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The
FRONTALIS-3 cruise in April – May 2005
A
French contribution to ARGO deployments in the western tropical Pacific
On average, the distribution of near-surface water in the equatorial
Pacific presents warm (>28°C) and low salinity (<34.5) waters in the
so-called warm and fresh pool. The eastern edge of the warm pool is dominated by
strong interannual zonal displacements related to the El Niño Southern
Oscillation (ENSO) event. It is most often characterised by a well-marked zonal
front in surface salinity, partial pressure of CO2, concentration of
nutrients and plankton. Such zonal front is the result of a zonal convergence of
western and central Pacific water masses. The zonal front is connected with
changes in mixed-layer temperature and salinity, with barrier-layer thickness,
formation and variability, as well as with the location of the world’s largest
tuna harvest. The zonal displacements of the front induce an extension (El Niño)
or a reduction (La Niña) of the surface waters above 28°C, the temperature
threshold for organised atmospheric convection. Hence, they are the main source
for the ENSO coupled system.
The
FRONTALIS-3 cruise, conducted on board the IRD R/V l’Alis during April 22 –
May 20 (Fig. 1) in the western tropical Pacific ocean, carried on physical (CTD,
L-ADCP, S-ADCP, TSG, drifting buoys) and biochemical (NO2, NO3,
PO3, pCO2, plankton, chla) observations on this zonal
front in order to analyse the mechanisms responsible for its formation, its
persistence and potential role in the ENSO air-sea interaction coupled system.
During
the cruise, we farther repeated the sampling of the 165°E meridional section in
order to study a) the time-variability of the water masses between 0 and 2000 m
and b) the ventilation of the thermocline waters by the subtropical waters,
which is thought to be one possible mechanism at the origin of a decadal
variability of the ENSO phenomenon. This 165°E section has been sampled
regularly during 1984-2000, within 0-1000m. It represents one of the longest
time series of repeated hydrographic sections in the open ocean.
A
total of 20 PROVOR floats were also launched during that cruise (Fig.2), with 10
floats along the equator within 161°E and 172°E, and 10 floats along 165°E
within 19°S and 10°S. In line with the CLIVAR (www.clivar.org) objectives,
these floats will enable us, for the first time, to monitor the eastern edge of
the warm pool during a rather long time period, depending on the drifter drifts
at depth, as well as to initiate continuous measurements of the southern route
of the possible thermohaline anomalies related to the low frequency modulation
of ENSO.
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To display the FRONTALIS floats data, click on the left side buttons to :
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PI: Thierry Delcroix (IRD
Toulouse) & Christophe Maes
(IRD Nouméa)
Data management contact: codac@ifremer.fr
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