I The striking deep current reversal

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I
Surface layer currents (in red) and subsurface currents (blue) in the tropical Pacific. The South Equatorial
Current (CES/SEC) can be distinguished. When it arrives at the western margin (W) it feeds into the two
Western Pacific Boundary Currents (CBO/WPBC)and the Equatorial Undercurrent (SCE/ECU). The ECU flows
along the thermocline (sloping blue line) in the opposite direction to the South Equatorial Current. Under the
EUC runs the Equatorial Intermediate Current (CEI/EIC) and the Lower Equatorial Intermediate Current (CEIP/
LEIC) between about 300 and 1200 m depth. The EIC and LEIC reversed direction between October 1999
and April 2000. There is still no explanation for this reversal.
The CTD and the currentmeter
(yellow) used during the survey
cruises.
The ocean’s immense heat storage capacity means that it has a dominant role in the
regulation of heat exchange and of the Earth’s
climate. And it is the ocean’s currents that
drive thermal exchanges between ocean and
atmosphere and contribute to climate balance.
This they do in transporting warm- and coldwater masses from the Equator to the poles.
The near-surface currents are generated
essentially by the winds, whereas the deeper
ones (known as thermohaline currents) result
from water density variations induced by differences in temperature and salinity between the
distinct masses.
The prevailing winds in the tropical Pacific, the
trade winds, blow from the American continent towards Asia, causing the warm surface
© IRD/Th. Delcroix
n two oceanographic surveys conducted in the southwest Pacific Ocean, in October
1999 and April 2000, as part of
the IRD research programme
Etudes climatiques de l’océan
Pacifique (ECOP), scientists
from this Institute observed
changes in the intermediate
and deep current circulation.
They brought evidence of a
sharp change in direction
of equatorial intermediate
current between these two
dates. Such temporal variability is not new, but in this
case its amplitude is strikingly
strong. This reversal implies
a considerable variation in
mass transport of water in the
equatorial Pacific Ocean. This
effect will have to be taken
into account if better understanding of ocean/atmosphere
exchanges and climate variations is to be achieved.
Sheet n°251 - October 2006
© IRD
The striking deep current reversal
in the tropical Pacific Ocean
waters to drift in a general East-West direction.
As they approach the Asian continent, these
waters accumulate, then change direction,
part of them turning North and feeding the
Kuroshio (the equivalent in the Pacific of the
Gulf Stream), part going South to join up with
the East Australian current, another portion
flowing at depth and feeding the Equatorial
Undercurrent (EUC), which runs at between
100 and 150 m below the surface. The EUC
flows along the Equator, from Papua New
Guinea to the Galapagos Islands, counter to
the trade winds. That current extends over a
width of nearly 300 km and transports a large
mass of water eastwards (1), at a maximum
velocity of around 2 knots (1 m/s or 3.6 km/h).
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Sheet n°251 - Octobre 2006
For futher information
CONTACTS :
YVES GOURIOU :
IRD - US 025 "Interventions
à la mer et observatoires
océaniques" Centre IRD de
Bretagne.
Tel: +33 (0 )2 98 22 45 07 ;
[email protected]
THIERRY DELCROIX and
GÉRARD ELDIN
IRD - UMR065. Laboratoire
d'études en géophysique
et océanographie spatiales
(LEGOS), Toulouse
Tel: +33 (0)5 61 33 30 01
[email protected]
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REFERENCES:
GOURIOU, Y., T. DELCROIX,
G. ELDIN - Upper and intermediate circulation in the
western equatorial Pacific
Ocean in October 1999 and
April 2000, Geophys. Res.
Letters, 2006 (33), L10603,
doi:10.1029/2006GL025941.
KEY WORDS
THE DEEP CURRENTS,
OCEAN CIRCULTATION,
Scientists are currently seeking to describe ocean circulation and improve on data
acquired, aiming to identify the physical
mechanisms that regulate climate variability.
The impact of the ENSO (El Niño-Southern
Oscillation) event on the climatic situation in the
southern Pacific Ocean is still not well known,
for instance. In two oceanographic cruises
run in October 1999 and April 2000 as part of
the IRD’s ECOP programme, the Institute’s
researchers were able to study this region and,
in particular, the ENSO. The latter has a determinant effect on the distribution of ocean water
masses, ocean/atmosphere exchanges in the
tropical southern Pacific and many anomalies
of climate that occur on the continents that
border the Pacific. Physical determinations of
currents and masses of water under transport
were made from the surface down to 1 200
m over a large area, 1700 km in length, along
the Equator (between the Equator and 10° S
latitude, between 165° and 180° E longitude),
using a Lowered Acoustic Doppler Current
Profiler (L- ADCP) (2) installed aboard R/V Alis,
the IRD oceanographic research vessel.
These series of measurements give a welldefined picture of the tropical circulation in this
zone, for two specific dates. They show up in
particular the horizontal alternation of bands of
currents of opposing directions between the
Equator and 10° S latitude, from the surface
to 1200 m. Essentially, however, they reveal a
surprising variability of intermediate equatorial
currents (the equatorial intermediate current
(EIC) and the lower equatorial intermediate
current (LEIC)), which plunge at the Equator
under the Equatorial Undercurrent and flow in
the same direction, between about 300 and
1200 m (see Figure). Between October 1999
and April 2000, these equatorial intermediate
currents changed direction, between 2° S latitude and the Equator, over the 1 700 km of
the zone investigated. This reversal is already
known, but its amplitude in this case is striking.
The resulting variation in water mass transport
is considerable, around 100 Sv (50 Sv towards
the West in October 1999 and 50 Sv towards
the East in April 2000). The question is, what
causes this change-about? One hypothesis
put forward involves the passage of an oceanic instability wave, but no disturbance of the
EUC was detected during the research cruises
and the reversal remains unexplained. Further
current measurement campaigns in the future
should shed light on this event and bring clues
for unravelling the dynamics of these currents.
At present, such a change in ocean water
mass transport must be taken into account
in studies on the mass balance that exists in
the equatorial Pacific Ocean. Mathematical
models of ocean circulation are needed so that
the variations in water transport can be reproduced, and thereby facilitate assessment of their
impact on the climate variability, whether a seasonal, inter-annual or decennial temporal scales.
(1) The average transport of this current is
estimated at 30 Sv (1 Sverdrup, unit of volume
transport = 1 000 000 m3/s, a value commensurable with the average discharge of the
world’s major rivers). For comparison, the average transport at the mouth of the Amazon, the
most voluminous in the world, is about 0.3 Sv.
(2) Knowledge of ocean current circulation made
great advances from the 1980s with the general
use of current meters applying the Doppler
effect (Acoustic Doppler Current Profiler, ADCP)
which, fitted under the hull of the research
vessels, make continuous measurements of
currents (from ocean surface to an average 700
m depth). Since the 1990s investigation of deep
ocean circulation has been using a new generation of ADCP, the L-ADCP (Lowered Acoustic
Doppler Current Profiler), which are fixed on
CTD (conductivity-temperature-depth) probes
then lowered to a fixed point at depth.
The principle of the Doppler effect (which is also
employed in road and aviation radars) is founded on the instrument’s emission of an acoustic
signal. This signal is reflected by particles (zooplankton) transported by the ocean currents.
These particles are in movement and therefore
alter the frequency of the reflected sound wave.
The difference in frequency between the emitted wave and the reflected wave is a function
of the relative velocity between the instrument
and the particles, and therefore of the current
velocity.
Marie Guillaume-Signoret - IRD
Translation : Nicholas Flay
Océans et climat
CLIMATE VARIABILITY,
TROPICAL
PACIFIC
A book accessible to a large public, devoted to the relations between the oceans and the
climate, has just been published by IRD Editions. It includes a review of the technical advances, the human and scientific means deployed for the discovery of fundamental mechanisms
of ocean dynamics and the particular role of tropical oceans in climatic variability.
Océan et climat, by Jacques Merle (IRD), IRD Editions, 2006, 222 pages.
Marie Guillaume - Signoret, coordinatrice
Délégation à l’information et à la communication
Tél. : +33(0)1 48 03 76 07 - fax : +33(0)1 40 36 24 55 - [email protected]
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