Central South Pacific thermocline water circulation from a high-resolution ocean model validated against satellite data: Seasonal variability and El Nino 1997-1998 influence

The oceanic circulation in French Polynesia, central South Pacific, is investigated based on a high-resolution model validated with satellite data. We focus on the upper 500 m, where ocean current variability has been barely documented due to a lack of in situ measurements. Seasonal and El Nino-Southern Oscillation 1997-1999 variability of surface and subsurface currents are characterized. In the north, the South Equatorial Current (SEC) follows a seasonal cycle with winter enhancement corresponding to trade wind strengthening. During summer, the SEC slows and the eastward South Equatorial Countercurrent (SECC) appears across the domain north of an upward Ekman pumping region related to the South Pacific Convergence Zone development in the area. Around the Marquesas Islands, a wind curl dipole is suspected to create westward and eastward (Marquesas Countercurrent, MCC) jets. The eastward Subtropical Countercurrent (STCC) is found south of 20 degrees S. It is disturbed by eddies and broken into westward and eastward branches. During El Nino 1997-1998, due to the reversed winds in the equatorial band, the SEC weakened, while eastward countercurrents appeared. The SECC reinforced and moved northeastward in summer 1998. During La Nina 1998-1999, following the trade wind enhancement, the SEC strengthened. The countercurrents weakened or disappeared and the SECC moved southwestward in summer 1999. During both El Nino and La Nina events, the STCC eddy activity strengthened, while the MCC and associated westward jet were absent. These variations and their dynamical origins are discussed, and a coherent picture of the French Polynesian current system and its variations is proposed.