Vertically averaged velocities in the North Atlantic Current from field trials of a Lagrangian electric-field float

Abstract

A subsurface Lagrangian float that utilizes motional induction to calculate vertically averaged velocities was tested in the North Atlantic Current (NAC), taking advantage of existing cruises and infrastructure. The Electric Field Float (EFF) is a RAFOS float with horizontal electrodes that measures its own velocity by RAFOS tracking and calculates vertically averaged velocities when merged with the electrode system. The observations showed depth-averaged velocities that were fast in the core of the NAC (0.6–0.9ms −1) and moderate in adjacent recirculations and eddies (0.3–0.4ms−1). A float at 850dbar moved at close to the depth-averaged velocity, while shallower floats followed surface intensified flow on top of the depth-averaged motion. Integral time scales of depth-averaged velocity (1.3–1.6±0.4d) are slightly shorter than time scales of float velocity (1.6–2.0±0.3d), while integral length scales of depth-averaged water velocity (35±10km for u, 18±6km for v) are slightly shorter than length scales of float motion (53±12km for u, 28±6km for v). Velocity spectra of depth-averaged velocity show significant variance at inertial periods. Quantitative and qualitative validation with multiple independent data sets confirms the accuracy of the instrument and sampling strategy in the NAC, advancing the limited observational knowledge of depth-averaged circulation in subpolar regions.