Actualité scientifique
Actualidad científica
Scientific news
N° 482
June 2015
The West Pacific Ocean, at the heart of
the climate machine
Pandora oceanographic survey, in the western Pacific (© IRD / A. Ganachaud).
The Pacific’s “western boundary currents” convey huge quantities of warm water
containing salt and nutrients and have a considerable influence on the climate.
Although they were discovered long ago, still little is known about these ocean
systems. Significant research efforts have been undertaken in the last seven
years. An IRD team with Chinese, Australian and American partners recently
published a report in Nature magazine on the state of knowledge on these Pacific
currents, their variations, their impact and their response to climate change.
Key points
The western boundary circulation system is a set of intense currents that form in the west of major ocean basins such as the Atlantic, Indian
and Pacific Oceans. Pushed westwards by winds under the effect of the Earth’s rotation, water comes up against the continents – the
Americas, Africa, Asia and Australia – and is then diverted either north or south. These currents form veins 50 to 500 km wide along coasts
and can reach surface speeds of up to 1 metre per second.
Glossary
El Niño: a climate phenomenon that occurs every two to seven years in the equatorial Pacific and plunges the global climate into chaos
for several months with drought or heavy rainfall depending on the region.
Thermohaline circulation: very slow circulation of the ocean on a global scale linked to differences in temperature and salinity between
water masses.
Contacts
A little-understood system of currents
The Pacific’s western boundary currents were
among the first dynamic systems studied by
pioneering oceanographers. However, until
now, little was known about their complex
structure. Given their significance in oceanatmosphere interaction, scientists have been
redoubling their efforts for the last seven years.
Following the SPICE programme, focused on
the south-western Pacific, IRD researchers
and their Chinese, Australian and American
partners are continuing this work within the
CLIVAR programme across the whole Pacific.
Their objective is to understand exchanges of
heat and water masses, possible changes in
them as a result of increased greenhouse gas
effects, and their impact on climate. They have
recently published a report on the current state
of knowledge in Nature magazine.
on the intensity of the currents and their spatiotemporal variation in the inter-tropical zone,
this system can disrupt the world’s weather
quite considerably. The consequences of these
phenomena – cyclone formation, changes to
precipitation patterns and the functioning of
natural and agricultural ecosystems – strongly
affect societies around the world.
Coordination
Gaëlle Courcoux
Information and Culture
Department
With climate change, western boundary
currents in the Pacific, as in other oceans, have
intensified over the last century. They have
extended towards the poles, where they have
heated up two to three times more than in the
rest of the world’s oceans. How will the climate
respond to these fundamental changes? The
challenge for researchers now is to find out…
T. +33 (0)4 91 99 94 90
[email protected]
www.ird.fr/la-mediatheque
Great variability
A central role in world climate
In return, this system of currents affects
the climate in many ways. First of all, flows
towards the Indian Ocean, via the Indonesian
archipelago, contribute to the world’s
thermohaline circulation, the “conveyor belt”
spreading heat and regulating the climate
across the globe. In addition, it exchanges heat
and water masses with the equatorial zone and
its famous “warm pool”, a huge reservoir of warm
water in the middle of the Pacific. This is the
planet’s main “heat pump”, supplying flows of
heat and humidity for the majority of the Earth’s
atmosphere. Lastly, one branch of the current
peels off towards the South Pole. Depending
Partners
Chinese Academy of Sciences, Ocean University
of China, CSIRO and The University of New South
Wales in Australia, University of Hawaii, Columbia
University, Scripps Institution of Oceanography
and the NOAA in the United States and the Japan
Agency for Marine-Earth Science and Technology.
References
Dunxin Hu, Lixin Wu, Wenju Cai, Alex Sen
Gupta, Alexandre Ganachaud, Bo Qiu, Arnold
L. Gordon, Xiaopei Lin, Zhaohui Chen, Shijian
Hu, Guojian Wang, Qingye Wang, Janet Sprintall,
Tangdong Qu, Yuji Kashino, Fan Wang & William
S. Kessler. Pacific western boundary currents and
their roles in climate. Nature, 2015, 522, p.299308. doi:10.1038/nature14504
Contact
Alexandre Ganachaud, IRD researcher
[email protected]
T. +33 (0)5 61 33 29 66
Laboratoire d’études en géophysique et
océanographie spatiales - LEGOS (IRD / CNES
/ CNRS / université Toulouse 3)
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© IRD/DIC, Mars 2015 - Conception et réalisation graphique : L. CORSINI
The scientists show that this ocean system
functions as a single entity. For example, when
a warm El Niño episode occurs, the entire
western boundary currents system shifts to
higher latitudes. It is also affected by other
factors, such as the seasons, local monsoon
winds, etc. It consequently shows great
variability within seasons, from one year and
decade to the next, and indeed over the longer
term. In places, such as the Solomon Sea, the
amounts of warm, saline water conveyed can
double between a cold and a warm El Niño
phase.