Mark D. Prater

The Labrador Sea Deep Convection Experiment

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.

A Meddy off Cape St. Vincent. Part I: Description

Abstract A meddy, an eddy formed from Mediterranean source water, was surveyed in detail with two types of expendable profilers and a CTD instrument. The muddy comprised two distinct, vertically aligned tenses with a combined thickness of 650 m. Both lenses were stratification minima. The upper lens was warmer, fresher (12.25°C, 36.5 psu), and more circular., the lower lens was cooler, more saline (12.1°C, 36.65 psu), and more elliptical, oriented alone a northeast by southwest line. The upper lens, homogeneous out to a radius of 6 km, had a radius of maximum velocity of 9 km. Its relative vorticity was −0.85 f, and its Ertel potential vorticity, 4 × 10−12 (m s)−1, was 17 times below ambient levels due to the combined effects of negative relative vorticity and vortex stretching. The meddy contained more kinetic energy than available potential energy (energy Burger number of 2.5). Compared with historical meddies, it had a larger Burger number and a more negative vorticity Rossby number.

Eddies in the Labrador Sea as Observed by Profiling RAFOS Floats and Remote Sensing

Abstract 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 61.5°N, 52°W where the 3000-m isobath separates from the shelf, and in the center of the basin at 58°N, 52°W. 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...

Isopycnal Lagrangian statistics from the North Atlantic Current RAFOS float observations

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.

Pathways of inflow and dispersion of warm waters in the Nordic seas

[1] 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 Voring 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.

An Alternative Hypothesis for the Origin of the “Mediterranean” Salt Lens Observed off the Bahamas in the Fall of 1976

Abstract 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, but instead in the vicinity of the northwest corner (51°N, 43°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 advect...

The Labrador Sea Deep Convection Experiment data collection

Between 1996 and 1998, a concerted effort was made to study the deep open ocean convection in the Labrador Sea. Both in situ observations and numerical models were employed with close collaboration between the researchers in the fields of physical oceanography, boundary layer meteorology, and climate. A multitude of different methods were used to observe the state of ocean and atmosphere and determine the exchange between them over the experiments period. The Labrador Sea Deep Convection Experiment data collection aims to assemble the observational data sets in order to facilitate the exchange and collaboration between the various projects and new projects for an overall synthesis. A common file format and a browsable inventory have been used so as to simplify the access to the data.