Sheet n°280 - November 2007
El Niño affected by global warming
© IRD/ Ortlieb Luc
l Niño climatic and
oceanic events, which
exert their influence particularly on coastal regions
stretching from the South
of Ecuador down to central Chile, have occurred with a periodicity of
around 2 to 7 years for
several thousand years.
Many of the secrets of
this climatic phenomenon, also called El Niño
Southern Oscillation
(ENSO), remain intact.
Recently, however, a team
of IRD scientists working jointly with Chilean
research organizations (1)
made some unexpected
findings about the recent
evolution of the ENSO
system. Investigation of
marine sedimentary drill
cores enabled them to
retrace the changes in the
ENSO’s functioning since
the XVIIth Century to the
present. Results showed
a 2°C fall in temperature of the waters of the
Humboldt current system
for the period 1820-1878.
This time corresponds to
the end of the Little Ice
Age yet coincides with
a warming of the Earth.
The explanation of this
paradox, drawn from
the detailed analysis of
various constituents of
the sedimentary cores,
hinges on the involvement
of complex interactions
that exist between the
atmospheric circulation,
the ocean currents and
trade winds which cause
upwellings of cold water
along the Pacific coasts
of South America.
E
Mejillones Bay in northern Chile.
The climatic event El Niño, literally
“the Baby Jesus”, was given its
name because it generally occurs at
Christmas time along the Peruvian
coasts. This expression of climatic
variability, also called El Niño Southern
Oscillation (ENSO), results from
a series of interactions between the
atmosphere and the tropical ocean. It
induces drought in areas that normally
receive abundant rain and, conversely,
heavy rainfall and floods in usually
arid desert zones. Scientists term this
phenomenon a “quasi-cyclic” variation
because its periodicity, which varies
from 2 to 7 years, shows no regular
time pattern. Research conducted over
the past 25 years, by oceanographers,
climatologists and meteorologists has
much improved knowledge on the
mechanisms generating an El Niño
event. However, possible influence of
other systems of climate variability on
the ENSO regime is more difficult to
fathom. More particularly, it is not known
if the intensity and frequency of the
event is susceptible to modification in a
situation of global warming.
The research work recently published
by a team of Chilean and IRD scientists
sheds new light on El Niño’s variability.
Several geochemical factors contained
in a drill core sediment sampled from
80 m depth under the Bay of Mejillones,
in northern Chile, were determined.
Analysis of breakdown byproducts from
diatoms, unicellular planktonic algae,
yielded an accurate trace of this region’s
trends in sea surface temperature
between 1650 and 2000. Data for the
period 1820-1878 showed a fall of over
2°C. This temperature decrease was
also detected in two cores collected
near the South-American coasts, over
1000 km to the North and South of
Mejillones. These samplings confirmed
that the decrease in ocean temperature
observed from 1820 affected the whole
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CONTACTS
Luc ORTLIEB
Director of research IRD
Research unit ‘Paléoenvironnements tropicaux
et variabilité climatique
(PALEOTROPIQUE)’
Address: IRD
32, Rue Henri Varagnat
93143 Bondy cedex
France
Tel : +33 (0)1 48 02 56 37
Fax : +33 (0)1 48 02 55 54
Email: Luc.Ortlieb@ird.fr
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REFERENCES :
VARGAS G., PANTOJA
S., RUTLLANT J. A.,
LANGE C. B., ORTLIEB
L. Enhancement of
coastal upwelling and
interdecadal ENSO-like
variability in the PeruChile Current since
late 19th century - art.
no. L13607 Geophysical
Research Letters, 2007,
34 (13).
doi :10.1029/2006GL028812
KEY WORDS :
El Niño, Little Ice Age,
Humboldt current,
climatology
to the North of Peru. All the oceanic area
situated on the path of the Humboldt
current system was therefore the scene
of significant cooling during this period.
This conclusion brings a paradox, seeing
that the beginning of the XIXth Century
coincided with the end of the Little Ice
Age which came at the same time as a
warming of the Earth.
Complementary analyses on certain
minerals contained in the sediment
samples confirmed that these minerals
were transported by the winds from the
continent. Therefore the reinforcement
of such prevailing winds, the trade
winds, would have favoured the rise of
colder waters up from deeper reaches,
along the Pacific coasts of South
America, by pushing the ocean surface
layer westwards. Confirmation of this
hypothesis came from measurement of
the organic carbon flux which is directly
linked to growth in nutrient concentration.
The increase in this flux accords with
the phase of falling sea temperatures
between 1820 and 1878 which proves
that the rise in nutrient concentration
stems from a rising up of cold water by
the process of upwelling (2).
The hypothesis the researchers
postulate suggests that, in a situation
of climate warming like the one that
followed the end of the Little Ice Age,
the large continent–ocean temperature
(and hence thermal) contrast would
be responsible for this accentuation of
the trade wind regime. Whereas the
Atacama, a coastal-zone desert, warmed
rapidly during this period, the sea surface
temperature would have risen much
more slowly. The long-term persistence
of a substantial temperature difference
between ocean and continent would
have caused an intensification of the
prevailing winds. Then by pushing the
surface water towards the west, these
winds would have induced cooling of
the coastal waters, changing the normal
feature of the El Niño regime which is a
warming of the waters. Between the end
of the Little Ice Age and the beginning of
the global warming attributable to human
activities the ENSO regime was modified.
Historical climatology studies founded on
chroniclers’ accounts and descriptions of
floods caused by these El Niño events
also showed an abrupt change, around
1820, in the ENSO system along Pacific
seaboard of South America. Since the
beginning of the XIXth Century, in other
words the final phase of the Little Ice
Age, the characteristic feature of El Niño
events was abnormal rainfall, both in
central Chile during the southern winter
and on the northern coast of Peru during
the subsequent southern summer.
These results as a whole emphasize
the complexity of the interactions at
work between the global-scale climate
changes, the diverse behaviour of the
ENSO system and regional climate
changes. It remains to be determined if
the extreme intensity of the two events
which occurred at the end of the XXth
Century, in 1982-1983, then in 19971998, is effectively linked to recent
intensification of global warming. If that
turned out to be the case, the El Niño
phenomenon could become more and
more intense and destructive, not only
on the South American coasts, but also
in other regions of the world.
Grégory Fléchet – DIC
Translation : Nicholas Flay
(1) This research was conducted jointly with the universities of Chile and
Concepción (Chile). It follows-up work
for a doctoral thesis by Gabriel Vargas presented at the University of
Bordeaux I and financed by the IRD.
(2) This term designates the phenomenon of an uprising of cold water
which occurs when the strong oceanic
winds displace the sea surface waters.
©IRD/ Ortlieb Luc
Sheet n°280 - November 2007
For further information Pacific seaboard, from central Chile up
Corer used for sediment sampling
Grégory Fléchet, coordinator
Délégation à l’information et à la communication
Tél. : +33(0)1 48 03 76 07 - fax : +33(0)1 40 36 24 55 - fichesactu@ird.fr