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Showing papers in "Journal of Physical Oceanography in 1990"


Journal ArticleDOI
TL;DR: In this paper, a subgrid-scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non-eddy-resolving ocean circulation models.
Abstract: A subgrid-scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non-eddy-resolving ocean circulation models. The mixing is applied in isopycnal coordinates to isopycnal layer thickness, or inverse density gradient, as well as to passive scalars, temperature and salinity. The transformation of these mixing forms to physical coordinates is also presented.

3,107 citations


Journal ArticleDOI
TL;DR: The mean annual cycle in surface wind stress over the global oceans from surface wind analyses from the European Centre for Medium Range Weather Forecasts (ECMWF) for seven years (1980-86) is presented in this article.
Abstract: The mean annual cycle in surface wind stress over the global oceans from surface wind analyses from the European Centre for Medium Range Weather Forecasts (ECMWF) for seven years (1980–86) is presented. The drag coefficient is a function of wind speed and atmospheric stability, and the density is computed for each observation. Annual and seasonal mean climatologies of wind stress, wind stress and Sverdrup transport and the first two annual harmonies of the wind stress are presented. The Northern and Southern hemispheres are contrasted as an the Pacific and Atlantic basins. The representativeness of the climatology is also assessed. The main shortcomings with the current results are in the topics. The wind stress statistics over the southern ocean are believed to be the moon reliable because of the paucity of direct wind observations. Annual mean values exceed 2 dyn cm−2 over the eastern hemisphere near 50°S and locally exceed 3 dyn cm−2 in the southern Indian Ocean; values much larger than in pre...

374 citations


Journal ArticleDOI
TL;DR: In this article, a generalized formula for the determination of the westward drift of mesoscale eddies under the planetary (beta) effect has been derived within the confines of a single-layer model.
Abstract: Since the pioneering work of Nof, the determination of the westward drift of mesoscale eddies under the planetary (beta) effect has been a recurrent theme in mesoscale oceanography, and several different formulae have been proposed in the literature. Here, recpatiulation is sought, and, within the confines of a single-layer model, a generalized formula is derived. Although it is similar to Nof's, the present formula is established from a modified definition and with fewer assumptions. It also recaptiulates all other formulae for the one-layer model and applies to a wide variety of situations, including cases when the vortex develops a wake of Rossby waves or undergoes axismmetrization. Following the derivation of the formula, a physical interpretation clarifies the migration mechanism, which can be divided between a self-induced propulsion and a reaction from the displaced ambient fluid. Numerical simulations with primitive and geostrophic equations validate the formula for a variety of length sc...

220 citations


Journal ArticleDOI
TL;DR: In this article, a model for the form of the directional wavenumber spectrum slice in the dominant wave direction was proposed for ocean gravity wind waves, assuming an empirical spectral directional spreading function and the linear gravity wave dispersion relation.
Abstract: Equilibrium spectral behavior for ocean gravity wind waves has been investigated actively over the past three decades, yet fundamental problems remain in reconciling theory with observations. Predicted equilibrium spectral forms from physical models proposed recently by Kitaigorodskii and by Phillips are examined in the light of wavenumber and frequency spectra reported by several investigators. While frequency domain observations appear to support the model predictions, observed wavenumber spectra are found to differ both in the spectral dependence on wavenumber and on the wind speed. Based on observed wavenumber and frequency spectra for fetch-limited condition a model is proposed for the form of the directional wavenumber spectrum slice in the dominant wave direction. Reduced wavenumber and frequency spectra are calculated from this model, assuming an empirical spectral directional spreading function and the linear gravity wave dispersion relation. These calculations reveal the underlying infl...

207 citations


Journal ArticleDOI
TL;DR: The transient response of climate to an instantaneous increase in the atmospheric concentration of carbon dioxide has been investigated by a general circulation model of the coupled ocean-atmosphere-land system with global geography and annual mean insulation as mentioned in this paper.
Abstract: The transient response of climate to an instantaneous increase in the atmospheric concentration of carbon dioxide has been investigated by a general circulation model of the coupled ocean-atmosphere-land system with global geography and annual mean insulation. An equilibrium climate of the coupled model is perturbed by an abrupt doubling of the atmospheric carbon dioxide. The evolution of the model climate during the 60-year period after the doubling is compared with the result from a control integration of the model without the doubling. The increase of surface air temperature in middle and high latitudes is slower in the Southern Hemisphere than the Northern Hemisphere The large thermal inertia of the ocean-dominated hemisphere is partly responsible for this difference. The effective thermal inertia of the oceans becomes particularly large in high southern latitudes. Owing to the absence of meridional barriers at the latitudes of the Drake Passage. a wind-driven. deep cell meridional circulatio...

195 citations


Journal ArticleDOI
TL;DR: In this article, hydrographic surveys were conducted off the Philippine coast in September 1987 and April 1988 as part of the United States/People's Republic of China cooperative research program.
Abstract: Hydrographic surveys were conducted off the Philippine coast in September 1987 and April 1988 as part of the United States/People's Republic of China cooperative research program. These cruises sampled the western Pacific Ocean where the North Equatorial Current (NEC) meets the western boundary and divides into the Kuroshio and Mindanao Currents. The requirement for mass conservation within a region enclosed by stations is utilized here to obtain absolute circulation fields for the two surveys. In both realizations, the surface flow of the NEC was observed to bifurcate near latitude 13°N; NEC flow poleward of this latitude turned north as the Kuroshio while flow to the south fed the Mindanao Current. Most striking was a twofold increase in the strength of the current system in spring 1988 as compared with fall 1987. We note that the observations in fall 1987 were obtained during the height of the 1986/87 El Nino, while those in spring 1988 were during a cold phase of the El Nino/Southern Oscillat...

190 citations


Journal ArticleDOI
TL;DR: In this article, the authors used the historical hydrographic data base for the South Atlantic Ocean to investigate the hydrography boundary between the subtropical gyre and the Antarctic Circumpolar Current (ACC), the Sub-tropical Front (STF), and the southern current band of the gyre, which is called the SAC.
Abstract: In this paper we use the historical hydrographic data base for the South Atlantic Ocean to investigate (i) the hydrographic boundary between the subtropical gyre and the Antarctic Circumpolar Current (ACC), the Sub-tropical Front (STF), and (ii) the southern current band of the gyre, which is called the South Atlantic Current (SAC). The STF begins in the west in the Brazil-Falkland (Malvinas) confluence zone, but at locations at and west of 45°W this front is often coincident with the Brazil Current front. East of 45°W the STF appears to be a distinct feature to at least the region south of Africa, whereupon it continues into the Indian Ocean. The associated current band of increased zonal speed is the SAC, which, except for one instance, is found at or north of the surface STF until Indian Ocean water from the Agulhas retroflection is reached. A reversal of baroclinicity in the STF is observed south of a highly saline Agulhas ring, causing the SAC to separate from the STF and turn north into the Benguela Current. Zonal flow south of the STF is generally weak and serves to separate the South Atlantic and circumpolar currents. In the Argentine Basin, the SAC has a typical volume transport of 30 Sv (1 Sv = 106m3s−1) in the upper 1000 m relative to a deep potential density surface (σ4 = 45.87 kg m−3), and can be as high as 37 Sv. It is thus comparable to, or stronger than, the Brazil Current. In the Cape Basin, the transport of the SAC is reduced to about 15 SY before it turns north to feed the Benguela Current. In late 1983 this flow was joined by about 8 Sv of water from the Agulhas Current.

170 citations


Journal ArticleDOI
TL;DR: In this paper, a two-layer numerical model indicates that the two-way exchange flow may indeed be subject to a series of internal hydraulic adjustments along the Bosphorus Strait due to morphological features such as sills, a contraction and abrupt expansion of the width of the strait.
Abstract: Recent hydrographic observations obtained in the Bosphorus Strait illustrate several features of the flow that may be related with the internal hydraulics. A two-layer numerical model indicates that the two-way exchange flow may indeed be subject to a series of internal hydraulic adjustments along the strait due to morphological features such as sills, a contraction and abrupt expansion of the width of the strait. The model identifies three distinct regions of the supercritical flow. The lower-layer flow of the Marmara Sea origin is directed to the north towards the Black Sea in a progressively thinning layer and is controlled by the sill located near the Black Sea entrance of the strait. The upper-layer water of the Black Sea origin flows in the opposite direction and is controlled upon reaching the constricted region located about 10–12 km away from the Marmara end of the strait. The upper-layer flow is then matched to the subsequent subcritical conditions by undergoing an internal hydraulic ju...

164 citations


Journal ArticleDOI
TL;DR: In this paper, the exchange of water between the Norwegian Sea and the North Atlantic around the Faroe Islands was investigated in 1987 and 1988 combining CTD data with shipborne Acoustic Doppler Current Profiler (ADCP)measurements.
Abstract: The exchange of water between the Norwegian Sea and the North Atlantic around the Faroe Islands was investigated in 1987 and 1988 combining CTD data with shipborne Acoustic Doppler Current Profiler (ADCP)measurements. Tests were devised to assess the accuracy of the ADCP measurements and the results are described; it is found that estimating a reference velocity for geostrophic measurements is limited by the presence of tides of order 10 cm s−1. In the outflow of cold water from the Faroe Bank Channel, which reaches speeds of nearly 1 m s−1 the error is acceptably small but strong time dependent components are revealed. The discharge of cold water is traced 75 km beyond the sill of this channel and changes in potential temperature, salinity and potential vorticity on an isopycnal surface are attributed to intense mixing. An effective diffusivity near 100 cm2 s−1 is derived and K-H instability demonstrated as a plausible source of the turbulent energy. Six current meter moorings were deployed from...

160 citations


Journal ArticleDOI
TL;DR: In this paper, the authors present evidence showing the likelihood that the influence of the wind waves can also be large over the oceans and propose an alternative expression for flow over growing wind waves, which are in local equilibrium with the wind, given by a form including the wind-wave spectral peak frequency explicity.
Abstract: Distribution of the wind stress over the oceans is usually estimated by using a bulk formula. It contains the squared 10-m wind speed multiplied by the drag coefficient, which has been assumed in many cases to be a weak function of the 10-m wind speed. Over land the important role of thermal stratification has been clearly recognized, but over the sea the influence of wind waves is less well documented. This paper presents evidence showing the likelihood that the influence of the wind waves can also be large. Charnock proposed an expression for the marine atmospheric boundary layer roughness parameter, z0, which depended only on the wind friction velocity, u☆ and the acceleration of gravity, g. Toba and Koga have recently proposed an alternative expression for flow over growing wind waves, which are in local equilibrium with the wind, given by a form including the wind-wave spectral peak frequency explicity. The criterion for local equilibrium of the wave field with the wind is its consistency wi...

158 citations


Journal ArticleDOI
TL;DR: In this article, a mathematical model that describes the formation and dilution of a frontally bounded river plume is presented, incorporating the effects of nonlinear advection, Coriolis acceleration, time dependency, mixing, friction, and a free frontal boundary.
Abstract: A mathematical model that describes the formation and dilution of a frontally bounded river plume is presented. Such features were first studied at the mouth of the Connecticut River during periods of high discharge and have subsequently been reported elsewhere. The model incorporates the effects of nonlinear advection, Coriolis acceleration, time dependency, mixing, friction, and a free frontal boundary. A numerical solution technique is employed to obtain approximate solutions to several problems which are interpreted to yield new insights to the dynamics of these phenomena. In particular, computed solutions for the growth of a plume discharged from a radial source into a steady crossflow are presented for a variety of crossflow velocities and physical scales. These demonstrate that the stability of the layer to vertical shear in the horizontal velocity is sensitive to the relative directions of the crossflow current and that of a free Kelvin wave. The effect of a reversing crossflow, represent...

Journal ArticleDOI
TL;DR: In this article, it was shown that the major low frequency velocity fluctuations in the lower 1000 to 2000 m of the water column in the three regions have the characteristics of topographic Rossby waves (TRWs).
Abstract: Direct current measurements using moored arrays have been made below 1000 m in the eastern, central and western Gulf of Mexico basin. The major low frequency velocity fluctuations in the lower 1000 to 2000 m of the water column in the three regions have the characteristics of topographic Rossby waves (TRWs). Spectral peaks are observed at periods of about 25 days and 40 to 100 days. Motions are highly coherent with depth. Variances increase toward the bottom despite the very weak stratification of the deep waters of the Gulf. Wave-lengths are about 150–250 km and phase propagation is offshore with energy propagation westward. A group velocity of about 9 km day−1 could be directly estimated from significantly coherent signals between eastern and western arrays. This value is consistent with estimates derived from the dispersion relation and is higher than the westward translation speed of 3 to 6 km day−1 of the large anticyclonic eddies shed from the Loop Current. It appears that a major source of...

Journal ArticleDOI
TL;DR: In this article, the authors examined topographic influences in an eddy-resolving model of oceanic channel flow forced by steady zonal winds and found that large wale topography provides the largest form stress.
Abstract: Topographic influences are examined in an eddy-resolving model of oceanic channel flow forced by steady zonal winds. With small explicit lateral friction, transient eddies generated by the baroclinic instability of the mean flow transfer momentum downward to the bottom layer. In the flat-bottom case, bottom friction is the only efficient sink of eastward momentum. When bottom topography is present, the topographic form stress can replace the bottom friction sink in the momentum budget, and a large decrease of the zonal transport results. Large wale topography (of the scale of the forcing) provides the largest form stress. Topographic effects decay with height as suggested by the Prandit scaling, and therefore only topographic scales larger than the Rossby radius can affect the whole water column. In that case, the interfaces are deformed by standing eddies on topographic length scales, and standing eddies replace transient eddies in transferring momentum downward. The bottom-layer mean streamfunc...

Journal ArticleDOI
TL;DR: Although computationally intensive, when it is carefully implemented, simulated annealing is found to give superior results to more traditional methods of nonlinear optimization.
Abstract: The oceanographic experiment design problem is discussed in the context of several simple examples drawn from acoustic tomography. The optimization of an objective function—chosen to characterize the array design— is carried out using the technique of simulated annealing. A detailed description of this method and its implementation for the examples above, is provided. Although computationally intensive, when it is carefully implemented, simulated annealing is found to give superior results to more traditional methods of nonlinear optimization.

Journal ArticleDOI
TL;DR: In this article, an extensive dataset collected in the Alboran Sea during the 1982 Donde Va? experiment is used to characterize the kinematics and dynamics of the inflow of Atlantic water into the Mediterranean Sea.
Abstract: An extensive dataset collected in the Alboran Sea during the 1982 Donde Va? experiment is used to characterize the kinematics and dynamics of the inflow of Atlantic water into the Mediterranean Sea. The veering of the inflow toward the ENE after leaving the Strait of Gibraltar and the existence of an anticyclonic gyre that fills much of the western Alboran Sea in the upper 200 m are confirmed in the mean. Inflow variability with periods of 2 to 10 days is described. Particularly striking is one interval of about 9 days during which the gyre was not present and the inflow adopted a southerly course after leaving the strait. The observations are interpreted in terms of vorticity conservation and in the light of earlier theoretical and numerical results.

Journal ArticleDOI
TL;DR: In this paper, it is shown that the Bryan-Cox General Circulation Model (GCM) can produce an artificial cell at the equator when typical values of the vertical diffusivity and viscosity parameters are used.
Abstract: In centered difference models of ocean circulation, two grid-point computational modes can be excited if grid Reynolds and Peclet numbers are greater than two. The Bryan-Cox General Circulation Model (GCM) is used to show the dramatic effect that this instability has on the equatorial thermohaline circulation. In many recent numerical calculations researchers have used 12 vertical levels. It is shown that this resolution produces an artificial cell at the equator when typical values of the vertical diffusivity and viscosity parameters are used. This artifical cell rotates counter to the primary cell driven by deep water formation at high latitudes, is driven by downwelling at the eastern boundary near the equator and is 40% the strength of the primary cell for the parameters used in the present study. When the vertical resolution is increased the cell vanishes. It is suggested therefore that higher vertical resolution should be used in Bryan-Cox GCM deep-ocean modeling studies when current values...

Journal ArticleDOI
TL;DR: The evolution of a Mediterranean salt lens (Meddy) over a two-year period is examined in this paper, where several non-dimensional numbers can be used to describe the overall decay in the structure of the Moddy.
Abstract: The evolution of a Mediterranean salt lens (Meddy) over a two year period is examined. Several nondimensional numbers can be used to describe the overall decay in the structure of the Moddy. Two Rossby numbers, one using the central relative vorticity and another using the radius and velocity of the azimuthal velocity maximum, did not change over the two year period. However, the Burger number N2H2/(f2L2) increased as the Meddy decayed. Another Burger number, the ratio of total kinetic energy to total available potential energy, decreased from 1.1 to 0.6 over a one year period. The rates at which the Meddy lost salt and heat are consistent with estimates of horizontal fluxes by intrusions. A horizontal diffusivity of O(5 m2 s−1) is needed if this flux by intrusions is parameterized by an eddy coefficient. Simple models of the evolution of an isolated eddy by horizontal and vertical mixing of mass and momentum are examined. These simple attempts to explain the evolution of the Meddy suggest more c...

Journal ArticleDOI
TL;DR: In this article, a coarse resolution, primitive equation general circulation model with idealized geometry and forcing is used to explore sensitivity to the assumption that vertical diffusion depends upon local stability, and the experiments suggest that it may be possible to calibrate the rate of deepwater formation of general circulation models, without affecting the poleward heat transport, by varying the magnitude of the vertical diffusivity below the thermocline.
Abstract: A coarse resolution, primitive equation general circulation model with idealized geometry and forcing is used to explore sensitivity to the assumption that vertical diffusion depends upon local stability. A case with constant diffusivity is compared with a case in which the diffusivity is inversely proportional to the local Brunt-V frequency. The stability-dependent parameterization of vertical diffusivity yields a poleward heat flux similar to that of a small, constant diffusivity. However, this parameterization increases the mean temperature in the deep ocean by about 0.8°C and the strength of the meridional circulation by over 40%. In addition, the stability-dependent diffusivity is found to increase the stratification in the deep ocean. The experiments suggest that it may be possible to calibrate the rate of deep-water formation of general circulation models, without affecting the poleward heat transport, by varying the magnitude of the vertical diffusivity below the thermocline. The explicit...

Journal ArticleDOI
TL;DR: In this paper, a four-mooring, transport-resolving current meter array was deployed in a section extending 70 km eastward from Abaco, Bahamas, at 26.5°N.
Abstract: During April 1987 to June 1988, a four-mooring, transport-resolving current meter array was deployed in a section extending 70 km eastward from Abaco, Bahamas, at 26.5°N. Mean currents in the upper layer (≤800 m) showed a clockwise rotation that appears to be associated with a small scale, quasi-permanent, anticyclonic gyre centered just northeast or the Bahamas. Deep mean currents were persistenly southward and indicated a strong, deep jet, the Deep Western Boundary Current (DWBC), reaching core speeds of 20 cm s−1at 2500 m depth about 25 km seaward of the western boundary. Vertically integrated meridional volume transport over the section showed a surprisingly large variability, ranging from approximately 20 Sv northward to 70 Sv southward (1 Sv = 106m3s−1). The mean meridional volume transport was 30 Sv to the south, with about 3 Sv flowing northward above 800 m, which could be produced by a weak Antilles Current and 33 Sv flowing, southward below 800 m. The deep southward transport of 33 Sv i...

Journal ArticleDOI
TL;DR: In this article, the authors describe variability in the western Pacific Ocean during the 1986-87 El Nino/Southern Oscillation (ENSO) event, with emphasis on time series measurements of currents, temperature, sea level and winds near the equator at 165°E.
Abstract: We describe variability in the western Pacific Ocean during the 1986–87 El Nino/Southern Oscillation (ENSO) event, with emphasis on time series measurements of currents, temperature, sea level and winds near the equator at 165°E. Zonal winds were anomalously westerly from mid-1986 to late 1987 and were punctuated by 2–10 m s−1 episodes of westerlies lasting about 10 days to 2 months. Zonal current in the upper 100-m surface layer responded to these wind variations typically within a week, in some cases with speeds exceeding 100 cm s−1 to the east. Zonal current variations in the thermocline below 100 m were generally less coherent with the local wires than currents near the surface. They were also generally less variable, although the Equation Undercurrent disappeared for 3–4 weeks in October-November 1987 at a time when the normal eastward directed zonal pressure gradient force reversed along the equator. Periods of intense and prolonged eastward flow in the surface layer were associated with a ...

Journal ArticleDOI
TL;DR: In this paper, temperature and salinity data obtained from the northwestern Weddell Sea during March 1986 reveal numerous thermohaline staircases in the thermocline separating warm deep water from the overlying colder, lower salinity winter water.
Abstract: Temperature and salinity data obtained from the northwestern Weddell Sea during March 1986 reveal numerous thermohaline staircases in the thermocline separating warm deep water from the overlying colder, lower salinity winter water Staircases in the upper, steeper portion of the thermocline were characterized by layers having vertical extents of 1–5 m Layer thicknesses in the deeper, weaker portion of the thermocline were far greater, sometimes exceeding 100 m The former staircases are referred to as Type A, and the latter as Type B Vertical gradients in temperature and salinity decreased abruptly across the boundary between Type A and Type B staircase regions Mean density ratios Rρ were 152 and 136 over the depth intervals containing Type A and Type B staircases, respectively Type A staircases were present at all sites sampled, whereas Type B staircases were present over approximately the central 50% of the area sampled Laboratory-derived results show that the observed time and vertical

Journal ArticleDOI
TL;DR: In this paper, the authors analyzed the energy propagation away from the jet, the Reynolds stress pattern, and the seasonal cycle of the eddy field in the first year of the Geosat Exact Repeat Mission (ERM) (starting in November 1986).
Abstract: Altimetric sea level data around the Kuroshio Extension, collected during the first year of the Geosat Exact Repeat Mission (ERM) (starting in November 1986), have been analyzed for energy propagation away from the jet, the Reynolds stress pattern, and the seasonal cycle of the eddy field. Space-time diagrams and wavenumber-frequency spectra are employed to discern the dominant direction of phase propagation, revealing westward and inward phase propagation towards the jet (i.e., energy propagation away from the jet), which is consistent with the notion that the widespread eddy variability from the jet is the result of transient Rossby wave response to forcing provided by growing, pulsating, and decaying meanders of the jet. It can be shown that any theory based on the linear instability cannot normally account for the energy propagation, because any linear instability that is wavelike away from the jet is by necessity not the fastest growing linear instability. Rather, a nonlinear instability mec...

Journal ArticleDOI
TL;DR: In this paper, a generalized geostrophic equation is used to study the energy cascade of unforced Geostrophic turbulence and the result of the cyclonic-anticyclonic asymmetry brought on by finite vertical displacements.
Abstract: Geostrophic turbulence has traditionally been studied within the framework of the classical, quasi-geostrophic equation. This equation, valid only when vertical displacements are weak, possesses a symmetry between cyclonic and anticyclonic vortices that is not present in the primitive equations. Moreover, previous studies were restricted by length scales not in excess of the deformation radius. In an attempt to advance the study of unforced geostrophic turbulence, we address here the following questions: How is the energy cascade toward longer length scales affected beyond the deformation radius? And, what is the result of the cyclonic-anticyclonic asymmetry brought on by finite vertical displacements? Some answers are provided by numerical experiments using a generalized geostrophic equation. The energy cascade is found to come to a halt beyond the deformation radius. There, a statistical equilibrium is reached at a length scale prescribed as a combination of the deformation radius, the beta eff...

Journal ArticleDOI
TL;DR: In their derivation of the lognormal probability density function for volume-averaged dissipation rates, Gurvich and Yaglom as mentioned in this paper assumed explicitly that these disipation rates are statistically homogeneous and that the averaging scale is small compared to the domain scale of the turbulent flow and large compared to a Kolmogorov scale.
Abstract: In their derivation of the lognormal probability density function for volume-averaged dissipation rates, Gurvich and Yaglom assumed explicitly that these dissipation rates are statistically homogeneous and that the averaging scale is small compared to the domain scale of the turbulent flow and large compared to the Kolmogorov scale. Estimates of dissipation rates in the oceanic thermocline reported by various researchers do not, in general, distribute lognormally because these datasets are often not homogeneous, nor is the averaging scale small compared to the scale of the turbulent patches. The conventional method of computing dissipation rates, a spectral technique, is incompatible with the assumptions for a lognormal distribution. Dissipation rates do distribute lognormally when they are computed with an alternative method that is consistent with the assumptions made by Gurvich and Yaglom. The shortest averaging scale that produced a lognormal distribution is three Kolmogorov length scales.

Journal ArticleDOI
TL;DR: An X-band marine radar on Windmill Hill, Gibraltar, was used to monitor the propagation of internal waves in the Strait of Gibraltar during March, April, and June 1986 as discussed by the authors.
Abstract: An X-band marine radar on Windmill Hill, Gibraltar, was used to monitor the propagation of internal waves in the Strait of Gibraltar during March, April, and June 1986. Surface roughness features of the waves were observed out to a range of ∼15 km. From photographic images of the radar screen, the positions of image features were measured manually using a digitizing table, and these data were then computer processed to give wave arrival times and phase speeds. During most tidal cycles, an internal undular bore generated at Camarinal Sill was observed to pass eastward (A-waves). These occurred throughout the experiments although less frequently near neap tides. The time of travel to Gibraltar varied significantly, so that the bore arrived between four and nine hours after high water. Arrival times were found to be closely related to the amplitude of the upper layer tidal current. This current had a strong diurnal component, which was manifested as an inequality between the arrival times of success...

Journal ArticleDOI
TL;DR: In this paper, the authors examined eddy energy in the central equatorial Pacific Ocean using acoustic Doppler Current Profiler velocities and CTD densities collected during the Hawaii-to-Tahiti Shuttle Experiment, in 1979-80.
Abstract: Eddy energetics in the central equatorial Pacific Ocean is examined using Acoustic Doppler Current Profiler velocities and CTD densities collected during the Hawaii-to-Tahiti Shuttle Experiment, in 1979–80. Three distinct sources of eddy energy are identified with varying degrees of statistical reliability, and are interpreted as evidence for three separate instabilities of the mean flow field. An instability at and just north of the equator occurs primarily in boreal summer and fall. It arises from the cyclonic shear between the Equatorial Undercurrent and the South Equatorial Current (SEC) north of the equator. The instability is present only when and where both currents are well developed, and there is little involvement of the shear between the SEC and the North Equatorial Countercurrent (NECC). The instability is characterized by local maxima in zonal and meridional eddy velocity variance, strong U*V* Reynolds stress, and large mean flow to eddy kinetic energy conversion. Despite seasonal va...

Journal ArticleDOI
TL;DR: Coastal sea level variations from six sites around South Africa are used to establish the characteristics of coastal-trapped wave (CTW) propagation Substantial amplitudes (>50 cm) are found along the south coast, but further propagation on the east coast against the Agulhas Current is inhibited Current measurements over the shelf in this region show barotropic reversals during the passage of the peak of a CTW, with essentially geostrophic flow occurring as discussed by the authors.
Abstract: Coastal sea level variations from six sites around South Africa are used to establish the characteristics of coastal-trapped wave (CTW) propagation Substantial amplitudes (>50 cm) are found along the south coast, but further propagation on the east coast against the Agulhas Current is inhibited Current measurements over the shelf in this region show barotropic reversals during the passage of the peak of a CTW, with essentially geostrophic flow occurring Comparisons are made with theoretically calculated first-mode CTW characteristics along various sections of the coast These calculated speeds fall within the range of speeds determined from observation, though an admixture of a mode 2 CTW is possible It is found that the speeds of wind systems moving along the coast also fall in the same range, probably leading to a resonance condition, and an explanation for the large CTW amplitudes

Journal ArticleDOI
TL;DR: In this article, the vertical mixing of momentum terms to the incompressible thermocline equations was investigated and it was shown that choosing the vertical eddy viscosity, ν = Af2/N2, where f is the Coriolis parameter and N is the local value of the buoyancy frequency, leads to isopycnal mixing of fQ, where Q is the reciprocal of potential vorticity, provided A is independent of the vertical coordinate.
Abstract: We investigate the consequence, at small Ekman number, of adding vertical mixing of momentum terms to the incompressible thermocline equations. We find that choosing the vertical eddy viscosity, ν = Af2/N2, where f is the Coriolis parameter and N is the local value of the buoyancy frequency, leads to isopycnal mixing of fQ, where Q is the reciprocal of potential vorticity, provided A is independent of the vertical coordinate. If, additionally, A is also independent of the north–south coordinate, then on a beta-plane, this implies homogenization of potential vorticity, q, within closed q-contours on isopycnal surfaces. This conclusion extends to spherical geometry if ν is also inversely proportional to β, the gradient of f with respect to latitude, i.e. ν = Af2/(N2β). The connection with the recent work of Gent and McWilliams and the consequences for coarse resolution numerical model studies are discussed.

Journal ArticleDOI
TL;DR: In this article, a feature model approach is used to initialize the primitive equation and quasi-geostrophic eddy resolving, open ocean models for hindcast studies in 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...

MonographDOI
TL;DR: In this article, the scattering of freely-propapting coastal-trapped waves (CTWs) by large variations in coastline and topography is studied using a numerical model which accomodates arbitrary density stratification, bathymetry and coastline.
Abstract: The scattering of freely-propapting coastal-trapped waves (CTWs) by large variations in coastline and topography is studied using a numerical model which accomodates arbitrary density stratification, bathymetry and coastline. Particular attention is paid to the role of stratification which in moderate amounts can eliminate backscattered free-waves which occur. theoretically, in a barotropic ocean. Numerical simulations using widening and narrowing shelf topographies show that the strength of the forward scattering into transmitted CTW modes is proportional to a topographic warp factor which estimates the severity of the topographic irregularities. The forward-scattering is further amplified by density stratification. Within the scattering region itself, the strengths of the scattered-wave-induced currents exhibit substantial variation over short spatial scales. There is generally a marked intensification of the flow within the scattering region, and rapid variations in phase. On narrowing shelves...