In this study, we use 22 acoustically tracked RAFOS floats to examine the routes and spreading of warm North Atlantic waters entering the Norwegian Sea between Iceland and the Faroes. The majority of floats crossed the Iceland‐Faroe Ridge at the eastern end where it is deepest. They joined the Iceland‐Faroe Front, but rather than continue north with the outer branch of the Norwegian Atlantic Current into the Nordic seas, most of them jumped over to the inner branch, which continues the inflow through the Faroe‐Shetland Channel northeast over the Vøring Plateau toward the Lofoten Basin. Indeed, 17 floats, whether deployed near Iceland or the Faroes, did so; only 2 floats continued north along the outer branch. Despite the small numbers, these results highlight (1) the strong influence of topography on flow patterns, (2) the strong crossover of Iceland‐Faroes waters to the inner branch, and (3) the rapid and structured spreading into the Nordic seas.

Neutrally buoyant floats, deployed across the northern slope of the Iceland‐Faroe Ridge at 800m depth, reveal tight topographic control of their movement: a cluster of 22 floats drifts southeast to the Faroe‐Shetland Channel where it bifurcates such that floats deployed over the upper slope turn south and eventually exit the Norwegian Sea through the Faroe Bank Channel, and floats over the deeper slope turn north in the Norwegian Sea. A subset of the latter group moves quickly north along the western slope of the Vøring Plateau and divides with most of the floats turning east into the Lofoten Basin and the remainder circulating cyclonically around the Norwegian Basin. This study establishes that i) the Faroe Bank Channel overflow waters must come from along the slope north of the Faroes, not the interior of the Norwegian Sea, and ii) exchange of intermediate waters between basins takes place along topographically controlled routes.

We present the results of six dye tracer experiments that measured the mixing and circulation at the shelfbreak front on the New England Shelf. The last three were conducted during the New England Shelfbreak Productivity Experiment (NESPEX) with concurrent isopycnal float deployments. The results are consistent with the Chapman and Lentz [Chapman, D.C., and Lentz, S.J. (1994). Trapping of a coastal density front by the bottom boundary layer. Journal of Physical Oceanography, 24, 1465–1479.] model prediction of the separation and upwelling along the shelfbreak front of bottom boundary layer (BBL) water forced by an Ekman buoyancy flux, but show considerable variability. Cross-shelf velocities at the detachment point are 2–3 × 10−2 m/s. But seaward, over the slope region, dye tagged water was sheared from the main patch into small filaments that upwelled along the front with cross-shelf speeds up to 0.1 m/s. Cross-shelf diffusion was of order 10 m2/s in the mixed bottom layer and 1 m2/s in the interior along the front. Within the stratified front, the mean vertical diffusivity was Kz ∼ 4 × 10−6 m2/s. The dispersion of shelfwater in the slope region is effected by turbulent flow with advective speeds exceeding the small scale diffusive mixing. The mean flux of the detached BBL water is sufficient to account for the net loss of shelf water during its transit from Cape Cod to Cape Hatteras.

Present research suggests that the North Atlantic Oscillation (NAO) has a significant impact on the circulation in the Northwest Atlantic. An ocean general circulation model was used to quantify the changes of circulation patterns and transport on this region during a high NAO period (between 1992 and 1995) when compared to climatological mean forcing. Upstream of the study region, the maximum barotropic transport in the Labrador Sea was decreased by 8Sv (1Sv=106m3s−1) during the high NAO conditions, leading to a reduction in transport of the Labrador Current by ∼1.6Sv east of Newfoundland and by ∼0.4Sv south of Newfoundland. In addition, a Lagrangian analysis was performed to examine the pathways of transport that advect water properties across the Grand Banks and how those pathways are modified owing to the high-NAO forcing. The potential implications of these changes for biological production processes in the adjacent downstream shelf regions such as the Scotian Shelf, Gulf of Maine, and Georges Bank are discussed.

Overflows do not easily lend themselves to study by Lagrangian floats that remain on a constant isobaric (pressure) or isopycnal (density) surface, since the mixing, entrainment, and descent of an overflow plume result in an increase of the pressure and typically a decrease in the density of the overflow waters. A simple technique to maintain the float's altitude above the bottom was developed, and 12 “bottom-following” RAFOS floats were deployed at or downstream of the sill in the Faroe Bank Channel in the summer of 2000 from the R.S.S. Discovery. A technical problem resulted in the majority of the floats becoming stuck to the bottom; nevertheless several floats were able to traverse the Iceland Basin and surface near the southeastern slope of Iceland. These floats made a descent from the mouth of the Faroe Bank Channel, only to shoal along the southern slope of the Iceland-Faroe Ridge before descending again when passing through the northwest corner of the Iceland Basin. Typical current speeds through the Basin were 0.20–0.30ms-1, with peaks of 0.40–0.50ms-1. Although the floats that were stuck on the bottom provided no trajectory information, they were able to provide a time-series of bottom or near-bottom temperature. In addition, a crude estimate of the flow regime could be made by interpreting the pressure signals from these stuck floats as a response to strong or weak currents. Floats that were bottom stuck near the mouth of the Channel experienced large fluctuations in temperature (0–5∘C) and height of the bottom (and thus presumably speed) on scales from 1 to 4 days. Another float stuck 100km downstream of the sill underwent temperature and speed excursions on similar time scales, albeit over a smaller range. The behavior of the floats is assumed to be the result of the mesoscale variability of the overflow plume downstream of the Faroe Bank Channel.

By causing an isopycnal float to rise and sink to neighboring density surfaces, one can measure layer thickness along a float trajectory and thus variations in static stability. We report here on layer thickness measurements that were obtained from the nearly 100 isopycnal floats that were deployed in the 1993–1995 Lagrangian study of the North Atlantic Current. Layer thickness variations depend significantly upon location and process: In the high velocity core of the meandering North Atlantic Current, layer thickness changes correlate with curvature of the flow—stretching in troughs, compression in crests, but this correlation fails in unsteady flows or outside the high velocity center of the current. Floats also register significant changes in stratification when they cross between the warm waters in the Newfoundland Basin and the cold waters from the Labrador Sea. As floats enter and exit what appear to be coherent eddy structures, significant but not necessarily predictable changes in layer thickness can occur. While the technology to measure layer thickness variations performed as well and reliably as we had hoped for, the presence of ubiquitous small-scale features in the density field together with the fact that the thickness measurements sampled only O(100) m of the total water column limited our ability to isolate and identify the cause of thickness variations to only relatively energetic features.

This paper presents a numerical study of the warm water pathways, transports, and water mass transformation in the Newfoundland Basin region and an investigation of the rectification effects of a series of cold air outbreaks (CAOs) on the above processes. An initial “mean state” simulation was compared with observations and showed good agreement. Then, repeated CAO events were explicitly included in the surface fluxes and illustrated the following rectification effects. The thermal regime of the entire baroclinic layer was impacted, as the thermocline deepened and the temperature anomaly was noticeable down to ∼500 m. Different mechanisms were responsible for the propagation of the temperature anomaly at different stages. During the onset of the CAO, vertical diffusion propagated the temperature anomaly downward near the surface, then vertical advection further propagated the anomaly downward between CAO events to about 500 m depth. With CAOs, the North Atlantic Current carried more warm water, and its pathway in the Northwest Corner shifted towards the southeast. The volume‐averaged mean and eddy kinetic energy increased by 30% and 20%, respectively, and the water mass transformation rate in the Newfoundland basin was doubled. The Gulf Stream carried more heat, but heat transport to the eastern basin decreased owing to the increase in heat release to the atmosphere.

the reversible transformation of the two phases. Taken together, our findings strongly argue for the HDA–LDA transformation being a first-order phase transformation. Structurally, HDA and LDA resemble high-density liquid water (HDL) and low-density liquid water (LDL), respectively, and the present results thus also support the liquid–liquid critical-point theory, which holds that HDL and LDL transform into each other through a discontinuous, first-order process. Finally, we note that a clear polyamorphic phase separation has already been observed in the Al2O3–Y2O3 system , and may be seen in other network-forming amorphous materials. A

Data from profiling RAFOS floats, TOPEX/Poseidon altimetry, and the alongtrack scanning radiometer (ATSR) aboard ERS-1 have been used to describe the spatial and seasonal patterns of eddy variability in the Labrador Sea. Peaks in sea surface height (SSH) variability appear in two regions: off the west Greenland shelf near 61.5degreesN, 52degreesW where the 3000-m isobath separates from the shelf, and in the center of the basin at 58degreesN, 52degreesW. Both locations show seasonal ranges in SSH variability of up to 40 mm, with the Greenland site, having largest variability in January-March, leading the central site by 50 days. A sea surface temperature image from the ATSR at the Greenland site shows numerous eddies, both cyclonic and anticyclonic, being formed by injection of West Greenland Current water into the Labrador Sea interior. Data from profiling RAFOS floats launched in 1997 as part of the Labrador Sea Deep Convection Experiment are used to describe three of the West Greenland Current eddies in detail. One of the sampled eddies was anticyclonic, while the other two were cyclonic. The eddies contained various mixtures of Irminger Sea Water. Peak azimuthal velocities ranged from 22 to 42 cm s(-1), and diameters from 20 to 50 km. Although the floats were at a depth of 375 m, the surface elevations derived from cyclogeostrophy agreed with those obtained from TOPEX/Poseidon. The temporal and spatial patterns in SSH variability are thought to be caused primarily by seasonal variations in the strength and stability of the West Greenland Current and, less likely, by eddy formation following deep convection in the basin interior.

The circulation of water masses in the northeastern North Atlantic Ocean has a strong influence on global climate owing to the northward transport of warm subtropical water to high latitudes1. But the ocean circulation at depths below the reach of satellite observations is difficult to measure, and only recently have comprehensive, direct observations of whole ocean basins been possible2,3,4. Here we present quantitative maps of the absolute velocities at two levels in the northeastern North Atlantic as obtained from acoustically tracked floats. We find that most of the mean flow transported northward by the Gulf Stream system at the thermocline level (about 600 m depth) remains within the subpolar region, and only relatively little enters the Rockall trough or the Nordic seas. Contrary to previous work5,6, our data indicate that warm, saline water from the Mediterranean Sea reaches the high latitudes through a combination of narrow slope currents and mixing processes. At both depths under investigation, currents cross the Mid-Atlantic Ridge preferentially over deep gaps in the ridge, demonstrating that sea-floor topography can constrain even upper-ocean circulation patterns.

One hundred isopycnal floats were tracked on the 27.2 and 27.5 σθ surfaces in the Newfoundland Basin (NFB) from July 1993 to July 1995 to study the current structure and exchanges of waters between the subtropical and subpolar gyres. The float‐mapped mean flow consists of weak flows in the NFB and a strong boundary current (the North Atlantic Current (NAC)), which separates from the boundary at the Northwest Corner, becoming a diffusive zonal drift. The NAC meanders are linked to topography and have similar patterns on the two isopycnals despite the fact that the upper layer velocities are twice as fast as the lower layer ones. Perturbation velocity from the mean is used to compute isopycnal turbulent dispersion and diffusivity. This large data set allows us to resolve a narrow mean NAC and results in a Gaussian turbulence. The turbulence approximately follows the classic Taylor dispersion theory. Integral timescales and length scales and turbulent isopycnal diffusivity are of 1.5–2.5 days, 20–30 km, and (1 − 7) × 103 m2 s−1, respectively. The timescale increases with depth and decreases with latitude, the length scale decreases with depth and longitude, and the diffusivity decreases with depth and from NAC to NFB. Compared to previous results from surface drifters and isobaric floats, our isopycnal statistics are more isotropic and agree better with the Taylor dispersion theory because (1) the mean velocity has a better resolution and (2) the isopycnal floats are better Lagrangian followers. The diffusivity scales better with the rms velocity and length scale than with the velocity variance and timescale.

An instrument has been developed that measures finescale velocity and vorticity in seawater based on the principles of motional induction. This instrument, the electromagnetic vorticity meter (EMVM), measures components of the gradient and Laplacian of the electrostatic potential held induced by the motion of seawater through an applied magnetic field. The principal innovation described here is the development of a sensor for measuring small-scale vorticity, The sensor head consists of a strong NdFeB magnet, a Ave-electrode an ay, low-noise preamplifiers, and 20-Hz digitizers. The main electronics includes attitude sensors, batteries, a microprocessor, and a hard disk. The vorticity sensors are usually carried on a heavy towed vehicle capable of vertically profiling to 200 m and at tow speeds of several knots.The theoretical response functions of the EMVM are evaluated for velocity and vorticity. Extensive measurements were obtained in Pickering Passage, Washington, as the sensor vertically profiled in an unstratified tidal channel. During periods of strong flow, the vertical structure of all properties confirmed expectations for a fully developed turbulent bottom boundary layer. EMVM observations of velocity and vorticity are shown to be in agreement with the theoretical response function for isotropic turbulence. A principal result is that the vertical flux of spanwise vorticity (i.e., <(w'omega(y)')over bar>) is positive (i.e., flux is away from seabed) and vertically uniform. The vertical eddy diffusivity for vorticity is about 5 x 10(-2) m(2) s(-1), which is about the same value as for momentum.

A hypothesis is presented that the original salt lens, or "meddy," observed off the Bahamas in the fall of 1976 may have been formed, not near the Mediterranean outflow, bur instead in the vicinity of the northwest corner (51 degrees N, 43 degrees W) of the North Atlantic Current. An eddy was observed near the northwest corner by an isopycnal RAFOS float deployed during the 1993-95 North Atlantic Current Experiment, and had nearly identical temperature/salinity properties as those of the Bahamas lens. Hydrographic evidence of thick homogeneous layers with similar properties near the northwest corner suggest a possible formation mechanism by which surface eddies containing warm and saline waters are cooled and subducted. A plausible scenario is made whereby a northwest corner eddy might be advected southward in the Newfoundland Basin by the flow around the high pressure ridge east of the North Atlantic Current and then enter the recirculation gyre immediately south of the Gulf Stream. Such an eddy could be advected to the site of the Bahamas lens in just three years, perhaps much more quickly than an eddy of Mediterranean origin and without encountering the topographic barrier of the Mid-Atlantic Ridge. This conclusion is ironic because the Bahamas lens is considered the first observation of an eddy of Mediterranean origin, and led to the coining of the term "meddy.".

In the autumn of 1996 the field component of an experiment designed to observe water mass transformation began in the Labrador Sea. Intense observations of ocean convection were taken in the following two winters. The purpose of the experiment was, by a combination of meteorological and oceanographic field observations, laboratory studies, theory, and modeling, to improve understanding of the convective process in the ocean and its representation in models. The dataset that has been gathered far exceeds previous efforts to observe the convective process anywhere in the ocean, both in its scope and range of techniques deployed. Combined with a comprehensive set of meteorological and air-sea flux measurements, it is giving unprecedented insights into the dynamics and thermodynamics of a closely coupled, semienclosed system known to have direct influence on the processes that control global climate.

: The goal of this project is to develop a truly three-dimensional Lagrangian follower of water parcels for the coastal ocean. This COastal Ocean Lagrangian (COOL) float is based on an isopycnal float which would follow fluid parcels on a constant density surface. However, in the coastal ocean, there is significant mixing occurring such that a water parcel can change its density over a relatively short time. To account for this effect, we have designed a float which measures vertical (diapycnal, if the float is isopycnal) velocity past it. The COOL float has the capability to change its volume, hence its density, and follow the water parcel. -- We have completed two short cruises to test the COOL float. The first one (July 1997) was just off the continental shelf south of Rhode Island. We did six deployments of the COOL float using a variety of vane angles and either at an isobaric or isopycnal float. During August 1997, we completed two deployments of an isopycnal COOL float off the coast of Oregon in an attempt to look at water that might be upwellinq there.