Showing papers in "Journal of Physical Oceanography in 1992"

Latent and sensible heat flux anomalies over the northern oceans : driving the sea surface temperature

Abstract: A part of the large-scale thermodynamic forcing of the upper ocean is examined by relating monthly anomalous latent and sensible heat flux to changes in sea surface temperature (SST) anomalies over the North Atlantic and North Pacific. The fluxes are estimated using bulk formulas from a set of about four decades of marine observations from the COADS dataset from 1946 to 1986. Monthly anomalies are constructed by removing the long-term monthly means. The latent and sensible flux anomalies are strongly correlated over most of the ocean, so they are considered together as a sum. The heat flux estimates contain large spatial-scale anomalies consistent with both atmospheric circulation anomalies and with month-to-month changes (tendencies) in monthly SST anomalies. The monthly flux anomalies and the SST anomaly tendency are significantly correlated over much of the oceans, with anomalous positive/negative fluxes associated with anomalous cooling/warming. The connection between the flux and the SST ten...

Salinity-driven Thermocline Transients in a Wind- and Thermohaline-forced Isopycnic Coordinate Model of the North Atlantic

Abstract: An isopycnic-coordinate oceanic circulation model formulated with the aim of simulating thermodynamically and mechanically driven flow in realistic basins is presented. Special emphasis is placed on the handling of diabatic surface processes and on thermocline ventilation. The model performance is illustrated by a 30-year spinup run with coarse horizontal resolution (2° mesh) in a domain with North Atlantic topography extending from 10° to 60°N latitude. The vertical structure encompasses 10 isopycnic layers in steps of 0.2 σ units, capped by a thermodynamically active mixed layer. From an initially isohaline state with isopycnals prescribed by zonally averaged climatology, the model is forced by seasonally varying wind stress, radiative and freshwater fluxes, and by a thermal relaxation process at the surface. After a mechanical spinup time of about 15 years, a quasi-stationary pattern of mean circulation and annual variability ensues, characterized by pronounced subtropical mode-water formation...

Modeling Pack Ice as a Cavitating Fluid

Abstract: Polar ocean circulation is influenced by fluxes of salt and freshwater at the surface as ice freeze in one location, is transported by the winds and currents, and melts again elsewhere. The motion of sea ice, moreover, is strongly affected by internal stresses that arise from the mechanical strength of the ice cover. A simple sea-ice dynamics model, allowing these effects to be included in large-scale climate studies, is presented. In this model a cavitating fluid behaviour is assumed whereby the ice pack does not resist divergence or shear, but does resist convergence. While less realistic than other rheologies that include shear strength, this assumption has certain advantages for long-term climate studies. First, it allows a simple and efficient numerical scheme, in both rectangular and spherical coordinates, which as developed here along with a generation to include shear strength via the Mohr-Coulomb failure criteria. Second, realistic ice transport is maintained, even when the model is driv...

Vorticity and Vertical Circulation at an Ocean Front

Abstract: Density and velocity data with 4-km horizontal resolution from a survey of a front during FASINEX (Frontal Air-Sea Interaction Experiment) are combined to describe the structure of the top 300 m of the ocean. The geostrophic velocity field is derived and is used to examine the relative importance of stratification, relative vorticity, and twisting terms in Ertel's potential vorticity Q. Tenfold isopycnic changes in Q are found across a horizontal scale of only 10 km. These changes are confined to isopycnals that outcrop from the seasonal thermocline into the mixed layer. The ageostrophic velocity field is quantified by solution of the omega equation, and vertical velocities of up to 40 m day−1 are found. Small (40 km) surface-trapped (top 200 m) eddies are found to play a crucial role in the transport and effective diffusion of properties across the thermocline out of the mixed layer.

Rates of Water Mass Formation in the North Atlantic Ocean

Abstract: North Atlantic air-sea heat and freshwater flux data from several sources are used to estimate the conversion rate of water from one density to another throughout the range of sea surface density. This cross-isopycnal mass flux varies greatly over the ocean, with a maximum of 32.2 × 106 m3 s−1 at σ = 26.1 kg m−3 (toward greater densities) and a minimum of −7.6 × 106 m3 s−1 (toward lesser densities) at σ = 23.0 kg m−3. The air-sea fluxes force water to accumulate in three density bands: one at the lowest sea surface densities generated by heating; one centered near the density of subtropical mode water; and one spanning subpolar mode water densities. The transfer of water to the highest and lowest densities is balanced by mixing, which returns water to the middle density range, and also by boundary sources or sinks. Integrating the cross-isopycnal flux over all densities gives an annual average sinking of about 9 × 1O6 m3 s−1, which presumably escapes across the equator and must be balanced by a s...

Transport of freshwater by the oceans

Abstract: The global distribution of freshwater transport in the ocean is presented, based on an integration point at Bering Strait, which connects the Pacific and Atlantic oceans via the Artic Ocean. Through Bering Strait, 0.8 × 106 m3 s−1 of relatively fresh, 32.5 psu, water flows from the Pacific into the Arctic Ocean. Baumgrtner and Reichel's tabulation of the act gain of freshwater by the ocean in 5° latitude intervals is then integrated from the reference location at Bering Strait to yield the meridional freshwater transport in each ocean. Freshwater transport in the Pacific is directed northward at nearly all latitudes. In the Atlantic, the freshwater transport is directed southward at all latitudes, with a small southward freshwater transport out of the Atlantic across 35°S. Salt transport, which must be considered jointly with the freshwater transport, is northward throughout the Pacific and southward throughout the Atlantic (in the same direction as the freshwater flux) and is equal to the sal...

Reconciling Discrepancies in the Observed Growth of Wind-generated Waves

Abstract: Spectra from various wave-growth experiments have been collected into a database, and the data have been reanalyzed to explain the differences in the observed growth rates. For one of the experiments (Joint North Sea Wave Project: JONSWAP), the extensive wind data allowed the authors to perform a detailed analysis of the time history of the wind at a point moving with the group velocity of the peak of the wave spectrum. Using the average of the wind speed at this moving point, removing spectra that according to this wind estimate were not measured in steady or increasing wind, and taking into account the atmospheric stratification, the previously large discrepancy in the growth rates between JONSWAP and two other experiments (Bothnian Sea, Lake Ontario) was removed. The analysis revealed significant differences between the spectra in different groups. Peak enhancement was higher in the Bothnian Sea data than in the other datasets at the same dimensionless fetch. Equally high peak enhancement coul...

Interannual Variability in the Mid- and Low-Latitude Western North Pacific

Abstract: Twenty-two years (1967–88) of hydrographic data collected by the Japan Meteorological Agency along the 137°E meridian and surface wind data compiled by Florida State University (FSU) were analyzed to study the interannual variability in the western North Pacific. In the midlatitude region north of 22°N, the dominant signal in the dynamic height field was the interannual path variations of the Kuroshio. Whereas the eastward transport of the Kuroshio itself had no significant changes between the straight-path and meander-path years, the net transport of the Kuroshio system including recirculations showed a 30% increase during the meander-path years. In the straight-path years when the net transport was small, the Kuroshio tended to take a straight path with a strong recirculation developed to the south. The interannual path variations of the Kuroshio strongly influenced the water-mass movement in the midlatitudes. During the Kuroshio meander years, we found that a significant portion of the North P...

Representing topographic stress for large-scale ocean models

Abstract: Interaction of eddies with seafloor topography can exert enormous, systematic forces on the ocean circulation. This interaction has been considered previously under idealized circumstances. Theoretical results are here simplified and extended toward practical application in large-scale ocean circulation models. Among the suggestions is that coarse resolution models can “correct” a depth-independent part of the velocity field toward a velocity given by −z × ∇fLH, where z is the vertical unit vector, f is Coriolis form, L is a length scale O(10 km), and H is the total depth. Absence of this tendency may be implicated in a number of systematic defects that appear in present ocean models.

Vertical Mixing in the Indonesian Thermocline

Abstract: Western Pacific central and tropical waters characterized by a subsurface salinity maximum spread into the Indonesian seas as part of the Indonesian throughflow. Within the Indonesian seas this salinity maximum is attenuated and, in some places, completely removed. A simple advection-diffusion model verifies the importance of vertical mixing in the transformation of Western Pacific waters to Indonesian thermocline water. The profiles indicate a predominant North Pacific presence in most of the seas, although some South Pacific water is present in the eastern ses of Halmahera, Seram, and Banda. The main interocean route is through the western seas of Sulawesi, Makassar, and Flores, while the flow pathway in the eastern seas is less certain. The Banda Sea can be renewed from either the northern passages (Halmahera and Maluku) or from the south via the Flores Sea. Using representative basin property profiles derived from the archieved data allows determination of a range of vertical diffusivities an...

FLuid exchange across a meandering jet

Abstract: The motion of fluid parcels in a two-dimensional kinematic model of a meandering jet is investigated using Melnikov's method. The study is motivated by a recent analysis of float trajectories in the Gulf Stream. The results indicate that the efficiency of cross-jet exchange induced by fluctuating meander amplitudes depends strongly on the frequency of the fluctuations. For high frequencies (≳0.04 cpd), exchange between the core of the jet and regions of “trapped” fluid moving with the meander is preferred, while for low frequencies (≲0.04 cpd), exchange between the “trapped” fluid and the slow-moving fluid surrounding the jet is preferred. Propagating waves superimposed on the meandering jet can efficiently cause exchange between regimes when their phase speeds roughly match the basic flow velocities along the regime boundaries. Numerical results suggest that exchange across the center of the jet is less efficient than exchange between adjacent regimes so that the meandering jet will tend to stir...

The Surface Boundary Layer in Coastal Upwelling Regions

Abstract: Observations from the Oregon, northwest Africa, Peru, and northern California shelves are used to examine the characteristics of the surface boundary layer in coastal regions during the upwelling season. The observations from these four regions yield a consistent picture of the structure of the surface boundary layer. Both CTD and moored observations reveal the presence of surface mixed layers that are typically 0–20 m thick with variability at diurnal and subtidal (periods longer than 36 hours) frequencies. The subtidal surface mixed-layer depth variability scales as u*/(NIf)½, where u* = (τS/ρ0)½ is the shear velocity, NI is the buoyancy frequency below the surface mixed layer, and f is the Coriolis frequency. Surprisingly, this relationship indicates that the subtidal variability of surface mixed-layer depth does not depend strongly on either the surface heat flux or advection of heat, both of which are large in coastal upwelling regions. Within the surface mixed layer the cross-shelf current ...

Interpretations of the JEBAR Term

Abstract: In diagnostic calculations of the ocean's circulation, the so-called JEBAR (joint effect of baroclinicity and relief) term may induce a significant depth-average current field, as has been noted in the oceanographic literature. Here we present two consistent interpretations of this term. In the equation governing the vorticity of the depth-averaged current, a topographic vortex-stretching term proportional to the dot product of depth-averaged velocity and the depth gradient arises. We show that JEBAR corrects this term by removing the contribution of the geostrophic flow referenced to the bottom, which cannot generate topographic vortex stretching. In the evolution equation for the vorticity of the depth-integrated flow, a bottom-torque term is present. We show that the JFBAR term discussed above enters here as a contribution to the bottom torque, emphasizing the role of JEBAR as a forcing term. We also show that when a diagnostic calculation is formulated completely in terms of the transport str...

Effects of Numerics on the Physics in a Third-Generation Wind-Wave Model

Abstract: Numerical errors in third-generation ocean wave models can result in a misinterpretation of the physics in the model. Using idealized situations, it is shown that numerical errors significantly influence the initial growth, the response of wave fields to turning winds, the scaling behavior of model results with wind speed, and the propagation of swell. Furthermore, the numerics may influence the dynamic interaction between wind sea and swell. Surprisingly, fetch-limited model behavior is hardly influenced by numerical errors in wave propagation. Simple modifications of the numerics are presented to reduce or eliminate such errors. The impact of numerical improvements for realistic conditions is illustrated by performing hindcasts for the Atlantic basin and for a smaller region off the east coast of the United States.

Eddy dynamics in a primitive equation model: Sensitivity to horizontal resolution and friction

Abstract: A primitive equation model of an idealized ocean basin, driven by simple, study wind and buoyancy forcing at the surface, is used to study the dynamics of mesoscale eddies. Model statistics of a six-year integration using a fine grid (1/6° × 0.2°), with reduced coefficients of horizontal friction, are compared to those using a coarser grid (1/3° × 0.4°), but otherwise identical configuration. Eddy generation in both model cases is primarily due to the release of mean potential energy by baroclinic instability. Horizontal Reynolds stresses become significant near the midlatitude jet of the fine-grid case, with a tendency for preferred energy transfers from the eddies to the mean flow. Using the finer resolution, eddy kinetic energy nearly doubles at the surface of the subtropical gyre, and increases by factors of 3–4 over the jet region and in higher latitudes. The spatial characteristics of the mesoscale fluctuations are examined by calculating zonal wavenumber spectra and velocity autocorrelation functions. With the higher resolution, the dominant eddy scale remains approximately the same in the subtropical gyre but decreases by a factor of 2 in the subpolar areas. The wavenumber spectra indicate a strong influence of the model friction in the coarse-grid case, especially in higher latitudes. Using the coarse grid, there is almost no separation between the energetic eddy scale and the scale where friction begins to dominate, leading to steep spectra beyond the cutoff wavenumber. Using the finer resolution an inertial subrange with a k−3 power law begins to emerge in all model regions outside the equatorial belt. Despite the large increase of eddy intensity in the fine-grid model, effects on the mean northward transport of heat are negligible. Strong eddy fluxes of heat across the midlatitude jet are almost exactly compensated by changes of the heat transport due to the mean flow.

An equivalent-barotropic mode in the fine resolution Antarctic model

Abstract: Both the 6-year time-mean flow and the eddy kinetic energy in the Fine Resolution Antarctic Model are found to be approximately self-similar in the vertical, with the velocity showing a rapid decay with depth. A simple argument is given as to why this would be expected to occur in this and other numerical models, and an analysis is given as to the degree of steering of the mean flow by topography.

Boundary Current Separation in a Quasigeostrophic, Eddy-resolving Ocean Circulation Model

Abstract: The response of a rectangular, flat-bottom, eddy-resolving, quasigeostrophic ocean to a steady, double-gyre wind stress is studied to assess the sensitivity of the solutions to a partial-slip lateral boundary condition in which tangential stress is proportional to tangential velocity. The constant of proportionality (α) has limiting values of zero and infinity, corresponding to free-slip (no-stress) and no-slip conditions, respectively. Seven numerical solutions—corresponding to the α values 0.0, 2.0, 3.5, 5.0, 6.5, 8.0, and 100.0—are obtained, which span the free-slip and no-slip limits. Significant qualitative changes in the time-mean behavior of the solutions are observed to occur with increasing α. These changes include a gradual retreat of the separation points of the western boundary currents in the subtropical and subpolar gyres, a dramatic reduction in the basin-integrated reservoirs of mean and eddy kinetic energy, a weakening of bottom dissipation and its replacement by lateral dissipat...

Intrusive Mixing in a Mediterranean Salt Lens—Intrusion Slopes and Dynamical Mechanisms

Abstract: Thermohaline intrusions found in Meddy “Sharon” were laterally coherent The slope of these intrusions across density surfaces was in opposite senses in the upper and lower part of the meddy, consistent with (i) the McIntyre instability for Prandtl number less than one, in which case mass flux (assumed equal in heat and salt diffusivity) dominates over viscosity, or (ii) double-diffusive intrusions, in which diffusive fluxes dominate in the upper, diffusively unstable portion of the meddy, and finger fluxes dominate in the lower, fingering-unstable part. The magnitudes of the intrusion slopes are outside the range of angles for which the McIntyre mechanism can provide energy to the intrusive motions, but within the range of angles for which double-diffusive mixing can drive the intrusions. Thus, the energy source for the intrusions is not the kinetic or potential energy of the geostrophic shear flow, but is the potential energy associated with the lateral salinity and temperature gradients. Furthe...

The Role of Stratification in the Formation and Maintenance of Shelf-Break Fronts

Abstract: A mechanism is described for the formation or a front at the edge of a continental shelf in an initially linearly stratified fluid lacking horizontal density gradients. A primitive equation numerical model is used with a specified vertically uniform inflow imposed at the upstream boundary, and the flow is allowed to evolve alongshelf under the influence of bottom friction. As the flow progresses downstream, the shelf water moves steadily offshore due to the Ekman flux concentrated in the bottom boundary layer. This offshore flow transports light water under heavier water, which leads to convective overturning and ultimately a vertically well-mixed density field over the shelf. Large cross-shelf density gradients appear along the bottom at the shelf break where the vertically well-mixed shelf water abuts the linearly stratified water on the upper slope. At the shelf break, the bottom boundary layer detaches and continues offshore along upward sloping isopycnals. Neutrally buoyant particles in the ...

The Superadiabatic Surface Layer of the Ocean during Convection

Abstract: Clear identification of the relatively weak superadiabatic potential temperature gradient in the ocean surface layer during convection has been made with the help of intensive vertical profiling measurements at an open-ocean site. In the surface layer the superadiabatic gradient, with a mean value of −1.8 × 10−4 K m−1, was a consistent feature of the convective boundary layer persisting throughout each of six consecutive nights. In the well-mixed layer, below the surface layer, the observed potential temperature was nearly constant and turbulent kinetic energy (TKE) dissipation rate approximately balanced the production of TKE by the buoyancy flux through the sea surface. In the surface layer the TKE dissipation rate was systematically larger than the production of TKE predicted by wind stress and mixed-layer similarity scaling.

A Numerical Study of the Variability and the Separation of the Gulf Stream, Induced by Surface Atmospheric Forcing and Lateral Boundary Flows

Abstract: A primitive equation regional model is used to study the effects of surface and lateral forcing on the variability and the climatology of the Gulf Stream system. The model is an eddy-resolving, coastal ocean model that includes thermohaline dynamics and a second-order turbulence closure scheme to provide vertical mixing. The surface forcing consists of wind stroll and beat fluxes obtained from the Comprehensive Ocean-Atmosphere Data Set (COADS). Sensitivity studies am performed by driving the model with different forcing (e.g., annual versus zero surface forcing or monthly versus annual forcing). The model climatology, obtained from a five-year simulation of each case, is then compared to observed climatologies obtained from satellite-derived SST and hydrocast data. The experiments in which surface best flux and wind stress were neglected show less realistic Gulf Stream separation and variability, compared with experiments in which annual or seasonal forcing are used. A similar unrealistic Gulf S...

The South Indian Ocean Current

Abstract: In this paper, the historical hydrographic database for the south Indian Ocean is used to investigate (i) the hydrographic boundary between the subtropical gyre and the Antarctic Circumpolar Current (ACC), the subtropical front (STF), and especially (ii) the southern current band of the gyre. A current band of increased zonal speeds in the upper 1000 m is found just north of the STF in the west near South Africa and at the surface STF in the open Indian Ocean until the waters off the coast of Australia are reached. As neither any other investigation of this current nor a name for it are known, the flow has been called the South Indian Ocean Current (SIOC). This name is anologous to the same current band in the South Atlantic Ocean, the South Atlantic Current. The STF is located in the entire south Indian Ocean near 40-degrees-S. The associated current band of increased zonal speeds is the SIOC, which is found at or north of the STF. East of 100-degrees-E the SIOC separates from the STF and continues to the northeast. The zonal flow south of the STF is normally weak and serves to separate the South Indian Ocean and Circumpolar currents. Near Africa the SIOC has a typical volume transport of 60 Sv (1 Sv = 10(6) m3 s-1) in the upper 1000 m relative to deep potential density surfaces of sigma(4) = 45.87 kg m-3 (2800-3500 m) or sigma(2) = 36.94 kg m-3 (1500-2500 m). Near western Australia the SIOC is reduced to about 10 Sv as it turns to the northeast.

Turbulence and Internal Waves at the Equator. Part I: Statistics from Towed Thermistors and a Microstructure Profiler

Abstract: High correlations between turbulent dissipation rates and high-wavenumber internal waves and the high values of turbulent dissipation associated with internal wave activity suggest that internal waves are the main direct source of mixing in the thermocline above the core of the Equatorial Undercurrent An extensive dataset obtained using a microstructure profiler and thermistor chain towed along the equator was analyzed to examine the correspondence between turbulent mixing and high-wavenumber internal waves In the low Richardson number (Ri) thermocline below the mixed layer but above the core of the Equatorial Undercurrent, and when winds were moderate and steadily westward, it was found that: • the spectrum of vertical isotherm displacement was dominated by a narrow wavenumber band (corresponding to 150–250-m zonal wavelength) of internal waves; • both turbulence and internal waves varied diurnally—hourly averaged values of turbulent dissipation rate and wave potential energy were greater by a

Fluid Dynamics of Oceanic Thermocline Ventilation

Abstract: A flux form of the Potential vorticity (PV) equation is applied to study the creation and transport of potential vorticity in an ocean gyre; generalized PV fluxes (J vectors) and the associated PV flux fines are used to map the creation, by buoyancy forcing, of PV in the mixed layer and its transport as fluid is subducted through the base of the mixed layer into the thermocline. The PV flux lines can either close on themselves (recirculation) or begin and end on the boundaries (ventilation). Idealized thermocline solutions are diagnosed using J vectors, which vividly illustrate the competing process of recirculation through western boundary currants and subduction from the surface. Potential vorticity flux vectors are then used to quantify the flux of mass passing invisidly through a surface across which potential vorticity changes discontinuously but at which potential density and velocity are continuous. Such a surface might be the base of the oceanic mixed layer or, in a meteorological context...

The Effect of Rain in Calming the Sea

Abstract: The effect of rain in damping surface waves appears to be significant in the estimation of wind speed from the backscatter of radar signal from the sea surface. The radar backscatter depends on the small-scale roughness of the sea surface. This is modified by the generation of capillary and gravity-capillary ripples by raindrops and by the increased damping of the wavelengths of 10-cm scale. The phenomenon is also important in wave generation and wave breaking, since in both cases the short wavelengths play a key role. A laboratory experiment has been performed to investigate the damping of water waves by rain in the absence of wind. Rain of intensity of 300 mm h−1 falls on mechanically generated progressive waves. The wave amplitude is measured before the wave enters and after it exits the rain section of a tank 2.35 m long. On the assumption of exponential damping, it is found that the effect of rain can be described by an eddy viscosity νE ≈ 0.3 cm2 s−1. The major part of the damping is attrib...

On the Formation of Antarctic Intermediate and Bottom Water in Ocean General Circulation Models

Abstract: A series of coarse-resolution models were integrated with a view to determining the most appropriate representation of the largest-scale water masses formed in the Southern Ocean In particular, it was hoped that the models could realistically simulate Antarctic Bottom and Intermediate Water The ocean model employed has a global domain with a realistic approximation of the continental outlines and bottom bathymetry The subgrid-scale variation of bottom bathymetry is removed by spatial averaging over each grid box The annual mean forcing at the sea surface is derived from climatological fields of temperature, salinity, and wind stress It is found that the salinity of shelf water in the Weddell and Ross seas is critical if the model is to appropriately simulate the world's intermediate and bottom water masses If the surface layer is too fresh in the Weddell and Ross seas, any bottom water formed adjacent to Antarctica is significantly less dense than in the real ocean Furthermore, surface wat

The Subtropical Mode Waters of the South Pacific Ocean

Abstract: The subtropical mode waters (STMW) of the southwestern Pacific Ocean are described, including their physical characteristics, spatial distribution, and temporal variability. STMW is a thermostad, or minimum in stratification, having temperatures of about 15°–19°C and vertical temperature gradient less than about 2°C per 100 m. Typical salinity is 35.5 psu at 16.5°C. The STMW layer is formed by deep mixing and cooling in the eastward-flowing waters of the separated East Australia Current. Surface mixed layers are observed as deep as 300 m north of New Zealand in winter, in the center of a recurring anticyclonic eddy. The STMW thermostad in the South Pacific is considerably weaker than its counterparts in the North Atlantic and North Pacific, a contrast that may help to discriminate between physical processes contributing to its formation. A quarterly time series of expendable bathythermograph transects between New Zealand and Fiji is used to study the temporal variability of STMW. Large fluctuatio...

An Adjoint Method for Obtaining the Most Rapidly Growing Perturbation to Oceanic Flows

Abstract: This work explores the formation and growth of waves on oceanic flows using a quasigeostrophic model. In particular, we consider flow regimes consisting of zonal oceanic jets, similar in fact to the westward extension of the Gulf Stream. Traditionally, the formation of waves has been ascribed to exponentially unstable modes, but rather than adopt this paradigm, we seek the most rapidly growing perturbation without restriction or its structure to normal-mode form. Optimal excitations are determined using the adjoint of the quasigeostrophic dynamic equations, and the perturbations found by this method are shown to grow mote rapidly than the unstable mode. Applications of the theory presented here include determination of a tight upper bound on perturbation growth rate, a constructive method for finding the most disruptive disturbance to a given flow, and a method for determining the relative predictability of flows. From the form of the most rapidly growing perturbation, resolution requirements for...

Scattering of Oceanic Internal Gravity Waves off Random Bottom Topography

Abstract: The scattering of oceanic internal gravity waves off random bottom topography is analyzed under the assumptions that (i) the height of the topography is smaller than the vertical wavelength and (ii) the slope of the topography is smaller than the wave slope. For each frequency, scattering redistributes the incoming energy flux in horizontal wavenumber space. The scattered wave field approaches an equilibrium state where the energy flux is equipartitioned in horizontal wavenumber space. For incoming red spectra, this implies a transfer from low to high wavenumbers. For typical internal wave and bottom spectra, about 6.8% of the incoming energy flux is redistributed. While this might be less than the flux redistribution caused by reflection off a critical slope, the scattering process transfers the energy flux to higher wavenumbers than the reflection process. Scattering might thus be equally or more efficient than reflection in causing high shears and mixing near the bottom.

Mesoscale Variability in the Atlantic Ocean from Geosat Altimetry and WOCE High-Resolution Numerical Modeling

Abstract: Characteristic of the mesoscale variability in the Atlantic Ocean are investigated by analyzing the Geosat altimeter signal between 60°S and 60°N. The rms sea-surface variability for various frequency bands is studied, including the high-frequency eddy-containing band with periods <150 days. Wavenumber spectra and spatial eddy characteristics are analyzed over 10° by 10° boxes covering both hemispheres of the Atlantic Ocean. A comparison, with solutions of a high-resolution numerical experiment, developed as the Community Modeling Effort of the World Ocean Circulation Experiment, aids interpretation of the Geosat results in the tropical and subtropical Atlantic and provides a test of the model fluctuating eddy field. Results from Geosat altimetry show a wavenumber dependence close to k1−5 (k1 being the alongtrack wave-number) over almost the entire Atlantic Ocean except for areas in the tropical and subtropical Atlantic where the rms variability in the eddy-containing band is less than 5 cm, that is, not significantly different from the altimeter noise level. Characteristic eddy length scales inferred from Geosat data are linearly related with the deformation radius of the first baroclinic mode over the whole Atlantic Ocean, except for the equatorial regime (10°S to 10°N). The data-model comparison indicates that the high-resolution model with horizontal grid size of ⅓° and ° in latitude and longitude is quite capable of simulating observed eddy characteristics in the tropics and subtropics. In mid- and high latitudes, however, the model fails to simulate the pronounced poleward decrease in eddy scales. This leads to systematic discrepancies between the model and Geosat observation, with model scales being up to 50% larger than deduced from altimetry.