Presence Of Bottom Topography Research Articles

Carriers of Sargassum and mechanism for coastal inundation in the Caribbean Sea

We identify effective carriers of Sargassum in the Caribbean Sea and describe a mechanism for coastal choking. Revealed from satellite altimetry, the carriers of Sargassum are mesoscale eddies (vortices of 50-km radius or larger) with coherent material (i.e., fluid) boundaries. These are observer-independent—unlike eddy boundaries identified with instantaneously closed streamlines of the altimetric sea-surface height field—and furthermore harbor finite-time attractors for networks of elastically connected finite-size buoyant or “inertial” particles dragged by ocean currents and winds, a mathematical abstraction of Sargassum rafts. The mechanism of coastal inundation, identified using a minimal model of surface-intensified Caribbean Sea eddies, is thermal instability in the presence of bottom topography.

Time-dependent Kelvin cat-eye structure due to current–topography interaction

Non-stationary, rotational, linear surface waves are considered where the underlying sheared current has constant vorticity. A time-dependent study is presented on the formation and persistence of a Kelvin cat-eye structure in the presence of bottom topography. The flow domain is two-dimensional, which allows for the use of a conformal mapping and working in a computational flat-bottom domain. In some cases an initial disturbance is prescribed, while in others the waves are generated from rest. Submarine particle dynamics numerically captures the horizontal critical layer, defined by closed orbits separating the fluid domain into two disjoint regions. In the wave’s moving frame, these recirculation regions are structured in the form of Kelvin cat-eyes. Owing to the interaction with topography, the usual travelling-wave formulation is abandoned and the critical layer is identified through a non-stationary set of equations. The respective time-dependent Kelvin cat-eye structure dynamically adjusts itself at the onset of wave–topography interaction, without losing its integrity. The formation of a Kelvin cat-eye structure is also studied in the case where the surface is initially undisturbed. Surface waves are generated from either the current–topography interaction or by a pressure distribution suddenly imposed along the free surface. Under the pressure forcing, an isolated cat-eye forms with a single recirculation region beneath the wave.

Diagnosing Subgrid Mesoscale Eddy Fluxes With and Without Topography

AbstractGeneral circulation models use subgrid‐scale (SGS) parameterizations to represent the effects of unresolved mesoscale eddies on large‐scale motions. Most of the current SGS parameterizations are based on a theoretical understanding of transient eddies, where the mean flow is a temporal average. In this work, we use a spatial filtering analysis to better understand the scale‐dependent characteristics of the SGS fluxes. Specifically, we apply the filtering approach to diagnose SGS eddy volume fluxes and eddy velocity scales in a hierarchy of model configurations from a flat bottom channel to an idealized Southern Hemisphere. Importantly, SGS volume fluxes include significant contributions from standing meanders; unlike for transient eddies, the vertically integrated SGS volume flux does not necessarily integrate to 0. To accommodate net vertically integrated eddy fluxes, we define a SGS eddy diffusivity based on planetary potential vorticity (PV) diffusion. We diagnose the transient and standing contributions to SGS fluxes and associated effective diffusivities. In the presence of bottom topography or continental barriers the standing component of the PV diffusivity becomes dominant at large filter scales in the westerly wind region, while the transient component remains dominant in the easterly wind region. Our results suggest that the diagnosed PV diffusivity can be parameterized using mixing length theory based on a priori estimates of SGS velocity and length scales.

Wave Scattering by a Partial Flexible Porous Barrier in the Presence of a Step-Type Bottom Topography

A semi-analytic model is presented for oblique wave scattering by a bottom-standing or surface-piercing flexible porous barrier in water of finite depth with a step-type bottom topography. The physical problem is solved using the methods of least-squares and multimode approximation associated with the modified mild-slope equation. Effects on the wave scattering due to bed profile, structural rigidity, compressive force, angle of incidence, barrier length, porosity, and height of the step are examined. The study reveals that under some special conditions, nearly zero/full reflection may occur in the case of wave scattering by a partial flexible porous barrier in the presence of an undulated bottom topography. Further, the study predicts that the Bragg resonance may not occur in the case of wave scattering by a topography of sinusoidal profile. The present study provides insights to help understand how waves are transformed in a marine environment with/without flexible porous barriers in the presence of a bottom topography. The concept and methodology can be generalized to analyze problems of similar nature arising in ocean engineering.

Effect of topographic barriers on the rates of available potential energy conversion of the oceans

Determining the energy budget of the oceans requires evaluating the rates of available potential energy conversion in the circulation. Calculating these conversion rates depends upon the definition of an appropriate “reference” state of the density field, but this definition is complicated in the oceans by the presence of bottom topography. The trapping of dense fluid by topographic barriers means that there are multiple definitions for the reference state. The approach taken in this paper is to examine the sensitivity of the available potential energy budget to several methods for defining the reference state. The first method makes allowances for restrictions imposed on the flow by topography, however it is computationally intensive. The second method is proposed as an inexpensive alternative to the first. These new methods are used to evaluate the energy budget of a model overturning circulation maintained by surface buoyancy forcing. The results are compared with those obtained from two existing methods; one which employs an adiabatic resorting procedure ignoring topography, and one which uses a reference profile developed from the horizontal average of the density field. In our model, the rates of available potential energy conversion are insensitive to the reference state definition providing the reference state is developed from an adiabatic resorting of the domain. These results suggest that any of the adiabatic resorting methods proposed here would be sufficient to evaluate the rates of energy conversion in the ocean.

Observed variability of the South Pacific westward sea level anomaly signal in the presence of bottom topography

This study investigates the behavior of westward propagating sea level anomalies across the South Pacific Ocean, with a focus on the long Rossby wave signal determined from filtered TOPEX/Poseidon and ERS satellite altimeter data. An evaluation of the energy variability of the signal using a two‐dimensional Radon Transform analysis suggests that Rossby waves interact with both ridges and seamounts at various locations across the basin. Anomalously slow Rossby wave phase speeds are found over steep, isolated bathymetric features in the tropical South Pacific and over the plateau around New Zealand. Interaction with ridges increases the energetic variability, range of dominant propagation speeds, and meridional deviations in the Rossby wave signal.

Noise-sustained currents in quasigeostrophic turbulence over topography

We study the development of directed mean currents in a nonlinear model of fluid flow appropriate for large-scale ocean dynamics in the presence of bottom topography and dissipation, and forced with a noise term. We show that the presence of noise leads to persistent average currents directed along isobaths. The effect requires the presence of both the nonlinear and the random terms, and can be though of as a rectification of the stochastic energy input by the combined effect of nonlinearity and topography. At variance with previous works we use a scale-unselective dissipation, so that the phenomenon can not be explained in terms of minimum enstrophy states. Despite the fact that the system is out of equilibrium the large-scales are well described by a generalized canonical equilibrium with mean energy and enstrophy determined by a balance between random forcing and dissipation. This result allows predicting the strenght of the noise-sustained currents. Finally we discuss the relevance that these noise-induced currents could have on real ocean circulation.

An adaptive discrete-velocity model for the shallow water equations

A new approach to solving the shallow water equations is presented. This involves using discrete velocities of an adaptive nature in a finite volume context. The origin of the discrete-velocity space and the magnitudes of the discrete-velocities are both spatially and temporally variable. The near-equilibrium flow method of Nadiga and Pullin is used to arrive at a robust second-order (in both space and time) scheme—the adaptive discrete velocity (ADV) scheme—which captures hydraulic jumps with no oscillations. The flow over a two-dimensional ridge, over a wide range of undisturbed upstream Froude numbers prove the robustness and accuracy of the scheme. A comparison of the interaction of two circular vortex patches in the presence of bottom topography as obtained by the ADV scheme and a semi-Lagrangian scheme more than validates the new scheme in two dimensions.

Observations and Modeling of Wind-driven Currents in the Northeast Pacific

Velocity measurements from the interior of the Alaskan gyre are presented from a current meter mooring deployed in 4000 m of water at 49°33′N, 138°38′W, in the vicinity of Ocean Weather Station P. The mooring held five current meters, which spanned the depth of the water column. The data reveal surface-intensified motions with flow fluctuations in the upper layers of the water column characterized by long-period, O(100 d) time scales of variability. At abyssal depths, the flow displays shorter, O(20 d) time scales of variability. The data are compared with observations from one of the NEPAC moorings in the northeast Pacific (42°N, 152°W). Similar characteristics in kinetic energy levels, in the vertical structure of the flow, and in the vertical variation of eddy time scales are found at this location. The current measurements are considered in terms of the linear theory of directly wind-driven variability of Müller and Frankignoul. A comparison with simulated currents from a quasigeostrophic numerical model demonstrates that stochastic atmospheric forcing of the ocean can account for the observed variability. The numerical experiments and a simple extension of the linear theory suggest that the presence of bottom topography is important for the partition of energy between vertical modes and for the vertical variation of the time scales of the flow.

Spinup of Source-driven Circulation in an Abyssal Basin in the Presence of Bottom Topography

Abstract Spinup of the circulation driven by an inflow into an abyssal basin containing a simple large-scale topographic feature is studied, using an inverted one-and-a-half-layer shallow-water model. Two types of topography, one a plateau and the other a depression, are considered. In both cases, the topography gives rise to a region of closed geostrophic contours. The character of the adjustment and the resultant flow differ markedly between the closed geostrophic contour region and the region outside. Long-wave processes set up a modified Stommel–Arons flow outside the closed contour region, while inside the closed contour region “frictional spinup” leads to a vigorous recirculation. It is shown that the ratio of the velocity of the recirculation to that of the Stommet–Arons flow is equal to half the ratio of the radius of the closed contour region to the Stommel boundary-layer thickness. Moreover, it is shown that the recirculation inside the closed contour region is always cyclonic, whether the topog...

On the Role of Topography in the Ocean Circulation: Diffusive and Inertial Effects

Abstract In an earlier paper it was shown that the presence of bottom topography can substantially alter the Sverdrup prediction for the large-scale wind driven flow. In their model the abyssal water is set in motion by the eddy field, parameterized as lateral diffusion of potential vorticity. If the topography has a structure in the east-west direction, then the solution found in the inviscid limit by Cessi and Pedlosky predicts the occurrence of strong jets in the interior of the model ocean. In this note I used a numerical model to test whether the jets predicted by the analytic solutions survive when inertia and diffusion are included explicitly. In the inviscid limit, according to the structure of the topography these internal jets can occur in both vertically homogeneous and stratified models. Specifically, if the topographic slope changes sign, then one kind of jets is observed both in baroclinic and barotropic models. In the present work it is shown that this phenomenon is observed with moderate a...

A New Class of Stratospheric Vacillations in a Highly Truncated Model Due to Wave Interference

A new class of vacillations is obtained in the Holton and Mass model with a different bottom boundary condition. The model is a highly truncated spectral model describing wave-zonal flow interactions in a forced-dissipative system. The mean zonal wind and the wave change their vertical structures periodically with a period of the wave progression (5-10 days for the parameters used in this study). The vacillations are interpreted as an interference between a stationary wave and a topographically modified Rossby wave. The modified Rossby wave is an eigenmode of baroclinic flow in the presence of bottom topography within the framework of the highly truncated system. Time variations of the mean zonal wind are essential for the modification of the Rossby wave.

Feedback between ice flow, barotropic flow, and baroclinic flow in the presence of bottom topography

Coupling between externally driven barotropic flow and locally driven baroclinic flow in the presence of an ice cover and topography is studied. The topography is a necessary ingredient in this coupling. This study shows that the observed mesoscale activity of the ice edge seen in satellite imagery does not necessarily reflect the mesoscale baroclinic activity in the ocean. Besides oceanic eddies, the ice cover can trace the topographic changes via the coupling to the barotropic flow. A two‐layer ocean model coupled to an ice model is constructed to simulate an ice‐ocean system with a varying bottom topography. In the absence of wind, forcing the ice cover reflects the externally driven barotropic ocean response, especially topographically forced Taylor columns by forming ice streamers or meanders. Some of these features propagate (advected with the ocean currents) as a whole downstream, creating an image of eddy propagation even though there is no baroclinic structure underneath. When downwelling favorable winds (ice on the left from the wind direction) are turned on, in addition to this background barotropic flow, the Ekman flow in the ocean (toward the open ocean) will enhance the meandering of the ice edge due to the barotropic flow. During upwelling favorable winds, the ice edge stays rather compact, including the case when a strong baroclinic, cyclonic vortex is developing beneath an ice meander supported by the Taylor column in the ocean.

Effects of a Discontinuous Surface Stress on a Model of Coastal Upwelling

Abstract We report on laboratory model experiments in which coastal upwelling was created by a radially discontinuous surface stress in the presence of bottom topography in the form of a ridge. An anticyclonic torque was thus applied to the surface of the fluid at the location of the stress discontinuity which, in turn, generated large, offshore cyclonic and anticyclonic eddies in the stress-free region. When the upwelled front interacted with this offshore eddy field, dense, upwelled water was transported far offshore as jetlike flows. The strong interaction between the stress discontinuity “front” and the upwelled front at the ridge resulted in an upwelling maximum and a downstream jet at the ridge that were weaker than those reported previously by Narimousa and Maxworthy. One observation of interest was that a large number of flow-marking particles accumulated in the region where the ridge intersected the stress discontinuity.

Two-layer model of shear-driven coastal upwelling in the presence of bottom topography

A stratified two-layer fluid is brought to solid-body counterclockwise rotation inside a cylindrical tank having a conical bottom with a radial topographic ridge. A stress is then applied to the top surface by means of a clockwise differentially rotating disk. The resulting Ekman-layer flux causes the top layer to spin-down and the interface to rise near the wall and to descend a t the centre of the tank. As this process continues, the interface (front) between the two layers intersects the disk surface, and after that migrates away from the wall and allows the bottom fluid to contact the disk directly. The migration of the front continues until a steady state is reached, the front becomes stationary and the system is in geostrophic balance. The first sign of upwelled flow at the surface always occurs as a high-speed jet-like plume at the bottom topography, and only at a later time does a uniform upwelled flow appear a t the surface upstream of the topography. This plume, which always forms near the downstream edge of the topography, migrates in advance of the upstream front to produce an upwelling maximum at the bottom topography.The final width of the upstream flow in steady-state conditions is estimated by a simple theoretical model, which is in good agreement with the experimental results. We have an experimental criterion to predict the occurrence of travelling baroclinic waves on the upstream front. On the downstream side of the ridge, large standing waves are observed, with significant upwelling within them. Under some circumstances cyclones pinch-off from the upstream front, the plume on the ridge and the crests of the large downstream standing waves. We present criteria to predict the occurrence of these pinch-off processes.

Instabilities of a Baroclinic Flow Related to Topographic Forcing

Abstract The presence of bottom topography in a baroclinic flow modifies the properties of the propagating baroclinic unstable modes and allows for the appearance of new unstable modes which are nonpropagating, as first shown by Charney and Straus. Mountain form-drag, which provides a coupling mechanism between the zonal flow and the waves, is the essential ingredient for topographic instability. In this paper, the properties of instability for both zonally symmetric and asymmetric baroclinic basic states in the presence of topographic forcing are investigated. The results in a two-layer and a continuously stratified atmosphere are also compared and discussed. We find that two different types of topographic instability exist, one which is essentially baroclinic and is present in symmetric and asymmetric basic states, the other which is mixed barotropic-baroclinic and is present only in asymmetric basic states.

Stationary Barotropic Flow Induced by a Mountain over a Tropical Belt

Abstract Stationary solutions in the presence of bottom topography are computed for a tropical atmospheric belt. A primitive divergent barotropic model is assumed. An iterative time-averaging method proposed by Edelmann (1972b) is used for the purpose of suppressing gravitational oscillations and synoptic transient perturbations. The spatial finite differencing scheme is analogous to the one adopted in the Mintz-Arakawa two-level general circulation model and documented by Gates et al. (1971). Time integration is performed with the leapfrog scheme using a time stop of 10 min. The initial state consists of realistic zonal flow and the initial geopotential height of the free surface is determined from the wind by assuming steady-state conditions. First. we obtain quasi-stationary solutions using an idealized mountain profile over a 36-day integration. The results are used to interpret stationary solutions in the case involving real African orography. When an idealized mountain profile is used, a quasi-stati...

A wind-driven zonal channel with stratification and bottom topography

The wind-driven flow in a zonal channel on a rotating Earth is analysed in the presence of stratification and bottom topography. The velocity field related to the surface input of wind stress, heat and precipitation. The barotropic transport in the channel is reduced by the presence of bottom topography. Narrow jets and counter currents are formed over a zonal ridge in the baroclinic state.