Dale B. Haidvogel

Model evaluation experiments in the North Atlantic Basin: simulations in nonlinear terrain-following coordinates

Abstract A primitive equation ocean circulation model in nonlinear terrain-following coordinates is applied to a decadal-length simulation of the circulation in the North Atlantic Ocean. In addition to the stretched sigma coordinate, novel features of the model include the utilization of a weakly dissipative, third-order scheme for tracer advection, and a conservative and constancy-preserving time-stepping algorithm. The objectives of the study are to assess the quality of the new terrain-following model in the limit of realistic basin-scale simulations, and to compare the results obtained with it against those of other North Atlantic models used in recent multi-model comparison studies. The new model is able to reproduce many features of both the wind-driven and thermohaline circulation, and to do so within error bounds comparable with prior model simulations (e.g., CME and DYNAMO). Quantitative comparison with comparable results obtained with the Miami Isopycnic Coordinate Model (MICOM) show our terrain-following solutions are of similar overall quality when viewed against known measures of merit including meridional overturning and heat flux, Florida Straits and Gulf Stream transport, seasonal cycling of temperature and salinity, and upper ocean currents and tracer fields in the eastern North Atlantic Basin. Sensitivity studies confirm that the nonlinear vertical coordinate contributes significantly to model fidelity, and that the global inventories and spatial structure of the tracer fields are affected in important ways by the choice of lateral advection scheme.

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.

Generation and unstable evolution of a density-driven Eastern Poleward Current: The Iberian Poleward Current

[1] The generation and evolution of a density-driven Eastern Poleward Current is investigated using a high-resolution primitive equation numerical model. The simulations focus on the Iberian Poleward Current (IPC) as a case study. The flow is generated by a meridional upper ocean density gradient balanced by an eastward surface-intensified flow that adjusts at the coastal margin. The resulting current system has a baroclinic character with poleward flow at the surface layer, and equatorward flow underneath. A few weeks after initialization, the sheared along-slope flow generates several vorticity structures downstream of the main topographic features. In the lee of the topography, persistent anticyclones are observed and deep cyclogenesis is induced in relation to the meandering of the upper layer jet. These structures evolve preferentially as cyclone/ anticyclone eddy pairs, and after interaction some dipoles are ejected offslope. Within a period of a few months, the initial meridional gradient evolves into a complex system of fronts, eddies and slope flows. The dynamics of flow topography interaction is analyzed. A comparison with satellite imagery of the IPC is conducted and similarity in scales and patterns is noted.

Formation of Taylor caps over a tall isolated seamount in a stratified ocean

Abstract A primitive equation numerical model is used to examine various aspects of the formation of Taylor caps over a tall isolated seamount in a steady, rotating, nearly inviscid, stratified flow. The flow is characterized by four nondimensional parameters: the Rossby number, R0 = U/fL; the Burger number, S = NH/fL; the fractional seamount height, δ = hm/H; and the aspect ratio, Δ = H/L. Here U is the uniform inflow velocity, f the Coriolis parameter, L the horizontal length scale of the seamount, N the initial buoyancy frequency, H the ocean depth away from the seamount, and h m the maximum height of the seamount above the otherwise flat bottom. For both unstratified (S = 0) and stratified (S= 1) flows over a tall seamount which ultimately form a Taylor cap, the initial response is similar to that found in previous studies which considered short seamounts (δ ≪ 1) in weakly nonlinear flows (R0 ≪ 1). Two eddies form over the seamount and co-rotate clockwise around the seamount until one is swept away wi...

DAMEE-NAB : the base experiments

The results of an intercomparison experiment performed with five numerical ocean models of different architecture are presented. While all models are able to simulate the large-scale characteristics of the North Atlantic circulation with a fair degree of realism, they also exhibit differences that can be attributed to the choices made in vertical coordinates, domain size, and boundary conditions.

Phenomenology of the Low-Frequency Variability in a Reduced-Gravity, Quasigeostrophic Double-Gyre Model

Abstract The low-frequency variability of the oceanic wind-driven circulation is investigated by use of a reduced-gravity, quasigeostrophic model with slight variations on the classic double-gyre wind forcing. Approximately 30 eddy-resolving simulations of 100–1000 years duration are analyzed to determine the types of low-frequency variability and to estimate statistical uncertainties in the results. For parameters close to those leading to a stable antisymmetric solution, the system appears to have several preferred phenomenological regimes, each with distinct total energy levels. These states include a high-energy quasi-stable state; a low-energy, weakly penetrating state; and a state of intermediate energy and modest eddy/ring generation. The low-frequency variability of the model is strongly linked to the irregular transitions between these dynamical regimes. For a central set of reference parameters, the behavior of the system is investigated for each period in which the total energy remains in certa...

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...

The Long-term Ecosystem Observatory: an integrated coastal observatory

An integrated ocean observatory has been developed and operated in the coastal waters off the central coast of New Jersey, USA. One major goal for the Long-term Ecosystem Observatory (LEO) is to develop a real-time capability for rapid environmental assessment and physical/biological forecasting in coastal waters. To this end, observational data are collected from satellites, aircrafts, ships, fixed/relocatable moorings and autonomous underwater vehicles. The majority of the data are available in real-time allowing for adaptive sampling of episodic events and are assimilated into ocean forecast models. In this observationally rich environment, model forecast errors are dominated by uncertainties in the model physics or future boundary conditions rather than initial conditions. Therefore, ensemble forecasts with differing model parameterizations provide a unique opportunity for model refinement and validation. The system has been operated during three annual coastal predictive skill experiments from 1998 through 2000. To illustrate the capabilities of the system, case studies on coastal upwelling and small-scale biological slicks are discussed. This observatory is one part of the expanding network of ocean observatories that will form the basis of a national observation network.