Aike Beckmann

Numerical ocean circulation modeling

The continuous equations the 1D heat and wave equations considerations in two dimensions three-dimensional ocean models subgridscale parameterization process-oriented test problems simulation of the North Atlantic the final frontier.

Numerical Simulation of Flow around a Tall Isolated Seamount. Part I: Problem Formulation and Model Accuracy

Abstract A sigma coordinate ocean circulation model is employed to study flow trapped to a tall seamount in a periodic f-plane channel. In Part I, errors arising from the pressure gradient formulation in the steep topography/strong stratification limit are examined. To illustrate the error properties, a linearized adiabatic version of the model is considered, both with and without forcing, and starting from a resting state with level isopycnals. The systematic discretization errors from the horizontal pressure gradient terms are shown analytically to increase with steeper topography (relative to a fixed horizontal grid) and for stronger stratification (as measured by the Burger number). For an initially quiescent unforced ocean, the pressure gradient errors produce a spurious oscillating current that, at the end of 10 days, is approximately 1 cm s−1 in amplitude. The period of the spurious oscillation (about 0.5 days) is shown to be a consequence of the particular form of the pressure gradient terms in th...

Dynamical simulations of filament formation and evolution in the Coastal Transition Zone

Using the semispectral primitive equation model of Haidvogel et al. (1991), the evolution of a forced, surface-intensified, eastern boundary current is studied in the presence of both finite-amplitude topography and irregular coastline geometry. The model domain is 1000 km in alongshore length, and extends on average 700 km in the cross-shelf direction. A representative cape, as well as smoothed continental shelf-slope topography, are included. The model is forced by inclusion of nudging terms in the equations of motion which relax the fluid system back to a prescribed reference state on a time scale of 45 days. The reference state chosen is a broad, geostrophically balanced, equatorward flow having a maximum current at the surface of 0.45 m s−1 and a transport of approximately 10 Sv. No explicit wind forcing is included. Initialized with the smooth surface current, the model quickly approaches a turbulent, time-dependent equilibrium featuring an intense, meandering alongshore jet with local velocities of 0.8–1.0 m s−1. A deep, poleward undercurrent also forms. Subsequent interaction with the protruding cape geometry causes an offshore deflection in the steepening frontal meanders, some of which produce elongated filaments which penetrate significant distances (400–500 km) offshore. The emerging filaments are characterized by a strong downwelling signal (maximum vertical velocities of 30–40 m d−1). The simulated filaments ultimately pinch off, typically within 40–50 days, to form a corotating pair of detached eddies. The existence of the cape geometry, as well as the southward surface flow, appears to be necessary in this model to produce filament generation; removal of the irregular coastal geometry or reversal of the sense of the surface circulation is shown to inhibit filament formation. Detailed analysis of both instantaneous and time-mean momentum balances show dynamical similarity to observations taken during the Coastal Transition Zone experiment and elucidate the eddy transport mechanisms responsible for the formation of the poleward undercurrent in these experiments. Offshore transport of heat by the filaments, found to be O(0.06) PW, is substantial.

A numerical model of the Weddell Sea: Large‐scale circulation and water mass distribution

A circumpolar model for regional studies of the wind-driven and thermohaline circulation of the Southern Ocean including the major sub-ice shelf areas is described. A first series of numerical experiments focusing on the Weddell Sea reveals a pronounced and persistent double-cell structure of the Weddell Gyre with a maximum transport of ∼60 Sv, in agreement with observations. Experiments with artificial passive tracers point to the shallow shelf areas off the Filchner-Ronne Ice Shelf as main locations for bottom water production. The trajectories of Lagrangian floats are used to determine the pathways and timescales of water mass spreading in the model. In addition, the effect of sub-ice shelf forcing on the water mass characteristics is evaluated. It is shown that water modified in the sub-ice cavities contributes significantly to the deep and bottom water formation along the continental slope, and affects the water mass characteristics throughout the Weddell Sea, by increasing the stability of the near-surface stratification and preventing deep convection.

Effects of Increased Horizontal Resolution in a Simulation of the North Atlantic Ocean

Global mean and eddy fields from a four-year experiment with a 1/6° × 1/5° horizontal resolution implementation of the CME North Atlantic model are presented. The time-averaged wind-driven and thermohaline circulation in the model is compared to the results of a 1/3° × 2/5° model run in very similar configuration. In general, the higher resolution results are found to confirm that the resolution of previous CME experiments is sufficient to describe many features of the large-scale circulation and water mass distribution quite well. While the increased resolution does not lead to large changes in the mean flow patterns, the variability in the model is enhanced significantly. On the other hand, however, not all aspects of the circulation have improved with resolution. The Azores Current Frontal Zone with its variability in the eastern basin is still represented very poorly. Particular attention is also directed toward the unrealistic stationary anticyclones north of Cape Hatteras and in the Gulf of Mexico.

On the generation and role of eddy variability in the central North Atlantic Ocean

Sources of near-surface oceanic variability in the central North Atlantic are identified from a combined analysis of climatology, surface drifter, and Geosat altimeter data as well as eddy-resolving 13° and 16° Community Modeling Effort North Atlantic model results. Both observational and numerical methods give a consistent picture of the concentration of mesoscale variability along the mean zonal flow bands. Three areas of high eddy energy can be found in all observational data sets: the North Equatorial Current, the North Atlantic Current, and the Azores Current. With increasing horizontal resolution the numerical models give a more realistic representation of the variability in the first two regimes, while no improvement is found with respect to the Azores Current Frontal Zone. Examination of the upper ocean hydrographic structure indicates baroclinic instability to be the main mechanism of eddy generation and suggests that the model deficiencies in the Azores Current area are related to deficiencies in the mean hydrographic fields. A linear instability analysis of the numerical model output reveals that instability based on the velocity shear between the mixed layer and the interior is also important for the generation of the mid-ocean variability, indicating a potential role of the mixed layer representation for the model. The 16° model successfully simulates the northward decrease of eddy length scales observed in the altimeter data, which follow a linear relationship with the first baroclinic Rossby radius. An analysis of the eddy-mean flow interaction terms and the energy budget indicates a release of mean potential energy by downgradient fluxes of heat in the main frontal zones. At the same time the North Atlantic Current is found to be supported by convergent eddy fluxes of zonal momentum.

On the seasonal variability and eddies in the North Brazil Current: insights from model intercomparison experiments

The time dependent circulation of the North Brazil Current is studied with three numerical ocean circulation models, which differ by the vertical coordinate used to formulate the primitive equations. The models are driven with the same surface boundary conditions and their horizontal grid-resolution (isotropic, 1/3° at the equator) is in principle fine enough to permit the generation of mesoscale eddies. Our analysis of the mean seasonal currents concludes that the volume transport of the North Brazil Current (NBC) at the equator is principally determined by the strength of the meridional overturning, and suggests that the return path of the global thermohaline circulation is concentrated in the NBC. Models which simulate a realistic overturning at 24°N of the order of 16–18 Sv also simulate a realistic NBC transport of nearly 35 Sv comparable to estimates deduced from the most recent observations. In all models, the major part of this inflow of warm waters from the South Atlantic recirculates in the zonal equatorial current system, but the models also agree on the existence of a permanent coastal mean flow to the north-west, from the equator into the Carribean Sea, in the form of a continuous current or a succession of eddies. Important differences are found between models in their representation of the eddy field. The reasons invoked are the use of different subgrid-scale parameterisations, and differences in stability of the NBC retroflection loop because of differences in the representation of the effect of bottom friction according to the vertical coordinate that is used. Finally, even if differences noticed between models in the details of the seasonal mean circulation and water mass properties could be explained by differences in the eddy field, nonetheless the major characteristics (mean seasonal currents, volume and heat transports) appears to be at first order driven by the strength of the thermohaline circulation.

Vertical Structure of Midlatitude Mesoscale Instabilities

Abstract Instability processes in frontal jet regions outside the western boundary currents are examined for their ability to produce mesoscale variability comparable to that observed in the open ocean. The important of sufficient numerical resolution for a correct modeling of these processes is considered. Using a local β-plane quasi-geostrophic multilevel model with periodic horizontal boundary conditions, the combined effects of special density and current profiles typical for the midlatitude eastern North Atlantic are studied. The linearized approach yields a set of vertical shear modes with different vertical structure depending on the zonal wavelength of the perturbation. Some of these shear modes are unstable; the mesoscale range (50–500 km) reveals two different types; a surface intensified shear mode and a deep-sea intensified one. The growth rates of the former are usually largely exceeding those of the latter due to the larger velocities and shears. Their exponential amplification time scale ra...

Tidal Mixing in the Southern Weddell Sea: Results from a Three-Dimensional Model

Abstract A three-dimensional primitive equation ocean model is used to study the magnitude and distribution of tidal mixing in the southern Weddell Sea. The contributions of (i) semidiurnal barotropic constituents M2 and S2, (ii) internal tides, and (iii) diurnal barotropic tides are considered. Computed elevation cotidal charts are generally consistent with our knowledge and observations for the region. The model reproduces maximum semidiurnal tidal elevations of about 1.6 m and 1.0 m for M2 and S2, respectively. An internal tide of moderate strength is generated over the slope, propagating in the alongslope direction but rapidly dissipating. On the continental shelf, a thick benthic boundary layer (150 m) develops due to the proximity of the critical latitude for the M2 constituent. Typical M2 and S2 baroclinic tidal currents at the shelf break are 7 and 4 cm s−1, respectively. In general, the tidal currents produce strong mixing in the bottom boundary layer at the shelf break and on the shelf leading t...

Circumpolar Influences on the Weddell Sea: Indication of an Antarctic Circumpolar Coastal Wave

Abstract The Antarctic circumpolar wave is now a well-known feature that can be detected in atmospheric, oceanic mixed layer, and sea-ice data. In this coupled ice–ocean model driven by 40 yr of daily atmospheric forcing data, it represents a significant part of the interannual variability, linking the sea-ice and water mass formation processes in the Weddell Sea with other areas in the Antarctic water ring. In addition, these model results show a decadal-period wavelike anomaly pattern near the coast of Antarctica, propagating westward at about 2 cm s−1. This coastally trapped, bottom-intensified phenomenon likely has important effects on the dense water formation rate in the Weddell Sea and even the occurrence of the Weddell polynya.