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John M. Bane Jr.

Abstract

Four types of stable, subinertial Rossby-like waves have been found to propagate alongshore in a model baroclinic boundary current similar to the Gulf Stream as it flows along the continental margin of the southeastern United States. The two-layer model incorporates a general bottom topography with continental shelf and slope, and a thermal-wind mean current confined to the upper layer. The four wave components are the familiar continental shelf wave, the quasi-geostrophic edge wave and complementary mode edge wave, plus a new frontal-trapped wave, which has wave amplitude maxima within the cyclonic side of the Stream. The dispersion diagram for a particular topography/density/current setting may be interpreted as a composite of the four components’ dispersion curve families. As in other similar problems a mode coupling between two components allows the characteristics of the two to be interchanged. The cases studied show the phase speeds of the various components to be affected by the location of the surface density front, the width of the continental shelf, and whether the inshore boundary is a vertical wall or a sloping beach. In general, the continental shelf waves (the quasi-geostrophic edge waves) are faster (slower) for a shelf with a coastal wall than for one with a sloping beach. The complementary mode edge waves and the frontal-trapped waves are fastest when the surface front is farthest from shore.

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David A. Brooks and John M. Bane Jr.

Abstract

Gulf Stream fluctuations observed over the 200 and 400 m isobaths off Onslow Bay, North Carolina have a prominent weekly time scale. The principal fluctuations observed during the 4-month winter experiment are consistent with Webster's (1961a) description of downstream propagating, skewed, lateral meanders of the Gulf Stream over the upper continental slope. The subtidal velocity fluctuations were highly coherent over the vertical extent (∼120 m) and over the horizontal extent (64 km) of our array. The implied downstream propagation speed was ∼30 km day−1 for the weekly period meanders. Concurrent satellite images of a sea surface temperature (SST) meander pattern indicate that subsurface temperature, salinity, velocity and relative-vorticity maxima occurred as meander crests (shoreward SST-front excursions) passed over the experiment site. The meandering currents were not coherent with nearby wind stress or coastal sea level fluctuations. Eddy-flux estimates indicate energy conversion from the fluctuations to the mean Stream.

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Mark E. Luther and John M. Bane Jr.

Abstract

A numerical model study is presented of the unstable normal modes of oscillation of a boundary current. The model background current approximates the Gulf Stream south of Cape Hatteras, North Carolina. Both vertical and horizontal shear in current velocity and a sloping bottom topography are included. The study seeks small amplitude, alongshore propagating perturbations with real frequency and complex alongshore wavenumber. A nonzero imaginary part of the wavenumber ensures that the wave amplitude either grows or decays in the alongshore direction. The first four eigenmodes are identified and their dispersion relations are investigated. Higher order modes are not resolved by the model. The dispersion surfaces (eigenvalues of frequency as a function of complex wavenumber) appear to bifurcate with increasing values of real wave number.

Observations in the Gulf Stream south of Cape Hatters have revealed a persistent wave-like meander pattern in the Stream with a period of 7–8 days. This wave form propagates in the downstream direction with a phase speed of about 40 km day−1 and is uncorrelated with local wind forcing. An 8-day wave also appears as an eigenmode in the model, and the perturbation velocity and buoyancy fields are consistent with observations. The instability mechanism of the model wave is of the mixed barotropic-baroclinic type, with the majority (about 80%) of the perturbation energy derived from the potential energy of the background flow. The model 8-day wave consists of a side-to-side meandering of the core of the current with filament-like structures of warm water (positive perturbation buoyancy) trailing the shoreward-most excursion of the core of the current. These filaments are separated from the core of the current by a cold dome of upwelled water.

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John M. Bane Jr. and Y. Hsueh

Abstract

Free, stable, coastal-trapped waves propagating in a continental shelf regime typical of coastal upwelling areas are studied using a two-layer analytical model. Two cases of interfacial geometry and mean current simulate the hydrographic and current fields before and after upwelling. The completely stratified shelf representing conditions before upwelling allows baroclinic waves to be trapped near the coastline with a cross-shelf scale of l −1, where 1 is the alongshore wavenumber. A geostrophic, baroclinic mean flow is found to give rise to stationary waves for which the phase speed is zero. After-upwelling conditions are modeled by allowing the density interface to warp upward and intersect the sea surface some distance away from the coast (∼10 km). The band of homogeneous water between the surface density front and the coast is found to support a barotropic wave motion that co-oscillates with the baroclinic waves trapped offshore of the front. At the surface front, surface wave elevation and cross-shelf transport matching conditions lead to the dispersion relation for this “complementary mode.” The complementary mode has the appearance of a nearshore oscillatory barotropic jet that is coherent with vertical pycnocline motions farther offshore. The phase speeds for complementary mode waves are found to be between those of a before-upwelling baroclinic wave and a purely barotropic wave. Before and after upwelling, the purely barotropic mode is essentially the same as a quasi-geostrophic edge wave.

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William K. Dewar and John M. Bane

Abstract

During 1984, five current meter moorings measured velocity and temperature in the Gulf Stream anticyclonic flank at a location approximately 250 km downstream of Cape Hatteras. Here, these data are used to analyze the energy budgets of the Gulf Stream mean flow with a view towards examining gulf stream vertical structure and inertial character.

We find that Gulf Stream dynamics exhibits considerable vertical structure at our experiment site. At 380 m, the kinetic energy flux appears to be convergent, but the eddies augment mean kinetic energy. Order of magnitude estimates of processes involving vertical velocity appear to be too small to account for this mismatch; hence, we conclude that a conversion of mean kinetic to mean potential energy, via flow up a mean pressure gradient, must be occurring, Opposite tendencies are found at 880 m, leading to conclusion that the Gulf Stream is flowing down a mean pressure gradient at this depth. Evidence supporting a situation similar to the observations in terms of northward shifts of the gyre structure with depth. Of course, our observations are undoubtedly influenced by lateral topography, bottom topography and eddies and the effects of these are poorly understood from a theoretical point of view.

We also present evidence that the Deep Western Boundary Current is restoring energy to the deep potential energy field by flow up a mean pressure gradient. The rates are considerably smaller than those in the Gulf Stream but structurally resemble our results at 380 m.

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William K. Dewar and John M. Bane Jr.

Abstract

The energy budgets of the eddies and the mean flow in the Gulf Stream near a topographic feature known as the Charleston bump are computed. First, we consider these results in the context of the amplification hypothesis for the development of Gulf Stream meanders. According to this hypothesis, the finite amplitude Gulf Stream fluctuations observed offshore of Onslow Bay are the result of the destabilizing effect of the bump on the Stream. The present dataset was obtained both immediately upstream and downstream of the bump, and the results of our analysis suggest: 1) Immediately south of the Charleston bump, the eddies perform net work on the Gulf Stream at a rate of (1.02 ± .66) × 10−2 ergs cm−3 s−1 by transporting momentum offshore; 2) The net work performed by the eddies south of the bump is not used locally to accelerate the mean; rather, it is exported to the rest of the ocean at a rate of (1.58 ± 1.39) × 10−2 ergs cm−3 s−1; 3) In spite of the net work performed by the eddies south of the bump, eddy kinetic energy apparently does not decrease; 4) Immediately north of the Charleston bump, the flow appears to be both barotropically and baroclinically unstable. These results support the amplification hypothesis by demonstrating the destabilizing effect of the bump on the eddies (points 1 and 4) and that upstream perturbations may survive to encounter the bump topography (point 3). Other results of our analysis are that the mean of mean kinetic energy by the eddies constitutes the dominant form of energy conversion and that eddy pressure work may be an important factor in the fluctuation energy budget.

The second application of our calculations is to a characterization of the mean Gulf Stream in the South Atlantic Bight (SAB). The results of this analysis indicate the following: 1) The mean Gulf Stream kinetic energy flux increases downstream at a rate of (2.17 ± .98) × 10−2 ergs cm−3 s−1; 2) The eddies tend to decelerate the mean flow at a rate of (-0.57 ± 1.3) × 10−2 ergs cm−3 s−1; 3) In order that the mean energy equation be balanced, the Gulf Stream in the SAB must be releasing mean potential energy by flowing down a mean pressure gradient. Thus we have evidence suggesting the existence of a component of the pressure gradient associated with the Gulf Stream which is not geostrophically balanced. The downstream pressure gradient inferred at our array site is consistent with published estimates of mean alongshore pressure gradients in the SAB; however, the partitioning of the pressure force between mean acceleration and eddy Reynolds stress most likely holds only near the bump. We also estimate the net loss from the mean potential energy in the SAB using our measured conversion rate and demonstrate that it compares in magnitude but is opposite in sign to that thought to occur downstream of Cape Hatteras. Thus we argue that the Gulf Stream in the SAB is exhibiting some of the properties of the inflow regions of western boundary layers in inviscid inertial models of the general ocean circulation. Our measurements, however, also indicate the presence of vigorous eddies whose effects in the mean energy equation are potentially sizeable. Such eddies are, of course, not contained in strictly inviscid, inertial models of the western boundary layer.

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William K. Dewar and John M. Bane

Abstract

Pointwise energy balances in the Gulf Stream System at 73°W (downstream of Cape Hatteras) are examined. Five current meter moorings obtained verlocity and temperature data at four different depths for approximately one year. These data were used to estimate eddy kinetic and potential energy flux divergences and the rates of energy conversion between the eddy and mean fields in both the Gulf Stream and the Deep Western Boundary Current.

Based on our results and those of others, we argue that (i) the Gulf Stream is on average baroclinically unstable in this region, although energy release from the horizontal shear dominates near the surface; (ii) eddy pressure work is an important component of Gulf Stream eddy energetics and acts as a sink of eddy energy at 73°W; (iii) the Deep Western Boundary Current appears to be baroclinically unstable; (iv) Gulf Stream eddies are affected by the “far-field"; and (v) the eddy-field at 73°W is significantly different in its effects on the Gulf stream than that upstream on Hatteras or that at 68°W, although the interaction rates are of comparable magnitude. We propose a hypothesis of eddy field dynamics in the vicinity of the Gulf Stream from the Florida Straits through the Gulf Stream extension.

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Huijie Xue and John M. Bane Jr.

Abstract

The three-dimensional Princeton Ocean Model is used to examine the modification of the Gulf Stream and its meanders by cold air outbreaks. Two types of Gulf Stream meanders are found in the model. Meanders on the shoreward side of the Gulf Stream are baroclinically unstable. They are affected little by the atmospheric forcing because their energy source is stored at the permanent thermocline, well below the influence of the surface forcing. Meanders on the seaward side of the stream are both barotropically and baroclinically unstable. The energy feeding these meanders is stored at the surface front separating the Gulf Stream and the Sargasso Sea, which is greatly reduced in case of cold air outbreaks. Thus, meanders there reduce strength and also seem to slow their downstream propagation due to the southward Ekman flow. Heat budget calculations suggest two almost separable processes. The oceanic heat released to the atmosphere during these severe cooling episodes comes almost exclusively from the upper water column. Transport of heat by meanders from the Gulf Stream to the shelf, though it is large, does not disrupt the principal balance. It is balanced nicely with the net heat transport in the downstream direction.

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Huijie Xue, Ziqin Pan, and John M. Bane Jr.

Abstract

The two-dimensional, Advanced Regional Prediction System (ARPS) has been coupled with the Princeton Ocean Model to study air–sea interaction processes during an extreme cold air outbreak over the Gulf Stream off the southeastern United States. Emphases have been placed on the development of the mesoscale front and local winds in the lower atmosphere due to differential fluxes over the land, the cold shelf water, and the warm Gulf Stream, and on how the mesoscale front and the local winds feed back to the ocean and modify the upper-ocean temperature and current fields. Model results show that a shallow mesoscale atmospheric front is generated over the Gulf Stream and progresses eastward with the prevailing airflow. Behind the front, the wind intensifies by as much as 75% and a northerly low-level wind maximum with speeds near 5 m s−1 appears. The low-level northerly winds remain relatively strong even after the front has progressed past the Gulf Stream. The total surface heat flux in the coupled experiment is about 10% less than the total surface heat flux in the experiment with fixed SST, suggesting that the oceanic feedback to the atmosphere might not be of leading importance. On the other hand, the response of the upper-ocean velocity field to the local winds is on the order of 20 cm s−1, dominating over the response to the synoptic winds. This suggests the modification in the atmosphere by air–sea fluxes, which induces the locally enhanced winds, has considerable impact on the ocean. That is, there is significant atmospheric feedback to the ocean through the heat-flux-enhanced surface winds.

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John M. Bane, Clinton D. Winant, and James E. Overland

A number of observational programs have been carried out on the United States continental shelf to describe coastal-ocean circulation with emphasis on mesoscale processes. In several of these studies the atmosphere was found to play a central role in determining the coastal circulation through either local or remote forcing. Because of these results, the Coastal Physical Oceanography (CoPO) planning effort has designated three coastal air-sea interaction areas to focus on in a national program to study the physical processes on the continental shelf. These areas are shelf frontogenesis, interaction of stable layers with topography, and forcing by severe storms. The long-term objective of the air-sea interaction component of CoPO is to better understand the structure, dynamics, and evolution of the various mesoscale and synoptic-scale processes that significantly affect coastal/shelf circulation through air-sea interactions. Within this body of knowledge will be an improved quantification of the air-sea exchanges of dynamically important quantities set in the framework of mesoscale and synoptic-scale processes.

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