming event, El
Niño, it involves a difference from average water temperatures of more than 0.5
ºC. La Niña can persist for 1-3 years, as seen, for example, in 1998–2000. </P>
<P minmax_bound="true"><B minmax_bound="true">What are its key global impacts?
</B></P>
<P minmax_bound="true">Ocean cooling affects tropical Pacific rainfall from
Indonesia to South America, says Gerry Bell, a NOAA climate scientist at the
Climate Prediction Center in Camp Springs, Maryland. Some places, such as
northern Australia, experience wetter than average seasons. "The changes are so
large that it affects wind, too," adds Bell, "and right downstream of this area
is the Atlantic." </P>
<P minmax_bound="true"><B minmax_bound="true">Does this have implications for
the Atlantic hurricane season? </B></P>
<P minmax_bound="true">La Niña reduces variations in wind speed and direction
throughout the atmosphere, which makes Atlantic hurricanes likelier, Bell says.
This year may be particularly active, he adds. On top of La Niña, Atlantic water
temperatures have remained unusually high over the past 15 years, and wind
patterns that have been in place since 1995 — such as weak easterly trade winds
and high pressure in the upper atmosphere — are particularly conducive to a
strong hurricane season. NOAA expects 8–12 Atlantic hurricanes this season, of
which it predicts 4–6 will be major hurricanes (that is, with sustained wind
speeds of more than 178 km h<SUP minmax_bound="true">–1</SUP>).</P>
<P minmax_bound="true"><B minmax_bound="true">Will La Niña help to lower global
temperatures after recent record highs? </B></P>
<P minmax_bound="true">Average temperature over land outside the tropical
regions of the Northern Hemisphere reached a new high in July 2010, according to
a report by the European Centre for Medium-Range Weather Forecasts. In addition,
the global average over land during the previous 12 months ranks as the second
highest on record. La Niña conditions have offset these highs, but have mainly
affected tropical Pacific waters. Global temperatures have been rising since the
turn of the twentieth century, most notably during the past 30 years, but La
Niña is part of a regular cycle that is simply overlaid onto that pattern.</P><!-- 300x250 ad -->
<DIV class="ad ad300x250" minmax_bound="true" sizset="21" sizcache="45">
<P minmax_bound="true"><B minmax_bound="true">How strong is this La Niña event
compared with previous ones? </B></P></DIV>
<P minmax_bound="true">So far, La Niña has been relatively strong this summer.
"That's not unheard of," says Bell, adding that similar conditions in 1998 made
headlines. The oscillation usually begins to strengthen during the autumn, so it
remains to be seen how much more powerful this year will be compared with
previous years.</P>
<P minmax_bound="true"><B minmax_bound="true">Why is it important to keep track
of these events? </B></P>
<P minmax_bound="true">Knowledge of how La Niña affects global weather patterns
can help governments and people to plan for drought or heavy rainfall. For
instance, researchers know that the Pacific Northwest is likely to be wetter
than normal in the late autumn and early winter of La Niña years, which helps to
refill parched aquifers in the wake of dry periods. Predicting
global-temperature oscillations is crucial to water management, as well as to
farmers seeking to mitigate potential losses.<SPAN class=end-of-item
minmax_bound="true"> </SPAN></P></DIV></FONT></DIV></BODY></HTML>