SOFAR float trajectories from an experiment to measure the Atlantic cross equatorial flow (1989-1990) (original) (raw)

Part I of this paper has given a descriptive view of the trajectories of 26 SOFAR floats drifting near 700-m depth in the central North Atlantic during the mid-1980s, as part of the TOPOGULF experiment. Here an Eulerian analysis of the 53.4 collected float years is performed by grouping data in 2 latitude 4 longitude boxes. The mean circulation lacks a significant southward Sverdrupian flow and shows instead zonal bands of alternating westward and eastward currents (except in the Canary Basin). West of the ridge and north of 38N, the general northeastward flow of the Gulf Stream system is recovered while, south of 38N the westward recirculation observed from historical float data between 70 and 55W is shown to extend as far as 40W. A mean Azores Current is observed both west of the ridge near 33N and east of 30W near 34N. In between, east of the ridge, a tongue of high eddy energy indicates stronger eddy activity and local instabilities of the Azores Current. Eddy kinetic energy and eddy potential energy (the latter inferred from temperature measurements) are equipartitioned on the scale of the eddy field and show a tenfold increase from 33N, 33W to 38N, 50W. Lateral diffusivity increases westward (1.5 10 3 m 2 s 1 in the Canary Basin, 3.5 10 3 m 2 s 1 in Newfoundland Basin, 4.1 10 3 m 2 s 1 near Corner Rise Seamounts) and scales approximately as eddy velocity times the first baroclinic Rossby radius of deformation.

Quasi-Lagrangian trajectories of 26 sound fixing and ranging (SOFAR) floats have been collected near a depth of 700 m in the Central North Atlantic between 1983 and 1989, aiming at studying the influence of the Mid-Atlantic ridge on the large-scale intermediate circulation. Launched as tight clusters (18 km near neighbor distance) on either side of the Mid-Atlantic ridge, the floats dispersed quickly over a few months, jumping from one mesoscale eddy to the next. By and large, cyclonic and anticyclonic eddy motions are equipartitioned. Apparently the Mid-Atlantic ridge remains a barrier even at that shallow depth, since only one float from either side drifted across the ridge. After a few years, floats have circulated through most of the western basin (west of the Mid-Atlantic ridge), between 30 and 45N; while east of the ridge and south of the Azores Plateau, floats advected east of the Great Meteor et al. Seamounts by the Azores current wandered more sluggishly. On this timescale, float dispersion is much more efficient zonally than meridionally, an anisotropy mainly seen west of the ridge, where floats spread westward over 30 longitude, while no float penetrated south of 30N and only two crossed 45N northward.

A comprehensive analysis of velocity data from subsurface floats in the northwestern tropical Atlantic at two depth layers is presented: one representing the Antarctic Intermediate Water (AAIW, pressure range 600–1050 dbar), the other the upper North Atlantic Deep Water (uNADW, pressure range 1200–2050 dbar). New data from three independent research programs are combined with previously available data to achieve blanket coverage in space for the AAIW layer, while coverage in the uNADW remains more intermittent. Results from the AAIW mainly confirm previous studies on the mean flow, namely the equatorial zonal and the boundary currents, but clarify details on pathways, mostly by virtue of the spatial data coverage that sets float observations apart from e.g. shipborne or mooring observations. Mean transports in each of five zonal equatorial current bands is found to be between 2.7 and 4.5 Sv. Pathways carrying AAIW northward beyond the North Brazil Undercurrent are clearly visible in the mean velocity field, in particular a northward transport of 3.7 Sv across 16°N between the Antilles islands and the Mid-Atlantic Ridge. New maps of Lagrangian eddy kinetic energy and integral time scales are presented to quantify mesoscale activity. For the uNADW, mean flow and mesoscale properties are discussed as data availability allows. Trajectories in the uNADW east of the Lesser Antilles reveal interactions between the Deep Western Boundary Current (DWBC) and the basin interior, which can explain recent hydrographic observations of changes in composition of DWBC water along its southward flow.