Eldin¹, Gérard, Thierry Delcroix¹, Martine Rodier² and Robert Le Borgne²

¹IRD/LEGOS, 14 Avenue Edouard Belin, 31400, Toulouse, France, Tel: (33) 5 61 33 28 72, Fax: (33) 5 61 25 32 05, E-mail: Gerard.Eldin@cnes.fr and ²IRD, Centre de Nouméa, 98848 Nouméa, New Caledonia

Observations of the frontal zone between the HNLC upwelling area and the oligotrophic Warm Pool in the equatorial Pacific

The Pacific Ocean spans almost half of the globe circumference at the equator. Its surface waters are, on a mean, divided into two dynamical and biogeochemical systems: an area of equatorial upwelling in the east, presenting cold and salty waters with HNLC characteristics, and an area of warm and fresh waters in the west (the “Warm Pool”) with oligotrophic features. Location, hydrological and biogeochemical features of the frontal area separating the two systems recently deserved numerous investigations, most of all for ENSO dynamical and carbon budget studies. This poster presents results from two oceanographic cruises which sampled contrasted frontal features. During El Niño, in October 1994, the frontal zone was observed near 172°W, with a sharp front in near-surface salinity and a few hundreds kilometres separation between the physical and biogeochemical fronts. During La Niña, in April 2001, the frontal zone was found about 3500 km farther to the west, near 157°E, without any no sharp variation in salinity, but coincidental physical and biogeochemical structures. While the gross characteristics of the front depend on the mean oceanic circulation, and its longitude is ENSO dependent, analysis of the two cruises data sets and of additional data indicate that the spatial distributions of physical and biogeochemical properties in the frontal zone are deeply conditioned by local and remote forcing at intraseasonal time-scales.