Gulf Stream Meanders Research Articles

Acoustic imaging of biological and physical processes within Gulf Stream Meanders

The western edge of the Gulf Stream has long interested oceanographers because of the dynamic biological and physical nature of its associated frontal region. Not only does the Gulf Stream meander as it flows northeast, but the physical and biological characteristics of the frontal region change within meanders. The dynamics associated with these meanders control the transition between the warmer, more oligotrophic Sargasso Sea water and the colder, seasonally nutrient‐rich Slope Water. Rossby et al. [1985] and Levine et al. [1986] used isopycnal floats to track water motion along constant density surfaces and have shown upwelling in anticyclonic/divergent regions of meanders and downwelling in cyclonic/convergent regions.

Regional Primitive Equation Studies of the Gulf Stream Meander and Ring Formation Region

Abstract Primitive equation and quasi-geostrophic eddy resolving, open ocean models are used for hindcast studies in the Gulf Stream meander and ring formation region. A feature model approach is used to initialize the models, based on one month of observations during November to December 1984. Flat bottom and topographic calculations are carried out using an initial Gulf Stream velocity profile based on the Pegasus dataset. All of the major events observed in the upper thermocline are reproduced by both numerical models. The addition of bottom topography is shown to significantly alter the character of the deep velocity fields. Large, basin scale circulations found near the bottom in both flat bottom calculations were replaced by energetic jets and eddies associated with the dominant spatial scales of the bottom topography. Use of the quasi-geostrophic model to dynamically adjust the initial conditions for the primitive equation model is shown to reduce the growth of large scale meanders on time scales o...

Modeled time variability of acoustic propagation through a Gulf Stream meander and eddies

A numerical modeling study of the time variability of acoustic propagation through a Gulf Stream meander and eddies is presented. Both a cold-core (cyclonic) eddy and a warm-core (anticyclonic) eddy are included. Time periods of 2, 4, and 7 days; a frequency of 25 Hz; source and receiver depths of 150 m; and propagation out to 180 km are considered. Propagation conditions assumed an absorbing bottom and no azimuthal coupling of energy. Results indicate that variations in the locations and propagation loss levels of convergence zones (CZ) in 2 days can be significant. In the first CZ, the losses can change by more than 5 dB and shift in their pattern range location by as much as 10 km. Changes in the environment over a 2-day period can be such that perturbations to the acoustic field caused by the environment within the first CZ can be nullified by the environment between the first and second CZs. The differences between propagation with the eddies present and not present were also observed for ranges out to the first CZ. For a source located in the center of a cold-core eddy, the propagation loss in the CZ increased by 3 dB and the range decreased by about 5 km. With the source located within the eddy but off center, the loss showed little change in level but the range again decreased by about 5 km. For a source located in the center of a warm eddy, the propagation loss in the CZ increased by about 3 dB and the range increased by 7–10 km. With the source located off center, the loss showed little change in level and range location.

Forecasting Gulf Stream meanders and rings

The deep open ocean exhibits energetic space‐time variability phenomena that are the counterpart of atmospheric weather systems. The internal weather of the sea occurs on the scale of the internal deformation radius (depth times the ratio of buoyancy to Coriolis frequencies), with features called “mesoscale” fronts and eddies, although the dynamical analogy is with the atmospheric synoptic scale [Charney and Flierl, 1980; Robinson, 1983]. Space scales range from tens to hundreds of kilometers and time scales from days to months; features extend from surface to bottom intensified in the upper ocean main thermocline.Mesoscale effects include the intermittent energization of regions; the shifting of currents; the location of air‐sea interaction events; the transport, entrapment and dispersion of such things as heat, chemicals, nutrients, larvae, and pollutants; and the alteration of sound propagation. Forecasting the internal weather of the sea is important scientifically and practically and is now feasible [Mooers et al, 1987; Robinson, 1987] due to rapid, recent advances in physical oceanography and related technologies, especially satellites and computers. We have established a forecast system for the Gulf Stream Meander and Ring (GSM&R) region, called GULFCAST, that provides predictions one week ahead in real time and on an ongoing basis.

Potential Vorticity Balances and Horizontal Divergence along Particle Trajectories in Gulf Stream Meanders East of Cape Hatteras

Abstract Trajectories of 37 isopycnal RAFOS floats launched in the Gulf Stream off Cape Hatteras have been analyzed to examine the dynamics of meanders from a Lagrangian viewpoint. Using the float data in conjunction with information on the structure of horizontal velocity shear from the PEGASUS study (Halkin and Rossby), variations in planetary, curvature, and shear vorticity have been estimated along the float trajectories. Changes in fluid layer thickness were then inferred assuming potential vorticity is conserved following the floats. This analysis shows that curvature vorticity changes are typically 10%–20% of f (planetary vorticity) as fluid parcels travel between meander troughs and crests. Lateral shear changes on the order of 20%–30% of f are common as parcels move laterally relative to the jet axis between meander extrema. Although changes in these two terms are usually of opposite sign and tend to compensate, significant layer thickness changes do occur, with some parcels exhibiting 30% change...

Evidence of Cross-Frontal Exchange Processes in the Gulf Stream Based on Isopycnal RAFOS Float Data

Abstract A unique set of Lagrangian observations has recently been collected in the Gulf Stream using the newly developed isopycnal RAFOS float. Between January, 1984 and October, 1985, thirty-seven of these drifters were launched in the main thermocline of the current off Cape Hatteras and lacked acoustically downstream for 30 or 45 days. Temperature and pressure were also recorded along each float trajectory. The isopycnal capability of this drifter allows it to follow fluid parcel pathways quite accurately along the sloping density surfaces of the Gulf Stream. The RAFOS drifters revealed a striking pattern of vertical and cross-stream motion in Gulf Stream meanders. Floats were consistently observed to upwell (downwell) and move onshore (offshore) as they approached anticyclonic (cyclonic) meander crests (troughs). The rms vertical velocity in the center of the stream was observed to be 0.08 cm s−1 on the 12°C surface. No mean vertical motion was detected in the main thermocline of the Gulf Stream betw...

Hydrographic variability of southeastern United States shelf and slope waters during the Genesis of Atlantic Lows Experiment: Winter 1986

Continental shelf waters are particularly responsive to winter storm events mainly because of their shallow depths. Those of the southeastern United States (the South Atlantic Bight (SAB)) are especially responsive because they are broad and shallow. Also, the Gulf Stream serves as a continual source of warm water at the outer boundary. Thus the SAB receives strong meteorological (wind stress and heat loss) and oceanographic (advective) forcing. During the Genesis of Atlantic Lows Experiment (GALE) the response of shelf waters to winter storm events and Gulf Stream forcing was observed. The mean conditions showed a mixed water column with areas of stratification near the coast and at the shelf break. The nearshore area was stratified only during weak offshore winds, and the shelf break area was stratified during southward winds with accompanying onshore Ekman flow. On the inner shelf, advective buoyancy flux was similar in value to heat flux buoyancy and the buoyancy equivalent of wind mixing. Over the shelf break the advective buoyancy flux was 4 times the other forms of buoyancy flux and controlled the observed potential energy variability. A simple box model heat budget used to separate the effect of Gulf Stream eddies and meanders, and Ekman flow and air‐sea heat exchange on the shelf heat content showed that the observed heat content variability was caused by intrusion of Gulf Stream water. The intrusions may be caused either by onshore Ekman flow during southward winds or Gulf Stream meander events.

Data assimilation and dynamical interpolation in GULFCAST experiments

GULFCAST is a forecast system for the Gulf Stream meander and ring region consisting of a dynamical open-ocean model and an observational network comprised of remotely sensed sea-surface temperatures (and recently, sea-surface height) and critically located air-dropped expendable bathythermographs (AXBTs). We present here the case study of a real-time forecast system out for 2 weeks in the Spring of 1986 during the development of GULFCAST methodology. The AXBT data from successive flights were assimilated and a frontal location was ‘nowcast’ and forecast within the error bounds of navigation, AXBT sampling and model resolution during a multiple ring-stream interaction event.

Acoustic tomography in the Florida Strait: Temperature, current, and vorticity measurements

Various aspects of near‐bottom ocean variability, including the alongshore temperature and current, the cross‐slope temperature, the onshore‐offshore flow, and the area‐averaged relative vorticity on the western slope of the Florida Strait near 27°N have been measured with a three‐point acoustic reciprocal transmission. Three pairs of reciprocal travel time data were extracted from acoustic pulse responses of a 1983 (August) triangular transmission experiment. Temperatures and currents were estimated from the sum and difference, respectively, of reciprocal travel times, and an area‐averaged vorticity was estimated from circulation following Stokes' theorem. The estimations agree well with the concurrent direct measurements at two nearby moorings. Low‐frequency fluctuations of temperatures, currents, and vorticity show a strong weekly period, clearly and consistently describing a near‐bottom onshore‐offshore water movement on the western slope in association with the Gulf Stream meanders.

Gulf Stream Simulations and the Dynamics of Ring and Meander Processes

We present here a regional, eddy resolving, numerical study of the dynamics of Gulf Stream Meander and Ring (GSMR) interaction processes. We initialize the Harvard quasi-geostrophic open-boundary model with realistic meander and ring locations as indicated by remotely sensed sea surface temperature (SST) data and predict the flow evolution for the period 23 November to 19 December 1984. The methodology of Feature-Model initialization is introduced to extend the surface information to the thermocline and deep levels in terms of climatological structures, which are then dynamically adjusted by the model. Six numerical simulators are carried out to explore the influence of initial and boundary conditions on the flow evolution. All of the major events observed in the SST data are simulated, including the birth of new warm and cold core rings. The results show the relevance of quasi-geostrophic dynamics for the GSMR region on these time scales in the thermocline. A set of parameter and sensitivity experiments then elucidate the dependence on physical parameters; ring births are nonlinear baroclinic processes. The dynamics of these realistic cold and warm core formation events are quantified via local energy and vorticity budget analyses (EVA). The cold core case involves a process of nonlinear baroclinic cascades that convert available gravitational energy to kinetic energy and vice versa. The warm core case involves a differential horizontal advection process.

Meandering and Eddy Detachment According to a Simple (Looking) Path Equation

Nonlinear meandering and “pinching off” process are investigated by solving the path equationAs shown by Pratt and Stern, this dimensioned equation determines the center line latitude l of a slowly-varying, equivalent barotropic, quasi-geostrophic, f-plane jet with cusped velocity profile and center line curvature κ = lxx/(1 + lx2)/. A class of exact solutions consisting of steadily propagating meanders is found having wavelength 2π/k and amplitude a. The meanders form a wave train which can be single-valued (for ak < 2.61) or multivalued (for 2.61 < ak < 8.30) with respect to the x (eastward) coordinate. For ak = 8.30 grazing contact occurs between neighboring meanders and a type of vortex street is formed. The amplitude-dependent dispersion relation for the meanders shows that phase propagation is eastward with speed that increases with decreasing wavelength and/or amplitude, trends observed for Gulf Stream meanders near 72 W by Vazquez and Watts. Numerical solutions are presented for isolated, single-valued initial disturbances having a characteristic wavenumber k0 and amplitude a0. When a0k0 is less than a critical value between 1.5 and 2.0, the disturbance disperses. For larger values of a0k0, the evolution leads to a “pinching off” phenomenon in which meanders begin to detach from the main portion of the jet and form roughly elliptical eddies.

Gulf Stream bimodality and variability downstream of the Charleston bump

Gulf Stream mean flow and variability have been studied in the Gulf Stream Deflection and Meander Energetics Experiment (DAMEX). This study was conducted between September 1981 and April 1982 in the area of the Charleston bump, off the coast of the southeastern United States. Using data from an array of current meters and satellite imagery, it has been shown that the seaward deflection of the Gulf Stream by the bump has a bimodal character and that the Gulf Stream meander field downstream of the bump differs between the two states of deflection. The two deflection conditions have been termed the weakly deflected state and the strongly deflected state in this paper. A weakly deflected Gulf Stream flows past the Charleston bump and close enough to the edge of the continental shelf that its shoreward sea surface temperature (SST) front remains inshore of the 600‐m isobath between Charleston and Cape Hatteras. A strongly deflected Gulf Stream turns sharply eastward at the bump and flows seaward far enough to have its shoreward SST front offshore of the 600‐m isobath for several tens of kilometers along the continental slope downstream of the bump. The transitions between deflection states may occur very rapidly, and once in a strongly deflected state the Gulf Stream may remain so for up to several months. Two transitions were observed during DAMEX, one from a weakly to a strongly deflected state and one in the other direction. The transition from weakly to strongly deflected state took only a few days at a given site, while the signatures of the other transition were not as definite in the data records. The strongly deflected state occurred during the fall and winter in DAMEX (October through January), and satellite observations from earlier years suggest that it is this time of year in which the strongly deflected state preferentially occurs. It was not clear from the DAMEX data alone how the transition from a weakly deflected to a strongly deflected state may be induced, but path or transport changes in the Gulf Stream as it approaches the bump and interactions between the current and rings or eddies are candidate mechanisms. The bimodal character of the stream's path affects the low‐frequency variability of the current between the Charleston bump and Cape Hatteras in the following manner. Typical meander/frontal eddy patterns dominate the mesoscale fluctuation field during times of weak deflection, while larger‐amplitude meanders of the Gulf Stream jet, which can have very sharp curvature and strong anticyclonic circulations in their crests, exist during times of strong deflection. Furthermore, these larger‐amplitude meanders, which we call the DAMEX biweekly meanders, have generally slower phase propagation speeds than do the typical meander/frontal eddy events (about 20–25 km/day compared with 35–60 km/day) as well as longer periods (about 16 days compared to 1 week).

Radiation of topographic waves from Gulf Stream meanders

In a two-layer model of the Gulf Stream and underlying cold water mass, meander formation and pinch-off generates substantial pressure torque on the interface. This transfers vorticity to the lower layer and acts, over a sloping sea floor, as a source term for topographic waves. The source intensity is proportional to the Jacobian of surface pressure and interface elevation. Other possible source terms (interfacial or bottom shear stress curl or horizontal momentum flux divergence) are dwarfed by the pressure torque. Model calculations show that individual meander pinch-off events radiate topographic waves with characteristics very much as observed. The radiation field is quite unlike a plain monochromatic wave: the perceived frequency of oscillations varies in proportion to distance from the source.

One-dimensional baroclinically unstable waves on the Gulf Stream potential vorticity gradient near Cape Hatteras

Application of linear baroclinic instability theory to the observed distributions of velocity, stratification, and potential vorticity in the Gulf Stream near 74° W is successful in predicting the time and length scales of the most rapidly growing disturbances. A continuously-stratified, one-dimensional model with realistic bottom slope predicts propagation speeds of 10–50 cm s −1 associated with two regimes of rapid temporal growth centered at periods of 28 days and 5–7 days. This prediction is consistent with observations of the propagation and growth of Gulf Stream meanders derived from inverted echo sounder measurements in this region. The instability model also predicts that for realistic bottom slopes the baroclinic energy transfer should be weakly negative (eddy-to-mean) in deep water, but for low-frequency waves should change to significant positive (mean-to-eddy) transfer above depths of ∼1500 m, consistent with observations.

A model for interpreting continental-shelf hydrographic processes from the stable isotope and cadmium: Calcium profiles of scallop shells

Stable oxygen and carbon isotope, and cadmium:calcium profiles from the shells of scallops collected from the outer continental shelf of the South Atlantic Bight and the Virginia Bight appear to trace the hydrographic processes of the two regions. A model is developed to distinguish two principal modes of nutrient and trace metal input to the outer shelf from the biogeochemical profiles of the shell. The oxygen isotope profiles record bottom-water temperature changes caused by seasonal temperature cycles and water-mass movements. Nutrient input and subsequent phytoplankton blooms are recorded in the carbon isotope and Cd:Ca profiles. The shell profiles from the South Atlantic Bight record the dominant effect of upwelling caused by Gulf Stream meandering. The shell profiles from the Virginia Bight show the influence of spring river runoff and summer water-column stratification. Biogeochemical profiles in mollusk shells are potentially important tools for studying the dynamics of both modern and ancient continental-shelf upwelling systems.

A Model of Gulf Stream Frontal Instabilities, Meanders and Eddies along the Continental Slope

Abstract For a simplified model of the Gulf Stream front along a vertical-walled continental slope of a constant-depth ocean basin, the dynamics governing frontal instabilities, meanders, and eddies depend primarily on (i) L0/ R0, the ratio of the cross-stream distance of the stream axis from the slope, L0, to the Roosby radius of deformation, L0, and (ii) h0/ H, the ratio of the depth L0 of upper layer of light water to ocean basin depth H. A nonlinear, time-dependent three-dimensional model is used to study this interdependency. Two different types of meander solutions are found, depending on the value of the (nondimensionalized) initial available potential energy per unit alongfront length and per unit “Rossby area,” HR0, APE = ( L0/ R0)(h0/ H)/2. For small APE, the meanders and their associated cyclones are slope-bounded as they propagate downstream. This solution has characteristics similar to observed Gulf Stream meanders south of a topographic feature at 31.5°N (the so-called “Charleston Bump”) on ...

Vertical velocities and dynamical balances in Gulf Stream meanders

A triangular array of current meter moorings was deployed within the cyclonic flank of the Gulf Stream in the South Atlantic Bight from September 1981 to April 1982. Using velocity and temperature data and the nondiffusive heat equation, a time series of vertical velocity was derived. A mean vertical velocity of −0.013 cm s−1 and standard deviation of 0.078 cm s−1 were obtained. To better understand the subsurface structure of Gulf Stream meanders, the time series of horizontal and vertical velocities and temperature were examined at times when events were passing through the array. The flow along the trailing edge of a Gulf Stream meander and within the leading portion of a cold core frontal eddy was found to have an upward component (positive w), while that within the trailing portion of the frontal eddy and along the leading edge of a meander had a downward component (negative w). Using the horizontal and vertical velocity time series, cross‐stream and along stream momentum balances were calculated. The downstream flow was found to be in geostrophic balance. In contrast, the Coriolis and nonlinear terms were found to contribute equally to the determination of the cross‐stream flow.

Processing and analysis of large volumes of satellite‐derived thermal infrared data

Reducing the large volume of TIROS‐N series advanced very high resolution radiometer‐derived data to a practical size for application to regional physcial oceanographic studies is a formidable task. Such data exist on a global basis for January 1979 to the present at approximately 4‐km resolution (global area coverage data, ≈2 passes per day) and in selected areas at high resolution (local area coverage and high‐resolution picture transmission data, at ≈1‐km resolution) for the same period. An approach that has been successful for a number of studies off the east coast of the United States divided the processing into two procedures: preprocessing and data reduction. The preprocessing procedure can reduce the data volume per satellite pass by over 98% for full‐resolution data or by ≈84% for the lower‐resolution data while the number of passes remains unchanged. The output of the preprocessing procedure for the examples presented is a set of sea surface temperature (SST) fields of 512 × 1024 pixels covering a region of approximately 2000 × 4000 km. In the data reduction procedure the number of SST fields (beginning with one per satellite pass) is generally reduced to a number manageable from the analyst's perspective (of the order of one SST field per day). This is done in most of the applications presented by compositing the data into 1‐ or 2‐day groups. The phenomena readily addressed by such procedures are the mean position of the Gulf Stream, the envelope of Gulf Stream meandering, cold core Gulf Stream ring trajectories, statistics on diurnal warming, and the region and period of 18°C water formation. The flexibility of this approach to regional oceanographic problems will certainly extend the list of applications quickly.

On Gulf Stream meander characteristics near Cape Hatteras

From 1979 to 1982, Gulf Stream path fluctuations within 375 km downstream of Cape Hatteras, North Carolina, have been monitored by inverted echo sounders. Histograms of the north wall locations along four cross‐stream sections change shape and increase in range dramatically downstream through the study area: Near Cape Hatteras, the histogram is peaked with the Gulf Stream found over half of the time within a narrow 10‐km range. The distributions become progressively more symmetric and the ranges widen downstream such that in the eastern portion of the study area the Gulf Stream can be found with equal probability throughout its 145‐km excursion range. From the 36‐month‐long time series near 73°W there is evidence of a seasonal cycle of Gulf Stream positions; northerly locations occur in the summer/fall, when transports are lower, and southerly locations occur in the winter/spring, when transports are higher. An observational dispersion relationship is presented for meander propagation and growth: Downstream propagation rates increase smoothly from about 14 km d−1 for meanders with periods and wavelengths (33 days, 460 km) to over 45 km d−1 for the (4 days, 180 km) meanders. Meander amplitudes show rapid growth rates in two separate bands, near (4–5 days, 180–230 km) and (10–33 days, 300–500 km).

The Formation and Maintenance of Density Fronts on the U.S. Southeastern Continental Shelf during Winter

Density fronts on the U.S. southeastern continental shelf, during winter, am formed by (i) breakdown of the shelf-break front by Gulf Stream meanders or strong southward winds or both, (ii) shoreward intrusion of upper Gulf Stream warm water by persistent southward winds, and (iii) mixing of this warm water with continental shelf water cooled by cycles of cold air outbreaks. A cross-shelf and depth, time-dependent numerical model, which includes a model Gulf Steam and vertical mixing calculated according to a second-moment, turbulence closure submodel, was used to study physical process involved in the formation and maintenance of continental shelf fronts. Model results using somewhat idealized atmospheric forcings suggest that three or more winter storms are required to establish fronts on the midshelf. Otherwise, the front is situated just inshore of, and is sometimes indistinguishable from, the shelf-break front. Once formed, the front is maintained by southward winds, which transport warm water converging at the front and prevent frontal weakening by balancing seaward advection of cooler water at the foot of the front with downward turbulent diffusion of warmer water. Mean along-front flow is about 5 cm s−1 northward, opposing the wind, and should contribute to the persistent northward flow observed on the continental shelf during winter. Model results agree qualitatively with available hydrographic data. One- and two-dimensional model simulators were performed for a two and a half month period during winter 1983/84 when wind, air and water temperature data available at a nearshore ocean station. Simulated circulation and density structures confirm the idealized run. Calculated temperature variations agree fairly well with observations, except for a discrepancy during a warm period from 1–18 December 1983, when incident solar radiation (minus longwave back radiation), neglected in the model, was significant.