SOFAR float trajectories from an experiment to measure the Atlantic cross equatorial flow (1989-1990) (original) (raw)
Deep cross-equatorial flow in the Atlantic measured with SOFAR floats
Journal of Geophysical Research, 1993
Neutrally buoyant SOFAR floats at nominal depths of 800, 1800, and 3300 m were tracked for 21 months in the vicinity of tropical boundary currents in the Atlantic near 6øN and at several sites near 11øN as well as along the equator. Trajectories at 1800 m show a swift (>50 cm/s), narrow (100 km wide), southward flowing deep western boundary current (DWBC) extending from 7øN to the equator.
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.
Journal of Physical Oceanography, 1999
Two neutrally buoyant SOFAR floats vigorously looped and meandered at depths of 950-1150 m in the eastern tropical Atlantic Ocean. The float trajectories illustrate a poleward flow along the tropical eastern boundary and significant intermediate-depth mesoscale variability in the low-latitude eastern basin. One float, caught within an energetic cyclonic eddy near the eastern boundary, looped cyclonically 14 times while translating 600 km northward parallel to the African coastline. A second float, launched near the Mid-Atlantic Ridge, meandered eastward with a Lagrangian zonal wavelength of 400 km and meridional amplitude exceeding 200 km. Satellite infrared imagery indicates that horizontal shear associated with the system of near-surface zonal equatorial currents may contribute to the observed intermediate-depth variability. * Woods Hole Oceanographic Institution Contribution Number 9691.
Deep cross-equatorial flow in the Altantic measured with SOFAR floats
Journal of Geophysical Research, 1993
Neutrally buoyant SOFAR floats at nominal depths of 800, 1800, and 3300 m were tracked for 21 months in the vicinity of tropical boundary currents in the Atlantic near 6øN and at several sites near 11øN as well as along the equator. Trajectories at 1800 m show a swift (>50 cm/s), narrow (100 km wide), southward flowing deep western boundary current (DWBC) extending from 7øN to the equator.
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.
Deep Sea Research Part II: Topical Studies in Oceanography, 1999
Fourteen neutrally buoyant SOFAR floats at a nominal depth of 1800 m were tracked acoustically for 3.7 yr in the vicinity of the western boundary and the equator of the Atlantic Ocean. The trajectories revealed a swift, narrow, southward-flowing deep western boundary current (DWBC) extending from 7N across the equator. Two floats crossed the equator in the DWBC and went to 10S. Two other floats left the DWBC and drifted eastward in the equatorial band (3S-3N). Three floats entered the DWBC from the equatorial current system and drifted southward. These results suggest that at times the DWBC flows directly southward across the equator with a mean velocity of 8-9 cm/s averaged over long distances (:2800 km). At other times DWBC water is diverted eastward near the equator for long periods (2-3 yr), which can reduce the mean along-boundary velocity to 1-2 cm/s. This is much less than the instantaneous along-boundary velocities in the DWBC, which are often above 25 cm/s and occasionally exceed 50 cm/s. Mean eastward-flowing jets were observed near 2N and 2S bounding a mean westward jet centered on the equator (1S-1N). The southern jet at 2S coincides with a CFC-rich plume centered south of the equator. The CFC plume is inferred to have been advected by the southern jet across the Atlantic and into the Gulf of Guinea.
The mid-depth circulation of the northwestern tropical Atlantic observed by floats
Deep-sea Research Part I-oceanographic Research Papers, 2009
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.
Recent moored current meter and SOFAR float observations in the eastern Atlantic near 32N
Journal of Marine Research, 1988
Basic flow statistics from the two-year deployment of a mooring in the vicinity of 32N and 24W are presented, along with intercomparisons between SOFAR float results concurrent with the first year of moored instrument data. Current-temperature meters were deployed in the main thermocline (-500 m depth), in Mediterranean Water (1000-1100 m depth) along with the SOFAR floats, and at an abyssal (-3000 m) level. The float and current meter averages over a common time interval are at least roughly the same, with eddy field intercomparisons being better than those for mean flow.
The intermediate depth circulation of the western South Atlantic
Geophysical Research Letters, 1999
The subsurface oceanic circulation is an important part of the Earth climate system. Subsurface currents traditionally are inferred indirectly from distributions of temperature and dissolved substances, occasionally supplemented by current meter measurements. Neutrally-buoyant floats however, now enable us to obtain for the first time directly measured intermediate depth velocity fields over large areas such as the western South Atlantic. Here, our combined data set provides unprecedented observations and quantification of key flow patterns, such as the Subtropical Gyre return flow (12 Sv; 1 Sverdrup = 106m3s4), its bifurcation near the Santos Plateau and the resulting continuous narrow and swift northward intermediate western boundary current (4 Sv). This northward flowing water passes through complex equatorial flows and finally enters into the North Atlantic.