Scientific bulletin n° 339 - January 2010
©IRD/Luc Ortlieb
The Indian Ocean
influences El Niño onset
rought in Southern
Africa, floods in Latin
America, weak monsoon in
South-East Asia: El Niño,
an oceanic and
atmospheric event resulting
from a temperature
anomaly in the tropical
Pacific, strongly disturbs
the climate global climate.
Every seven years, its
global-scale socioeconomic and
environmental
consequences can be
tragic. Substantial
advances have been made
towards understanding this
cycle warming of the waters
of the tropical Pacific, but
long-term prediction of this
phenomenon is still
challenging.
The IRD researchers and
their partners1 recently
identified an essential but
still little known factor for
the onset of such an event:
the Indian Ocean Dipole, a
climatic anomaly equivalent
to that of the Pacific. This
phenomenon modulates the
Indo-Pacific atmospheric
circulation and the winds
over the Equatorial Pacific.
It disappears rapidly at the
end of autumn. This favours
the development of an El
Niño episode, which
reaches its peak about
14 months later.
Consideration of this Indian
Ocean Dipole can help
arrive at improved
forecasting of El Niño’s
force
© Nature Geoscience.
D
Destruction of buildings in Paita Bay in the North of Peru during the very high intensity El Niño event in 198283. Figure 1 : in autumn (here, in October), the eastern part of the Indian Ocean can have a strong influence
(black arrow) on the winds of the Pacific.
El Niño, “Baby Jesus” in Spanish because
it comes into full force just after Christmas.
Also known as ENSO (El Niño Southern
Oscillation), it leads to drought in regions
which are usually arid desert. Currently
scientists can predict effectively the way
this event is going to evolve. Study of the
event earlier in its development requires
better ways of identifying the mechanisms responsible. IRD researchers and
their partners 1 recently showed the
decisive role played by the Indian Ocean
Dipole in triggering the episode. Integration of this anomaly in models of this other
ocean’s equivalent to El Niño, can help
extend the forecasting range to more than
one year beforehand.
El Niño gradually revealing its secrets
El Niño The origins of El Niño lie in the
ocean-atmosphere interactions in the tropical Pacific. The mechanisms which govern
the development at the end of an ENSO
episode are now quite well understood2
and represented by theoretical models.
The way its processes evolve can also be
predicted. However, many questions
remain about the mechanisms responsible
for the onset of the events and on its potential precursors.
The Indian Ocean was previously thought
to be relatively passive from the climatic
point of view and firmly tied to the influence
of the Pacific Ocean. This notion changed
with the discovery of the Indian Ocean
Dipole about 10 years ago.
The Indian Ocean sets the conditions
for El Niño…
Analysis of observations and model outputs enabled the research team to identify
mechanisms responsible for the influence
the Pacific Ocean exerts on the development of an El Niño even the following year.
The story begins in the Indian Ocean, when
a so-called negative dipole is established.
Characteristically this dipole features a
system with sea surface temperatures
(SST) which are above average near
Indonesia and below average nearer the
African coast. The South-East of the ocean
warms up, in an anomaly which reaches its
maximum in the autumn. In that season the
atmospheric circulation in that region is at
its maximum (see figure 1): small variations
of SST in the Indian Ocean exert a strong
influence on the atmosphere above the
Pacific. Thus, at the autumn peak of the
dipole, the East winds (see box Did you
know?) gain strength in the Pacific (see
figure 2) and hence favour the accumulation of warm water in the West of that
ocean. These conditions effectively predetermine the future development of an El
Niño event.
… and sets it off…
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IRD researchers currently on
assignment at the National
Institute of Oceanography
in Goa, India
Tel: +91-8322450212
Jerome.vialard@ird.fr
Matthieu.lengaigne@ird.fr
LOCEAN – Laboratoire d’océanographie et du climat : expérimentations et approches numériques (UMR – IRD, Université
Pierre et Marie Curie, CNRS,
MNHN)
Address:
Université Pierre et Marie Curie
Case 100, 75252 PARIS
CEDEX 05, France
Sophie CRAVATTE
researcher at the IRD
Tel: +33-5-6133-3005
sophie.cravatte@ird.fr
LEGOS – Laboratoire d’études en
géophysique et océanographie
spatiales (UMR – IRD, CNRS,
CNES, Université Toulouse 3 Paul Sabatier)
Address:
Observatoire Midi-Pyrénées
14 avenue Edouard BELIN
31400 Toulouse, France
Takeshi IZUMO
research scientist at the
University of Tokyo, Japan
Tel: +81-3-5841-4651
Izumo@eps.s.u-tokyo.ac.jp
* See Scientific news sheet n°314 – The western
tropical Pacific warming up while its salinity falls
Rédaction DIC – Gaëlle Courcoux
Translation – Nicholas Flay
1. This research work was conducted in conjunction
with researchers from the Japan Agency for MarineEarth Science and Technology (JAMSTEC), of the
University of Tokyo, and IFREMER
2. The easterly winds above the Pacific, called the
trades, sometimes die down, or even reverse direction. Warm waters in the area which are usually pushed westwards by these winds, act on the Earth’s
system like a heat pump, flowing back towards the
South American continent. These movements
consequently disturb the whole Earth’s climate.
3. Combined with the four-yearly trend linked to the
process of replenishment of the warm water of the
equatorial Pacific, this could explain the wide timescale of the ENSO period (from 2 to 7 years).
4. La Niña is the cold phase of the thermal oscillation in
the Pacific. It is characterized by an abnormally low
sea water temperature in the equatorial Pacific.
©/Laure Emperaire
CONTACTS:
Jérôme VIALARD
and Matthieu LENGAIGNE
At the end of autumn, the positive temperature anomaly in the East of the Indian
Ocean quickly disappears. Hence an
abrupt slackening of the easterly wind anomalies in the Pacific. That leads to anomalies in easterly-running current in winter
and spring, and therefore to a shift of the
western Pacific Warm pool towards the
east.* The onset of a classic El Niño event
then comes in February or March where
the ocean-atmosphere coupling favours an
increase in the anomalies. The warm anomalies in the central Pacific engenders
anomalies in the westerly winds, which in
turn reinforces the warm anomalies. And so
the process continues.
… every two years.
The research team also postulated a possible explanation for the biennial trend of
ENSO 3. A negative Indian Ocean dipole
favours the appearance of an El Niño event
in the Pacific the following year. Previous
work had shown that this El Niño often
generates a synchronous positive dipole
(SST higher than normal in the western
parts of the Indian Ocean and lower than
normal in the eastern side) The proposed
model indicates that the positive dipole
itself tends to favour a subsequent La Niña4
episode, which in turn is a prelude to a
negative Indian Ocean Dipole, and so it
continues.
This study brought an important element,
and up to now poorly known, in the understanding of El Niño onset. It emphasizes the
substantial role of the Indian Ocean forecasting the arrival of an episode and predict its repercussions. It also gives inspiration for a deepening of the study of the
Indo-Pacific interactions and to develop an
observation network system in the tropical
Indian Ocean on the lines of those already
deployed in the Pacific and Atlantic Oceans.
Desiccation crack in a lagoon floor in Peru after the
1983 floods induced by El Niño.
REFERENCES:
Izumo T., Vialard Jérome,
Lengaigne Matthieu, C. de Boyer
Montegut, S. K. Behera, J-J.
Luo, Cravatte Sophie, S. Masson,
and T. Yamagata. Influence of
the Indian Ocean Dipole on
following year’s El Niño, Nature
Geoscience (2010).
doi:10.1038/ngeo760
© Nature Geoscience.
Scientific bulletin n° 339 - January 2010
For further information
KEY WORDS:
El Niño, Pacific Ocean, Indian
Ocean
Figure 2 : Atmospheric circulation in the Indo-Pacific reinforced in this case by a negative Indian Ocean dipole
(sea surface temperatures lower than normal in the west and higher than normal in the east).
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Did you know ?
The terminology for the winds indicates their direction of origin: a wind from the east is an easterly
wind! Land-dwellers are concerned because of what the winds bring with them (clouds, rain etc).
However, for marine currents, the name indicates the direction in which they flow : an easterly
current is runs towards the east, because seafarers want to know the place where the currents
are taking their ship.
Oceanographers have adopted this nomenclature.
Gaëlle Courcoux, coordinatrice
Délégation à l’information et à la communication
Tél. : +33 (0)4 91 99 94 90 - fax : +33 (0)4 91 99 92 28 - fichesactu@ird.fr