Topographic steering of a mid‐depth drifter in an eddy‐like circulation region south and east of the Hawaiian Ridge

Abstract

A Profiling Autonomous Lagrangian Current Explorer (PALACE float) deployed in June 1997 at a “parking depth” of 1250 m depth to the south of Hawaii drifted westward within the North Equatorial Current for about a year before stalling in a region of directionally variable, eddy‐like flow centered near 21°N, 168°W, within the intersection region of the Pacific Seamounts and Hawaiian Ridge. The float remained within the 5‐km‐deep, eddying regime for more than 3 years before ending communication in October 2001. During its year in the North Equatorial Current, the float traveled roughly 1400 km westward along ≈18°N latitude at a mean mid‐depth speed of 4.39 ± 2.19 cm s−1. Within the eddying region, mid‐depth currents remained relatively strong but directions became highly variable and the mean westward drift diminished to 0.05 ± 2.67 cm s−1. Close examination of the 15‐day trajectory data reveals that the float entered the eddying region after making an abrupt northward turn along the shallow (∼2000 m deep) eastern limb of the Pacific Seamounts. Similar depth‐related directional changes were frequently observed throughout the 3.4‐year drift within the eddying region, consistent with modification of the mid‐depth circulation by the underlying seafloor topography. On the basis of the density structure obtained along the float trajectory, only topographic features with scale lengths of roughly 50 km or more have a significant effect on the mid‐depth flow.