Open Ocean Boundary Research Articles

Melting and freezing beneath ice shelves: implications from a three-dimensional ocean-circulation model

The thermohaline circulation below a regularly shaped ice shelf comparable in dimensions to the Ronne Ice Shelf, Antarctica, is investigated by means of a three-dimensional primitive-equation model. To handle the strongly inclined ocean surface as well as the bottom topography, we use scaled coordinates in the vertical direction. Preliminary results of two model versions containing open and closed open-ocean boundaries, respectively, prove the internal circulation is almost independent of the hydrography outside the ice shelf.Merely driven by the pressure-dependence of the sea-water freezing point (-0.000753 K dbar-1), a stationary circulation pattern evolves, forced by the buoyancy flux due to melting and freezing. The redistribution of ice from deep to shallow ice-shelf drafts resembles the ice-pump mechanism. Its strength depends on the ice-thickness gradients, namely, the differences of the freezing point at the ice-shelf base. As simulated melting and accumulation rates convincingly reproduce the interactions observed at the base of the Ronne Ice Shelf, the present model can act as a tool coupling ice-ocean dynamics.

Melting and freezing beneath ice shelves: implications from a three-dimensional ocean-circulation model

The thermohaline circulation below a regularly shaped ice shelf comparable in dimensions to the Ronne Ice Shelf, Antarctica, is investigated by means of a three-dimensional primitive-equation model. To handle the strongly inclined ocean surface as well as the bottom topography, we use scaled coordinates in the vertical direction. Preliminary results of two model versions containing open and closed open-ocean boundaries, respectively, prove the internal circulation is almost independent of the hydrography outside the ice shelf.Merely driven by the pressure-dependence of the sea-water freezing point (-0.000753 K dbar-1), a stationary circulation pattern evolves, forced by the buoyancy flux due to melting and freezing. The redistribution of ice from deep to shallow ice-shelf drafts resembles the ice-pump mechanism. Its strength depends on the ice-thickness gradients, namely, the differences of the freezing point at the ice-shelf base. As simulated melting and accumulation rates convincingly reproduce the interactions observed at the base of the Ronne Ice Shelf, the present model can act as a tool coupling ice-ocean dynamics.

Melting and freezing beneath ice shelves: implications from a three-dimensional ocean-circulation model

The thermohaline circulation below a regularly shaped ice shelf comparable in dimensions to the Ronne Ice Shelf, Antarctica, is investigated by means of a three-dimensional primitive-equation model. To handle the strongly inclined ocean surface as well as the bottom topography, we use scaled coordinates in the vertical direction. Preliminary results of two model versions containing open and closed open-ocean boundaries, respectively, prove the internal circulation is almost independent of the hydrography outside the ice shelf.Merely driven by the pressure-dependence of the sea-water freezing point (-0.000753 K dbar-1), a stationary circulation pattern evolves, forced by the buoyancy flux due to melting and freezing. The redistribution of ice from deep to shallow ice-shelf drafts resembles the ice-pump mechanism. Its strength depends on the ice-thickness gradients, namely, the differences of the freezing point at the ice-shelf base. As simulated melting and accumulation rates convincingly reproduce the interactions observed at the base of the Ronne Ice Shelf, the present model can act as a tool coupling ice-ocean dynamics.

M2 currents and residual flow in the Gulf of California

The full nonlinear, vertically-averaged equations of motion are time integrated through a finite difference scheme on a regular grid that reproduces the topography of the Gulf of California with a mesh size of 14 km. The model is forced through a remote, open-ocean boundary to the south by a linear superposition of the incoming and outgoing Kelvin waves of the M2 tide. Friction, parameterized by a quadratic depth-dependent law, and a viscosity close to 10 3·m 2·s −1, suffice to stabilize the model. The resulting maps of tidal elevations and phase agree adequately with those drawn by other authors from coastal observations. The computed tidal flows are dominated by the effects of the large bathymetric gradients; the currents accelerate markedly in the narrows and the straits around the Gulf's large islands, and effects of resonance and bottom friction are evident in the shallow northern Gulf. An average of the computed flow over an integral number of tidal cycles yields a residual circulation with typical values two orders of magnitude smaller than those of the instantaneous flow, organized in basin-wide closed cells with adjacent gyres rotating in opposite senses. The largest cell is a counterclockwise gyre found in the northern Gulf, which is probably driven by a transfer of tidal vorticity to the mean field through a bottom frictional torque. Residual flows are also important in the region of the large islands where the rectification is induced by the large topographic gradients in the channels and sills. The M2 residual circulation should remove a passive tracer from the upper Gulf in several months, or close to one year. No appreciable residual effects of the M2 tide are observed in the southern part of the Gulf.

Eclectic modelling of the North Atlantic. II. Transient tracers and the ventilation of the Eastern Basin thermocline

Renewal rates of the waters of the thermocline in the eastern North Atlantic are estimated by combining linear quasi-geostrophic dynamics with steady and transient tracers into a unified eclectic, reservoir model. The two-dimensional model first employed is finally rejected when it is found that it generates oxygen-utilization rates (OUR) that are, by conventional biological wisdom, too high. The three-dimensional model that replaces the two-dimensional one shows that the our is indeterminate, with possible ranges from zero to unacceptably high values. The region is flushed primarily from the north and east. The problem of using transient tracers is mathematically equivalent to that of distributed-system boundary-control theory, the open-ocean boundary conditions playing the role of the unknown control variables. The missing time histories of this new set of unknowns means that tritium and helium-3 distributions are only comparatively weak constraints on the flow field, but do set upper bounds on the vertical exchange with surface waters. Surface Ekman pumping is adequate to explain the interior distributions without additional buoyancy ventilation, although this latter process is possible. Some speculation is made about conditions under which transient tracers might play a more definitive role.

The potential for ocean prediction and the role of altimeter data

Abstract A skillful ocean forecasting capability will depend on (1) adequate data input, (2) adequate computing power, and (3) properly designed and adequately validated ocean models for data assimilation and forecasting. Once these conditions are satisfied, which is feasible with existing technology, forecasts of meandering currents and eddies up to a few months appear to be a reasonable expectation. Simulation studies suggest that the prediction of meandering currents and eddies requires high horizontal resolution (∼10 km), but only low vertical resolution. High vertical resolution can be obtained by using the circulation model to provide horizontal and vertical advection to a grid of one‐dimensional mixed‐layer models. Circulation models on subdomains of major ocean basins with extensive open ocean boundaries require accurate boundary information at all levels in the vertical and for the duration of forecasts longer than a few days. Basin‐scale models appear to be the most promising source for this information. To provide useful boundary conditions, the basin‐scale models must also resolve the meandering currents and eddies. Class 7 computers (∼1 gigaflop and 32–128 million words), expected on the market in the middle and late 1980s, are required for eddy‐resolving models of major ocean basins. Currently, satellites and atmospheric models provide the only prospects for oceanographic data and forcing functions with global coverage and resolution adequate for ocean circulation prediction models. For this purpose the most useful fields potentially available from satellites are current‐related variations in the sea‐surface elevation derived from altimeter data and scatterometer‐derived wind stress. Choosing an appropriate altimeter track pattern is critical. Successful ocean prediction appears feasible without extensive subsurface data acquisition by using the circulation model to convert the potentially well‐observed fields at the surface into subsurface information. This looks more promising than attempting to assimilate the extremely sparse subsurface data. Where adequate subsurface data are available (nowhere at present), they should, of course, be used. Subsurface data and major field programs are vital for local and regional subsurface validation of the forecast models, but altimeter data are essential for global prediction and representation of individual current meanders and eddies. Otherwise, in most regions it would be necessary to rely on the simulation skill of the model and the forcing functions, which are only weakly coupled to many of these oceanographic features. Within the next decade the appropriate satellite data, Class 7 computers, and eddy‐resolving basin or global‐scale models should become available, and should be used to form the heart of a global ocean prediction system. Part of this paper outlines a strategy for global ocean prediction based on these elements and on plans at the Naval Ocean Research and Development Activity.

The ice cover in the Greenland and Norwegian seas

The seasonal sea ice cover of the northern North Atlantic is reviewed, comprising the ice of the East Greenland Current, the ice cover around Svalbard, and the ice of the Barents Sea as far east as Franz Josef Land. The review considers various types of ice cover: (1) the fast ice along the coasts and in fjords, including ‘sikussak,’ or very old fast ice of exceptional thickness, (2) the transition zone between the fast ice and the main pack, including the positions and causes of recurring winter polynyas and the nature of the shear zone in NE Greenland, (3) the constitution, dynamics, and seasonal extent of the drifting pack ice, including interannual variations, (4) the processes of air‐ice‐ocean interaction which occur in the marginal ice zone, where the pack ice is influenced by an open ocean boundary.

Finite Element Grid Resolution Based OnSecond And Fourth-order Truncation ErrorAnalysis

Presently, many numerical modelers use the inadequate wavelength to grid size (Ax) ratio criterion as an aid in designing grids to solve the shallow water equations. Recent research has shown that grids designed using a local truncation error analysis (LTEA) are a very attractive alternative to the wavelength to Ax criterion. By computing grid spacing such that the local second-order truncation error is limited, we have shown the need for high resolution in areas on the continental shelf very near the coast and in the vicinity of the shelf break and slope. This paper will examine the effects of combining the first through the fourthorder truncation error terms. Here the finite element grid generation is accomplished by computing grid spacing such that the local truncation error is limited. Our analysis shows that the inclusion of the first through the fourth-order truncation errors has a dramatic effect on the allowable grid size throughout our idealized one-dimensional (1-D) domain. We show the variable grid generated using truncation error analysis to be highly accurate in capturing the physics of our idealized 1-D domain. We reaffirm that the wavelength to AJC criterion is insufficient. Introduction Coastal hydrodynamic models are including larger domains and increasing levels of localized detail. The Western North Atlantic Tidal (WNAT) model domain, Figure 1, provides an explicit example of just how large domain sizes have grown [Westerink et al*; Blain et al ]. This domain size is justified when one considers the simplicity of open ocean boundary conditions on deep ocean boundaries for both tide and storm surge computations. The total area, 8.347 xlO& kirr , coupled with the need for near shore detail indicates the use of a finite element based model. Generating a finite element grid for such large domains is typically accomplished using an inadequate criterion, namely the one-dimensional (1-D), linear, frictionless, constant topography wavelength to grid size ratio. This ratio is computed as: (i) Transactions on the Built Environment vol 9, © 1995 WIT Press, www.witpress.com, ISSN 1743-3509