The mid-depth circulation of the subpolar North Atlantic Ocean as measured by subsurface floats

Abstract

The multi-year mean, mid-depth circulation of the subpolar North Atlantic Ocean was estimated from direct velocity measurements obtained over a 7.5-year period by 211 neutrally-buoyant, profiling, subsurface floats. We present a statistical analysis of these drift velocity data, and describe the features of the circulation from 400–1500 m depth as measured by the floats. The time-lagged auto-covariances of velocity indicate a Lagrangian integral time scale of approximately 10 days at all drift depths and throughout the basin. Space–time averages of velocity and the associated error ellipses were constructed, and this information was used in an objective analysis to produce estimates of the multi-year mean flow. Estimates of eddy kinetic energy were also computed. The mapped flow depicts a basin-scale cyclonic gyre bounded in the south by the North Atlantic Current. The strongest currents are found along the coasts of Greenland and Labrador, where mean speeds exceed 10 cm s - 1 . Closed cyclonic recirculations appear just offshore of the boundary currents in the Labrador and Irminger Seas and in the Iceland basin, and also southwest of the Reykjanes Ridge. Topographic steering extends upwards to at least 400 m depth above bathymetry that is significantly deeper, and is most apparent in flow along basin boundaries, around the Reykjanes Ridge, and through gaps in the Mid-Atlantic Ridge. In the center of the basin-scale gyre and to the south of the North Atlantic Current flow is characteristically weak with few large-scale circulation features. There is no indication of a strong linkage along the western boundary from the subpolar to subtropical gyre at depths sampled by floats.