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 ( 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.

PI: Thierry Delcroix (IRD Toulouse) & Christophe Maes (IRD Nouméa)
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