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Accueil > Actualités > Séminaires > Séminaires 2009

Mardi 29 Septembre à 11:00 - Salle A103

William S. Kessler (NOAA / PMEL / OCRD)

Titre/Title :
Measuring South Pacific low-latitude western boundary currents with ocean gliders : A pilot study

Contact :
Jacques Verron

Résumé/Abstract :

Attention has focused on the equatorward western boundary current of the South Pacific as a pathway from the subtropics to the equator, and its potential role in the evolution of the ENSO cycle and longer-timescale variability in the equatorial Pacific. Water property diagnoses suggest that the majority of the water in the equatorial undercurrent comes from the South Pacific, principally via the Solomon Sea. The geography of the region is unusual in that it provides two western boundaries : a "real" one at the coast of New Guinea, plus the nearly-solid Solomon Islands chain about 600km to the east, stretching over more than 5 degrees of latitude. With only occasional previous snapshots of this system, we set out in mid-2007 to monitor the Solomon Sea circulation using ocean gliders, which measure absolute velocity, temperature and salinity, sampling densely at scales of about 4km and 4 hour intervals.

Five glider roundtrips across this system have been done in continuous rotation, encompassing the strong La Nina event of 2007-2008. The observations demonstrate the boundary-intensified flow as an undercurrent, about 80km wide, with substantial ENSO-related fluctuations. Consistent with linear Rossby dynamics driven by equatorial winds, equatorward transport dropped to nearly zero in the late stages of the La Nina. However, the observations have also exposed several mysteries that are the subject of ongoing work.

Although the Solomon Sea western boundary current is continuous with flow coming from further south along the coast of Australia, it is joined by substantial additional transport directly from the open Pacific. This is surface-trapped and associated with the thermocline slope, unlike the much deeper and thicker boundary current, and also seems more variable than the boundary current. What causes the South Equatorial Current to turn north into the Solomon Sea, inconsistent with the prevailing (strongly negative) wind stress curl forcing ? Do these two distinct elements of equatorward flow have different destinations, and does this imply different climate significance as they arrive at the equator ?

The talk will finish with some speculations as to why the New Guinea Coastal Undercurrent persistently reverses above about 200m, unlike the corresponding LLWBCs in the North Pacific. Although this fact has been known since the early WEPOCS surveys of the mid-1980s, it remains unexplained.