Deep Gulf Stream Research Articles

On the Deep Western Boundary Current south of Cape Cod

Abstract Using CTD/oxygen data from eight cruises in the decade from 1994–2003, we have constructed a mean ‘section’ of properties across the Deep Western Boundary Current (DWBC) south of Cape Cod near 70°W. Since all sections included direct velocity measurements, our composite section enables us to portray the flow field as well as the mean water mass structure. Inshore of the Gulf Stream between the 2500 and 4000 m isobaths, the flow is to the southwest along the bathymetry and is remarkably barotropic. The equatorward flowing Labrador Sea Water is shown to have high dissolved oxygen, low salinity, and low potential vorticity, while the underlying Overflow Water is high in oxygen. Transport estimates for the cold limb of the thermohaline circulation give a range of −14 to −19 Sv for the N. Atlantic Deep Water found on the section. The greatest uncertainty is due to the presence of a Warm-Core Ring on one of the sections, which apparently completely reversed the flow in the DWBC. Offshore of the DWBC, some of the deep source waters are returned to the north in the deep Gulf Stream. The section is compared to two other, widely separated locations (Abaco and 55°W) that have markedly different DWBC characteristics.

Barrier destruction and Lagrangian predictability at depth in a meandering jet

Numerical simulations of a jet with large amplitude meanders are used to explore chaotic advection processes and underlying geometry changes as functions of the ambient potential vorticity gradient β. Variations in β in the 2D model qualitatively simulate changes in depth in 3D, surface-intensified jets such as the Gulf Stream. As β is reduced, corresponding to motion on increasingly deep isopycnal surfaces, a number of geometrical transitions take place in the flanges and across the core of the jet. The most important is a joining (or separatrix reconnection) of heteroclinic cat’s eyes structures lying to the north and south of the jet core. The jet core acts as a barrier to transport, but this barrier is breached when the cat’s eyes merge. The subsequent chaotic transport across the jet is demonstrated by calculations of effective invariant manifolds (EIMs) originating in hyperbolic regions to the north and south of the core. Destruction of the central barrier occurs as β is lowered through a narrow window W about β = 0 and is marked by transitions form a meandering jet through a vortex street with no central meandering flow to a vortex street with a retrograde meander. Such small values of β are deemed reasonable in view of measurements of low potential vorticity gradients in the deep Gulf Stream. The strength of the central barrier for β outside W is tested by varying β about a mean value β0 and detecting the minimum amplitude of fluctuation necessary for destruction of the barrier. It is found that the barrier is stronger for β0 > 0, at least by this measure. A striking difference is that, for β 0, destruction of the barrier may only occur when β passes through W . Changes in underlying geometry also occur in the flanges of the jet and these changes alter the locations in which fluid is preferentially stirred and mixed.

Data Assimilation Experiments in the Gulf Stream Region: How Useful Are Satellite-Derived Surface Data for Nowcasting the Subsurface Fields?

Satellite-derived surface data have become an important source of information for studies of the Gulf Stream system. The question of just how useful these datasets are for nowcasting the subsurface thermal fields, however, remains to be fully explored. Three types of surface data—sea surface temperature (SST), sea surface height (SSH), and Gulf Stream position (GSP)—are used here in a series of data assimilation experiments to test their usefulness when assimilated into a realistic primitive equation model. The U.S. Navy’s analysis fields from the Optimal Thermal Interpolation System are used to simulate the surface data and to evaluate nowcast errors. Correlation factors between variations of the surface data and variations of the subsurface temperature are used to project the surface information into the deep ocean, using data and model error estimates and an optimal interpolation approach to blend model and observed fields. While assimilation of each surface data source shows some skill in nowcasting the subsurface fields (i.e., reducing errors compared to a control case without assimilation), SSH data reduce errors more effectively in middepths (around 500 m), and SST data reduce errors more effectively in the upper layers (above 100 m). Assimilation of GSP is effective in nowcasting the deep Gulf Stream, while the model dynamics produce eddies that are not included in the GSP analysis. An attempt to optimally combine SST and SSH data in the assimilation shows an improved skill at all depths compared to assimilation of each set of data separately.

Gulf Stream–Generated Topographic Rossby Waves

Abstract An inverse ray tracing model is applied to observations of 40-day topographic Rossby waves on the continental slope off of Cape Hatteras, North Carolina, to determine their origin. The rays are traced seaward and extend into the deep Gulf Stream, where the bottom slope remains strong enough that topographic β dominates planetary β. This enable coupling to occur between eastward propagating Gulf Stream meanders and topographic waves with eastward phase speed and matching zonal wavelength. Previous satellite observations indicate that the most frequently occurring Gulf Stream meanders have a period of 40 days and the model reveals that, as these meanders pass a topographic bend near 71°–72°W, they are able to couple to the observed topographic waves traced back from Cape Hatteras. The 40-day Gulf Stream meanders occur in bursts, which leads to associated bursts of the topographic waves.

Lateral entrainment in baroclinic currents

The strong mesoscale velocity fluctuations (''eddies'') observed in the deep Gulf Stream region maybe dynamically necessary to incorporate the Recirculation Gyres which account for the downstream increase in transport. The eddy-current interaction leading to entrainment is studied in a two layer quasi-geostrophic model with piecewise uniform potential vorticity. It is shown that the volume entrained by a bottom eddy of strength Gamma*(2) initially located near the edge of a bottom current depends mainly on the lateral shear st of this current, rather than on the much stronger shear of the upper layer. It is suggested that the ''mean entrainment velocity'' into an isopycnal layer of nearly uniform vertical thickness is proportional to (Gamma*(2s2)), where Gamma*(2) (cm(2)/sec) is proportional to the integrated potential vorticity anomaly of the eddy.

Space—Time Variability of the Deep Western Boundary Current Oxygen Core

Twelve historical CTD/oxygen sections between the Grand Banks and Cape Hatteras, occupied over the time period 1981–85, are analyzed to investigate the variability of the Deep Western Boundary Current (DWBC) property core. The sections are transformed into a coordinate system of bottom depth versus height above the bottom, then interpolated onto a regular grid in order to facilitate an inter-sectional comparison. The average sections show a high-oxygen core near 3200-m depth, θ = 2.2°C, which corresponds to a region of upward sloping isotherms against the boundary, inshore of the deep Gulf Stream. As the DWBC progresses toward Cape Hatteras it shoals significantly and becomes less dense as a result of mixing with surrounding fluid along its path. There is, however, much scatter about this general alongstream trend and large density fluctuations are correlated with changes in bottom depth and cross-sectional area of the DWBC core. This variability is most likely the downstream response to changes in the overflow source waters of the DWBC. Some of the DWBC appears to recirculate with the deep Gulf Stream near Cape Hatteras (where the two currents cross each other), forming a weaker offshore oxygen core.

Ageostrophy and pressure work in the Gulf Stream at 73°W

During 1984, velocity, bottom pressure and thermocline depth data were obtained from deployments of tall current meter moorings, bottom pressure gauges and inverted echo sounders at five sites on the anticyclonic side of the Gulf Stream at 73° W. An additional number of inverted echo sounders, deployed in several nearby sites, allowed us to construct daily regional maps of the thermocline topography. Using this data base we examined the momentum balance and the pressure work in the eddy kinetic energy balance at 500 m above the seafloor and at a depth of 400 m. At 500 m off the bottom, the correlations between fluctuating geostrophic currents, inferred from pressure differences between sites, and suitably averaged observed currents provide observational confirmation of the geostrophic balance in the deep Gulf Stream, within our ability to measure it. At 400 m depth, the ageostrophic current was computed using velocity measurements, curvature estimates from the thermocline maps, and the momentum equation in natural coordinates. Comparison of observed and ageostrophic velocities showed occasional departures from geostrophy of O(30% to 60%) with duration from 2 to 5 days. These ageostrophic events were caused by tangential and centripetal accelerations, mostly during the coalescence of two cold‐core rings with the stream. The mean departure from geostrophy at 400 m during the deployment period was 7%. The mean effect of the pressure work, on the anticyclonic side of the stream at 400 m, was to decrease the eddy kinetic energy at a rate of −3×10−3 ergs cm−3 s−1; a value with magnitude as large as that of the barotropic instability term. The pressure work record is very episodic with peaks, during the ageostrophic events, close to 30 times larger than the mean estimate and with both positive and negative values. At 500 m off the bottom, the pressure work was found to be insignificantly different from zero.

A tracer study of the deep Gulf Stream cyclonic recirculation

An advective-diffusive numerical model is used to interpret water sample data from beneath the thermocline in the region of the Gulf Stream and deep western boundary current (DWBC), west of the Grand Banks. The nearly homogenized distributions of salinity and oxygen suggest that substantial cyclonic recirculation of the deep Gulf Stream comes in close contact with DWBC, which is the source of the tracers. The exchange of properties between the two flows is studied in order to relate tracer distributions to particular characteristics of the Gulf Stream gyre. Using a corresponding diffusion box model, it is shown that spin-up and the onset of homogenization in the gyre is controlled by diffusion from the boundary, and that the level of tracer in the gyre can be insensitive to the strength of lateral diffusivity (the salt case), but is strongly sensitive to vertical mixing (the oxygen case). The presence of vertical mixing, however, does not inhibit the occurrence of homogenization. The lateral (κ) and vertical (ν) diffusivities of the Gulf Stream gyre can be deduced by applying the model results to the observed oxygen distribution, and this leads to estimates of κ ∼ 10 6 cm 2 s −1. The tight lateral extent of the recirculation, as suggested by the model results, agrees well with existing current meter data from the region.

Late Quaternary plume, nepheloid and turbidite sedimentation and effect of the Gulf Stream near the tail of the Grand Banks, Newfoundland

Cores collected from the tops of the Fogo Seamounts southwest of the Grand Banks of Newfoundland contain considerable amounts of detrital clay. The seamounts rise at least 1 km above the surrounding sea floor and are an unlikely place for sedimentation by turbidity currents. Sedimentological and micropaleontological evidence suggests that large amounts of detrital clay were injected into the system during glacial and interglacial stages by turbidity currents and surface sediment plumes. The main source of sediment was the Laurentian Channel, although some sediment was injected through the canyons around the Grand Banks. Surface sediment plumes were strongly influenced by the Labrador Current and by the surface circulation of the Gulf Stream. Fine-grained sediments injected into the system by turbidity currents through the Laurentian Channel were probably trapped by the deep circulation of the Gulf Stream and dispersed to the northeast. Effects of the sediment drift induced by the deep Gulf Stream circulation is also reflected in an abrupt eastward bent of the depositional lobes of the Laurentian Fan and in the shape of the Newfoundland Ridge.

The northern recirculation gyre of the gulf Stream

Results from two recent field programsin the western North Atlantic are presented with particular emphasis on the deep circulation. New long-term moored current measurements show that the flow north of the Gulf Stream and east of the New England Seamount Chain is toward the west from 500 m to the bottom with very little depth dependence. Nearly 40 × 10 6 m 3s −1 is transported to the west near 63°W, and half of this recirculates back to the east over the Seamount Chain to add a strong component to the deep Gulf Stream between the Chain and the Grand Banks. We call this current the “Northern Recirculation Gyre” in contrast with a similar feature to the south of the Stream popularly known as the “Worthington Gyre” ( Worthington, 1976, The Johns Hopkins Oceanographic Studies, 6, 110 pp.). The new gyre is similar to that proposed by Hogg (1983, Deep-Sea Research, 30, 945–961) but somewhat smaller in scale. Its relationship to the Gulf Stream and the Deep Western Boundary Current is made explicit by the new measurements. Tracer measurements show that the Northern Recirculation Gyre exchanges water properties with the Deep Western Boundary Current where the two are in close proximity along the northern boundary. The relatively high values of oxygen and freon, so imparted, are then advected to the interior where the gyre carries water eastward under the Gulf Stream. Beneath the thermocline these tracer fields are practically homogenous within the gyre, perhaps a reflection of the expulsion process described by Rhines and Young (1983, Journal of Fluid Mechanics, 133, 133–145). An advective-diffusive model is used to interpret some slight differences between the various tracer distributions.

Time scales and structure of topographic Rossby waves and meanders in the deep Gulf Stream

Time scales and structure of topographic Rossby waves and meanders in the deep Gulf Stream