Observations of Meso-Scale Structures at the Eastern Edge of the Western Equatorial Pacific Warm Pool

The FRONTALIS cruise was designed to observe and improve the understanding of the zonal front located at the eastern edge of the western Pacific warm pool. The cruise took place in April 2001, during the 1998-2001 La Nina event, within 155E-165E, 5N-20S. It provided CTD-derived temperature and salinity profiles (0-500 m) together with concurrent L-ADCP-derived current profiles (0-600 m), continuous ADCP profiles (0-250 m) and surface measurements (SST, SSS, pCO2). Bottle sampling further supplied nutrients and chlorophyll "a" measurements, and plankton casts were carried out on either side of the frontal zone.

Drifting buoys released during the cruise along the equator within 155E-165E suggest a 157E-158E zonal current convergence which usually marks the eastern edge of the warm pool. Consistently, a sharp drop in pCO2 and nutrient distributions was observed within 10-20 km near 157.5E, as expected from a frontal structure. West of this longitude, SST reached values in excess of 30C and a notable vertical salinity gradient appeared within the isothermal layer thus forming a well-defined barrier layer not present east of the front. Unexpectedly, a zonal SSS gradient was not evidenced across the front as was the case in the few existing high-resolution observations. Moreover, synoptic ADCP data revealed successive zonal convergences and divergences within 155E-165E. Likely mechanisms responsible of these features will be proposed, with help of additional observations providing the large-scale context for the cruise.