Eric P. Chassignet

North Atlantic Simulations with the Hybrid Coordinate Ocean Model (HYCOM): Impact of the Vertical Coordinate Choice, Reference Pressure, and Thermobaricity

The viability of a generalized (Hybrid) Coordinate Ocean Model (HYCOM), together with the importance of thermobaricity and the choice of reference pressure, is demonstrated by analyzing simulations carried out using the World Ocean Circulation Experiment (WOCE) Community Modeling Experiment (CME) Atlantic basin configuration. The standard hybrid vertical coordinate configuration is designed to remain isopycnic throughout as much of the water column as possible while smoothly making a transition to level (pressure) coordinates in regions with weak vertical density gradients, such as the surface mixed layer, and to terrain-following coordinates in shallow-water regions. Single-coordinate (pressure or density) experiments illustrate the flexibility of the model but also bring forward some of the limitations associated with such a choice. Hybrid experiments with potential density referenced to the surface ( su) and to 20 MPa (;2000 m) (s 2) illustrate the increased influence of pressure errors with increasing distance from the reference pressure. The su hybrid experiment does not properly reproduce the northward flow of Antarctic Bottom Water (AABW), and large errors in near-surface pressure gradients in the s 2 experiment produce a wind-driven gyre circulation that is too strong, when compared with observations, and a North Atlantic Current that follows an unrealistic path. These near-surface and nearbottom pressure errors are removed when thermobaric effects are included, resulting in a more accurate representation of the upper-ocean gyre circulation, the northward AABW flow near the bottom, and the meridional overturning circulation and heat flux.

Developments in ocean climate modelling

This paper presents some research developments in primitive equation ocean models which could impact the ocean component of realistic global coupled climate models aimed at large-scale, low frequency climate simulations and predictions. It is written primarily to an audience of modellers concerned with the ocean component of climate models, although not necessarily experts in the design and implementation of ocean model algorithms.

The Ocean's Response to North Atlantic Oscillation Variability

The North Atlantic Oscillation (NAO) is the dominant mode of atmospheric variability in the North Atlantic Sector. Basin scale changes in the atmospheric forcing significantly affect properties and circulation of the ocean. Part of the response is local and rapid (surface temperature, mixed-layer depth, upper ocean heat content, surface Ekman transport, sea ice cover). However, the geostrophically balanced large-scale horizontal and overturning circulation can take several years to adjust to changes in the forcing. The delayed response is non-local in the sense that waves and the mean circulation communicate perturbations at the air-sea interface to other parts of the Atlantic basin. A delayed and non-local response can potentially give rise to oscillatory behavior if there is significant feedback from the ocean to the atmosphere. We conjecture that, on decadal and longer time scales, changes in the oceans heat storage and transport should have an increasingly important impact on the climate. Finally, changes in the ocean circulation and distribution of heat and freshwater will also alter ventilation rates and pathways. Thus we expect a change in the net uptake of gases (e.g., O 2 , CO 2 ), altered nutrient balance, and changes in the dispersion of marine life. We review what is known about the oceanic response to changes in NAO-induced forcing from combined theoretical, numerical experimentation and observational perspectives.

Westward Motion of Mesoscale Eddies

Abstract Since the pioneering work of Nof, the determination of the westward drift of mesoscale eddies under the planetary (beta) effect has been a recurrent theme in mesoscale oceanography, and several different formulae have been proposed in the literature. Here, recpatiulation is sought, and, within the confines of a single-layer model, a generalized formula is derived. Although it is similar to Nofs, the present formula is established from a modified definition and with fewer assumptions. It also recaptiulates all other formulae for the one-layer model and applies to a wide variety of situations, including cases when the vortex develops a wake of Rossby waves or undergoes axismmetrization. Following the derivation of the formula, a physical interpretation clarifies the migration mechanism, which can be divided between a self-induced propulsion and a reaction from the displaced ambient fluid. Numerical simulations with primitive and geostrophic equations validate the formula for a variety of length sc...

Improving Oceanic Overflow Representation in Climate Models: The Gravity Current Entrainment Climate Process Team

Abstract Oceanic overflows are bottom-trapped density currents originating in semienclosed basins, such as the Nordic seas, or on continental shelves, such as the Antarctic shelf. Overflows are the source of most of the abyssal waters, and therefore play an important role in the large-scale ocean circulation, forming a component of the sinking branch of the thermohaline circulation. As they descend the continental slope, overflows mix vigorously with the surrounding oceanic waters, changing their density and transport significantly. These mixing processes occur on spatial scales well below the resolution of ocean climate models, with the result that deep waters and deep western boundary currents are simulated poorly. The Gravity Current Entrainment Climate Process Team was established by the U.S. Climate Variability and Prediction (CLIVAR) Program to accelerate the development and implementation of improved representations of overflows within large-scale climate models, bringing together climate model dev...

The North Atlantic Subpolar Gyre in Four High-Resolution Models

The authors present the first quantitative comparison between new velocity datasets and high-resolution models in the North Atlantic subpolar gyre [1U10° Parallel Ocean Program model (POPNA10), Miami Isopycnic Coordinate Ocean Model (MICOM), 1U6° Atlantic model (ATL6), and Family of Linked Atlantic Ocean Model Experiments (FLAME)]. At the surface, the model velocities agree generally well with World Ocean Circulation Experiment (WOCE) drifter data. Two noticeable exceptions are the weakness of the East Greenland coastal current in models and the presence in the surface layers of a strong southwestward East Reykjanes Ridge Current. At depths, the most prominent feature of the circulation is the boundary current following the continental slope. In this narrow flow, it is found that gridded float datasets cannot be used for a quantitative comparison with models. The models have very different patterns of deep convection, and it is suggested that this could be related to the differences in their barotropic transport at Cape Farewell. Models show a large drift in watermass properties with a salinization of the Labrador Sea Water. The authors believe that the main cause is related to horizontal transports of salt because models with different forcing and vertical mixing share the same salinization problem. A remarkable feature of the model solutions is the large westward transport over Reykjanes Ridge [10 Sv (Sv 10 6 m 3 s 1 ) or more].

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.

Atlantic Basin Seasonal Hurricane Simulations

Abstract An ensemble of seasonal Atlantic hurricane simulations is conducted using The Florida State University/Center for Ocean–Atmospheric Prediction Studies (FSU–COAPS) global spectral model (Cocke and LaRow) at a resolution of T126L27 (a Gaussian grid spacing of 0.94°). Four integrations comprising the ensembles were generated using the European Centre for Medium-Range Weather Forecasts (ECMWF) time-lagged initial atmospheric conditions centered on 1 June for the 20 yr from 1986 to 2005. The sea surface temperatures (SSTs) were updated weekly using the Reynolds et al. observed data. An objective-tracking algorithm obtained from the ECMWF and modified for this model’s resolution was used to detect and track the storms. It was found that the model’s composite storm structure and track lengths are realistic. In addition, the 20-yr interannual variability was well simulated by the ensembles with a 0.78 ensemble mean rank correlation. The ensembles tend to overestimate (underestimate) the numbers of storms...

Ocean modeling and parameterization

Preface. 1. Oceanic General Circulation Models J.C. McWilliams. 2. Forcing the Ocean B. Barnier. 3. Modeling and Parameterizing the Ocean Planetary Boundary Layer W.G. Large. 4. Parameterization of the Fair Weather Ekman Layer J.F. Price. 5. The Representation of Bottom Boundary Layer Processes in Numerical Ocean Circulation Models A. Beckman. 6. Marginal Sea Overflows for Climate Simulations J.F. Price, J. Yang. 7. Turbulent Mixing in the Ocean. Intensity, Causes, and Consequences J.M. Toole. 8. Parameterization of Processes in Deep Convection Regimes U. Send, R.H. Kase. 9. Double-Diffusive Convection. Its Role in Ocean Mixing and Parameterization Schemes for Large Scale Modeling R.W. Schmitt. 10. Interleaving at the Equator. Its Parameterization and Effect on the Large Scale Dynamics K.J. Richards. 11. Eddy Parameterization in Large Scale Flow P.D. Killworth. 12. Three-Dimensional Residual-Mean Theory T.J. McDougall. 13. Statistical Mechanics of Potential Vorticity for Parameterizing Mesoscale Eddies J. Sommeria. 14. Topographic Stress: Importance and Parameterization A. Alvarez, J. Tintore. 15. Large-Eddy Simulations of Three-Dimensional Turbulent Flows: Geophysical Applications O. Metais. 16. Parameter Estimation in Dynamical Models G. Evensen, et al. 17. On the Large-Scale Modeling of Sea Ice and Sea Ice-Ocean Interactions T. Fichefet, et al. 18. Ocean Modeling in Isopycnic Coordinates R. Bleck. Subject Index.

On the Connection between the Mediterranean Outflow and the Azores Current

As the salty and dense Mediteranean overflow exits the Strait of Gibraltar and descends rapidly in the Gulf of Cadiz, it entrains the fresher overlying subtropical Atlantic Water. A minimal model is put forth in this study to show that the entrainment process associated with the Mediterranean outflow in the Gulf of Cadiz can impact the upper-ocean circulation in the subtropical North Atlantic Ocean and can be a fundamental factor in the establishment of the Azores Current. Two key simplifications are applied in the interest of producing an economical model that captures the dominant effects. The first is to recognize that in a vertically asymmetric twolayer system, a relatively shallow upper layer can be dynamically approximated as a single-layer reduced-gravity controlled barotropic system, and the second is to apply quasigeostrophic dynamics such that the volume flux divergence effect associated with the entrainment is represented as a source of potential vorticity. Two sets of computations are presented within the 1‰-layer framework. A primitive-equation-based computation, which includes the divergent flow effects, is first compared with the equivalent quasigeostrophic formulation. The upper-ocean cyclonic eddy generated by the loss of mass over a localized area elongates westward under the influence of the b effect until the flow encounters the western boundary. In the steady state, the circulation pattern consists of bidirectional zonal flows with a limited meridional extent: eastward to the south of the sink and westward to the north of the sink. The localized sink drives a horizontal circulation in the interior ocean whose strength is approximately an order of magnitude greater than the sink’s strength. It is demonstrated that the induced circulation in the far field from a localized sink is insensitive to the neglect of the divergent flow component. A set of parameter sensitivity experiments is then undertaken with the quasigeostrophic model for an idealized midlatitude circulation, driven both by wind forcing and ‘‘thermohaline’’ flow through the open southern and northern boundaries. When a sink near the eastern boundary is superimposed on the idealized midlatitude circulation, it is shown to alter significantly the upper-ocean flow and induce an eastward zonal current, which resembles the Azores Current in location and transport. This mechanism also generates a westward current to the north of the sink location, which could be associated with the Azores Countercurrent. An extensive series of sensitivity experiments is conducted to determine the response of this current system to changes in the boundary layer processes, sink strength, sink distribution, model resolution, and wind forcing. The magnitude of the current transports is found to be sensitive to the sink intensity and to its distance from the coastline.