Lagrangian measurement of mid-depth currents in the eastern tropical Pacific

Helium tracer data and basin-scale steric height maps imply that there is a n eastward-flowing, intermediate-depth (approximate to 1000 m) countercurre nt near 20 degrees N in the eastern tropical Pacific. To measure this curre nt directly, we deployed Profiling Autonomous Lagrangian Current Explorer ( P-ALACE) drifters at depths of 1000 and 1300 m near Hawaii (approximate to 19 degrees N, 156 degrees W) in June 1997. The deployment site marks the su mmit of Loihi Seamount, an active hydrothermal venting region thought to be the source for the helium anomaly. Contrary to expectation, the drifters f irst moved to the southwest, then turned abruptly westward along approximat e to 18 degrees N. The drifters maintained their westward trajectories for one year before stalling in an eddying region near 165 degrees to 168 degre es W. The westward intermediate flow and location of the current reversal r egion observed by the drifters appear to be at variance with the average fl ow fields derived from the helium and steric height data.