Bifurcation of the Subtropical South Equatorial Current against New Caledonia in December 2004
from a hydrographic inverse box model
Ganachaud, A., L. Gourdeau, L. and W.S. Kessler
Journal of Physical Oceanography, accepted
The South Equatorial Current (SEC), the westward branch of the South Pacific subtropical gyre, extends from the equator to 30°S at depth. Linear ocean dynamics predict that the SEC forms boundary currents on the eastern coasts of the South Pacific islands it encounters. Those currents would then detach at the northern and southern tips of the islands, and cross the Coral Sea in the form of jets. The Vanuatu archipelago and New Caledonia are the major topographic obstacles on the SEC pathway to the Australian coast. Large-scale numerical studies, as well as climatologies, suggest the formation of three jets in their lee: the North Vanuatu Jet (NVJ), the North Caledonian Jet (NCJ) and the South Caledonian Jet (SCJ), implying a bifurcation against the east coast of each island. We present here the flow observed during the SECALIS-2 cruise in December 2004 between Vanuatu and New Caledonia. A box-inverse model is used to provide quantitative transport estimates with uncertainties and infer the pathways and boundary current formation. For that particular month, the 0-2000m SEC inflow was found to be 20±4 Sv between Vanuatu and New Caledonia. Of that, 6±2 Sv bifurcated to the south in a boundary current against the New Caledonia Coast (the Vauban Current), and the remainder exited north of New Caledonia, feeding the NCJ. The flow is comparable above and below the thermocline, while complex topography, associated with oceanic eddy generation, introduces several recirculation features. To the north, the NCJ, which extends down to 1500m, was fed not only by the SEC inflow, but also by waters coming from the north, possibly recirculated. To the south, a westward current rounds the tip of New Caledonia. The numerical simulation suggests a partial continuity with the deep extension of the Vauban Current (this current would then be the SCJ) while the hydrographic sections are too distant to confirm such continuity.
The revised/accepted version will be posted here shortly.
Dr. William S. Kessler
NOAA / PMEL / OCRD
7600 Sand Point Way NE
Seattle WA 98115 USA