Variability of the Equatorial Currents in the Western Pacific: Two Mooring Observations at 2°N and 4.7°N, 140°E

Two ADCP moorings were deployed at the core of mean Equatorial Undercurrent (EUC) and North Equatorial Countercurrent (NECC) at 140°E to continuously measured velocity profiles over entire upper 1100 m during January-August 2014 for the first time in this region. Significant intra-seasonal variability in equatorial currents with periods of 44-73 days are controlled by geostrophic balance to a large degree before mid-May and by the vertical propagation of intra-seasonal Rossby wave afterwards. The vertical propagation of intra-seasonal Rossby wave is characterized by the fact that the two mooring velocities are well correlated with only 10-day phase lag and both exhibit anti-clockwise rotation and upward phase propagation. The new observation captures two eastward reversal periods of the Equatorial Intermediate Current (EIC) and North Equatorial Subsurface Current (NESC) from mid-May to August. The intra-seasonal rather than annual Rossby wave appear to be a good candidate for driving the reversals of the EIC and NESC off the equator. However, the peak period of 90-day high-pass filtered wind stress is 8-20 days, shorter than that of currents. The vertical velocity shear, the peak period of which being consistent with that of wind stress, is found to be a bridge to connect currents and wind stress. Furthermore, the shear exhibits apparent upward phase propagation during the whole measurement period, suggesting that the equatorial currents derive their energy from the wind.