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

Abstract

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