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Robert S. Pickart

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.

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Robert S. Pickart

Abstract

The hydrographic properties of the bottom boundary layer (BBL) are investigated in a synoptic cross section of the Middle Atlantic Bight shelfbreak frontal jet. The dataset consists of closely spaced conductivity–temperature–depth stations and concurrent shipboard acoustic Doppler current profiler measurements. An extremum in BBL properties occurs in the frontal region where the layer becomes thinner (disappearing briefly), more stratified, and more strongly capped. These changes are apparently related to the significant cross-slope variation in interior stratification. Where the BBL vanishes, at the shoreward edge of the front, it detaches into the interior along an (nearly) isopycnal layer. This is revealed both by weak vertical stratification as well as weak isopycnal gradients of potential temperature (θ) and salinity (S) along the layer. An advective–diffusive model of the detachment in density space is used to explain the observed θ, S distribution as well as estimate the pumping speed along the detached BBL. The detided ADCP velocity fields are analyzed in light of the observed detached BBL. The mechanism of detachment is discussed in relation to existing models, and the secondary circulation in the cross-stream plane is inferred. This reveals a deep interior upwelling cell, apparently tied to the local bathymetry, which enhances the flow along the detached BBL.

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Robert S. Pickart

Abstract

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.

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Robert S. Pickart and Scott S. Lindstrom

Abstract

A geostrophic velocity section across the Gulf Stream and deep western boundary current near 35°N is referenced four different ways: using a POGO float (acoustically tracked transport float), shipboard acoustic Doppler current profiler (ADCP), and bottom current meters, and by assuming an isotherm level of no motion. The comparison between the first two techniques is emphasized because they are most easily applied. In general, reference velocities calculated using the float data agree well with those obtained from the ADCP data. However, there is disagreement at locations where the ADCP velocity is not in thermal wind balance, in which case the POGO value is deemed more accurate because the float samples deeper into the subsurface geostrophic flow. Disagreement is also caused by insufficient cross-stream POCSO spacing (although this could be avoided). The isotherm- and current meter-referenced sections, while similar to each other, both show unrealistic features. it is argued that the POGO method is preferable to the shipboard ADCP for a deep-water hydrographic experiment.

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Paula S. Fratantoni and Robert S. Pickart

Abstract

Twelve years of historical hydrographic data, spanning the period 1990–2001, are analyzed to examine the along-stream evolution of the western North Atlantic Ocean shelfbreak front and current, following its path between the west coast of Greenland and the Middle Atlantic Bight. Over 700 synoptic sections are used to construct a mean three-dimensional description of the summer shelfbreak front and to quantify the along-stream evolution in properties, including frontal strength and grounding position. Results show that there are actually two fronts in the northern part of the domain—a shallow front located near the shelf break and a deeper front centered in the core of Irminger Water over the upper slope. The properties of the deeper Irminger front erode gradually to the south, and the front disappears entirely near the Grand Banks of Newfoundland. The shallow shelfbreak front is identifiable throughout the domain, and its properties exhibit large variations from north to south, with the largest changes occurring near the Tail of the Grand Banks. Despite these structural changes, and large variations in topography, the foot of the shelfbreak front remains within 20 km of the shelf break. The hydrographic sections are also used to examine the evolution of the baroclinic velocity field and its associated volume transport. The baroclinic velocity structure consists of a single velocity core that is stronger and penetrates deeper where the Irminger front is present. The baroclinic volume transport decreases by equal amounts at the southern end of the Labrador Shelf and at the Tail of the Grand Banks. Overall, the results suggest that the Grand Banks is a geographically critical location in the North Atlantic shelfbreak system.

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Robert S. Pickart and D. Randolph Watts

Abstract

The methodology for converting the travel time measurement of the inverted echo sounder (IES) into an amplitude of the first baroclinic dynamical mode, A 1, is presented. For a Gulf Stream IES record the so-generated A 1(t) time series is used to compute a vertical profile of first mode temperature versus time by perturbing a basic state temperature profile. The basic state is constructed by averaging together historical CTD data collected near the IES site. Similarly the first mode amplitudes are used to perturb a basic state dynamic height profile, and, using neighboring IESs, a profile of alongstream geostrophic velocity is obtained at the same location. The resulting IES-derived temperatures and velocities compare favorably to independent current meter results, exhibiting most of the variability observed in both the current meter temperature and alongstream velocity.

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Robert S. Pickart and William M. Smethie Jr.

Abstract

The manner in which the deep western boundary current (DWBC) crosses the Gulf Stream is investigated using data from a hydrographic survey conducted in 1990. Absolute geostrophic velocity vectors are computed using in situ float data to obtain the reference level. Three density layers are considered in detail: two mid-depth layers, which together make up the shallowest water mass component of the DWBC (500–1200 m), and a deep layer consisting of the Norwegian–Greenland overflow water (2500–3500 m). The shallowest layer does not make it through the crossover and is completely entrained by the Gulf Stream; however, the resulting drop in equatorward transport is almost completely replenished by offshore entrainment just south of the crossover. In the intermediate layer, which is denser than the Gulf Stream coming off the shelf, part of the DWBC recirculates to the northeast while the onshoremost portion continues equatorward. In the deep layer only a small amount of recirculation occurs. The lateral fields of potential vorticity (Q) reveal a Q barrier associated with the Gulf Stream in the two mid-depth layers, which is partially lessened in the intermediate one allowing the equatorward continuation of flow. In the deep layer, the DWBC maintains its potential vorticity through the crossover.

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Annalisa Bracco, Joseph Pedlosky, and Robert S. Pickart

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This paper extends A. Bracco and J. Pedlosky’s investigation of the eddy-formation mechanism in the eastern Labrador Sea by including a more realistic depiction of the boundary current. The quasigeostrophic model consists of a meridional, coastally trapped current with three vertical layers. The current configuration and topographic domain are chosen to match, as closely as possible, the observations of the boundary current and the varying topographic slope along the West Greenland coast. The role played by the bottom-intensified component of the boundary current on the formation of the Labrador Sea Irminger Rings is explored. Consistent with the earlier study, a short, localized bottom-trapped wave is responsible for most of the perturbation energy growth. However, for the instability to occur in the three-layer model, the deepest component of the boundary current must be sufficiently strong, highlighting the importance of the near-bottom flow. The model is able to reproduce important features of the observed vortices in the eastern Labrador Sea, including the polarity, radius, rate of formation, and vertical structure. At the time of formation, the eddies have a surface signature as well as a strong circulation at depth, possibly allowing for the transport of both surface and near-bottom water from the boundary current into the interior basin. This work also supports the idea that changes in the current structure could be responsible for the observed interannual variability in the number of Irminger Rings formed.

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Michael A. Spall and Robert S. Pickart

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It is demonstrated that recently observed cyclonic recirculation gyres in the Irminger and Labrador Seas may be forced by the strong cyclonic wind stress curl that develops each winter seaward of the east coast of Greenland. Idealized analytical and numerical models forced with such variable winds over a sloping bottom reproduce the essential aspects of the observed gyres (strength, location, and horizontal and vertical length scales). The communication between the forcing region in the Irminger Sea and the recirculation to the west is achieved by baroclinic topographic Rossby wave propagation along potential vorticity contours. The circulation is characterized as a time-dependent, stratified, topographic beta plume. For weak stratification, as found in the subpolar North Atlantic, the recirculation strength exhibits only weak seasonal variability, consistent with the observations, even though the forcing is active only during the winter. Baroclinic Rossby waves that develop when the wind forcing ceases in springtime interact with the bottom to provide a source of cyclonic vorticity that maintains the circulation until the wind strengthens again in the following winter.

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Fiammetta Straneo, Mitsuhiro Kawase, and Robert S. Pickart

Abstract

Large buoyancy loss driving deep convection is often associated with a large wind stress that is typically omitted in simulations of convection. Here it is shown that this omission is not justified when overturning occurs in a horizontally inhomogeneous ocean. In strongly baroclinic flows, convective mixing is influenced both by the background horizontal density gradient and by the across-front advection of buoyancy due to wind. The former process—known as slantwise convection—results in deeper convection, while the effect of wind depends on the relative orientation of wind with respect to the baroclinic front. For the case of the Labrador Sea, wintertime winds act to destabilize the baroclinic Labrador Current causing a buoyancy removal roughly one-third as large as the air–sea buoyancy loss. Simulations using a nonhydrostatic numerical model, initialized and forced with observed fields from the Labrador Sea, show how the combination of wind and lateral gradients can result in significant convection within the current, in contrast with previous ideas. Though the advection of buoyancy due to wind in weakly baroclinic flows is negligible compared to the surface buoyancy removal typical of convective conditions, convective plumes are substantially deformed by wind. This deformation, and the associated across-front secondary circulation, are explained in terms of the vertical advection of wind-generated vorticity from the surface boundary layer to deeper depths. This mechanism generates vertical structure within the convective layer, contradicting the historical notion that properties become vertically homogenized during convection. For the interior Labrador Sea, this mechanism may be partly responsible for the vertical variability observed during convection, which modeling studies have until now failed to reproduce.

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