Résumé The ocean engine of NEMO (Nucleus for European Modelling of the Ocean) is a primitive equation model adapted to regional and global ocean circulation problems. It is intended to be a flexible tool for studying the ocean and its interactions with the others components of the earth climate system over a wide range of space and time scales. Prognostic variables are the three-dimensional velocity field, a linear or non-linear sea surface height, the temperature and the salinity. In the horizontal direction, the model uses a curvilinear orthogonal grid and in the vertical direction, a full or partial step z-coordinate, or s-coordinate, or a mixture of the two. The distribution of variables is a three-dimensional Arakawa C-type grid. Various physical choices are available to describe ocean physics, including TKE, GLS and KPP vertical physics. Within NEMO, the ocean is interfaced with a sea-ice model (LIM v2 and v3), passive tracer and biogeochemical models (TOP) and, via the OASIS coupler, with several atmospheric general circulation models. It also support two-way grid embedding via the AGRIF software. Le moteur océanique de NEMO (Nucleus for European Modelling of the Ocean) est un modèle aux équations primitives de la circulation océanique régionale et globale. Il se veut un outil flexible pour étudier sur un vaste spectre spatiotemporel l’océan et ses interactions avec les autres composantes du système climatique terrestre. Les variables pronostiques sont le champ tridimensionnel de vitesse, une hauteur de la mer linéaire ou non, la temperature et la salinité. La distribution des variables se fait sur une grille C d’Arakawa tridimensionnelle utilisant une coordonnée verticale z à niveaux entiers ou partiels, ou une coordonnée s, ou encore une combinaison des deux. Différents choix sont proposés pour décrire la physique océanique, incluant notamment des physiques verticales TKE, GLS et KPP. A travers l’infrastructure NEMO, l’océan est interfacé avec des modèles de glace de mer, de biogéochimie et de traceurs passifs, et, via le coupleur OASIS, à plusieurs modèles de circulation générale atmosphérique. Il supporte également l’emboı̂tement interactif de maillages via le logiciel AGRIF.

NEMO ocean engine

The past decade has seen a substantial amount of research on air-sea gas exchange and its environmental controls. These studies have significantly advanced the understanding of processes that control gas transfer, led to higher quality field measurements, and improved estimates of the flux of climate-relevant gases between the ocean and atmosphere. This review discusses the fundamental principles of air-sea gas transfer and recent developments in gas transfer theory, parameterizations, and measurement techniques in the context of the exchange of carbon dioxide. However, much of this discussion is applicable to any sparingly soluble, non-reactive gas. We show how the use of global variables of environmental forcing that have recently become available and gas exchange relationships that incorporate the main forcing factors will lead to improved estimates of global and regional air-sea gas fluxes based on better fundamental physical, chemical, and biological foundations.

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Advances in Quantifying Air-Sea Gas Exchange and Environmental Forcing*

Recent small-scale turbulence observations allow the mixing regimes in lakes, reservoirs, and other enclosed basins to be categorized into the turbulent surface and bottom boundary layers as well as the comparably quiet interior. The surface layer consists of an energetic wave-affected thin zone at the very top and a law-of-the-wall layer right below, where the classical logarithmic-layer characteristic applies on average. Short-term current and dissipation profiles, however, deviate strongly from any steady state. In contrast, the quasi-steady bottom boundary layer behaves almost perfectly as a logarithmic layer, although periodic seiching modifies the structure in the details. The interior stratified turbulence is extremely weak, even though much of the mechanical energy is contained in baroclinic basin-scale seiching and Kelvin waves or inertial currents (large lakes). The transformation of large-scale motions to turbulence occurs mainly in the bottom boundary and not in the interior, where the local shear remains weak and the Richardson numbers are generally large.

Small-scale hydrodynamics in lakes

Breaking serves to limit the height of surface waves, mix the surface waters, generate ocean currents, and enhance air-sea fluxes of heat, mass, and momentum through the generation of turbulence and the entrainment of air. Breaking may result from intrinsic instabilities of deep-water waves or through wavewave, wave-current, and wind-wave interactions. Observations in the field are made difficult by the fact that breaking is a strongly nonlinear intermittent process occurring over a wide range of scales. Controlled laboratory studies of breaking have proven useful in measuring the scaling relationships between the surface wave field and the kinematics and dynamics of breaking. Our inability to predict the occurrence and dynamics of breaking is a major impediment to the development of better wind-wave and mixed-layer models. Modern acoustic and electromagnetic oceanographic instrumentation should lead to significantly improved measurements of breaking in the near future.

The Role of Surface-Wave Breaking in Air-Sea Interaction

Second-order turbulence closure models for geophysical boundary layers. A review of recent work

Until recently, measurements below the ocean surface have tended to confirm “law of the wall” behavior, in which the velocity profile is logarithmic, and energy dissipation decays inversely with depth. Recent measurements, however, show a sublayer, within meters of the surface, in which turbulence is enhanced by the action of surface waves. In this layer, dissipation appears to decay with inverse depth raised to a power estimated between 3 and 4.6. The present study shows that a conventional model, employing a “level 2½” turbulence closure scheme predicts near-surface dissipation decaying as inverse depth to the power 3.4. The model shows agreement in detail with measured profiles of dissipation. This is despite the fact that empirical constants in the model are determined for situations very different from this near-surface application. The action of breaking waves is modeled by a turbulent kinetic energy input at the surface. In the wave-enhanced layer, the downward flux of turbulent kinetic en...

Modeling Wave-Enhanced Turbulence in the Ocean Surface Layer

Ocean boundary currents are poorly represented in existing coupled climate models, partly because of their insufficient resolution to resolve narrow jets. Therefore, there is limited confidence in the simulated response of boundary currents to climate change by climate models. To address this issue, the eddy-resolving Ocean Forecasting Australia Model (OFAM) was used, forced with bias-corrected output in the 2060s under the Special Report on Emissions Scenarios (SRES) A1B from the CSIRO Mark version 3.5 (Mk3.5) climate model, to provide downscaled regional ocean projections. CSIRO Mk3.5 captures a number of robust changes that are common to most climate models that are consistent with observed changes, including the weakening of the equatorial Pacific zonal wind stress and the strengthening of the wind stress curl in the Southern Pacific, important for driving the boundary currents around Australia.The 1990s climate is downscaled using air–sea fluxes from the 40-yr European Centre for Medium-Range...

Marine Downscaling of a Future Climate Scenario for Australian Boundary Currents

Observations are presented of the internal tide over the continental shelf and slope from a cross section on the Australian North West Shelf. Data collected from moored instruments and repeated profile measurements during the summer months of 1995 show an energetic, large amplitude, shoreward propagating semidiurnal internal tide. Multiple generation sites are suggested, coinciding with near-critical bottom slopes. In water less than approximately 200 m deep, the vertical structure of the internal tide is predominantly a first vertical mode, whereas in deeper water over the slope the vertical structure is more complicated with currents and vertical displacements intensified in the lower part of the water column. The internal tide is largely confined to a region approximately 100 km wide, from water depths between 70 and 1000 m. Strong generation and dissipation of the internal tide energy is observed over this region and there is evidence that the dissipated energy impacts on the vertical mixing ...

Internal Tide Observations from the Australian North West Shelf in Summer 1995

We consider the vertical current structure forced by a periodic surface wind acting on a homogeneous, constant-eddy-viscosity ocean. The vertical dynamics are described by a simple extension of Ekman's (1905) steady-wind solution. At forcing frequencies other than the inertial frequency, the energy from the surface wind stress propagates down into the water column as a pair of counter-rotating waves. At the inertial frequency, which is the frequency of free waves of the system, the forced current profile is quadratic in the depth, and, for a given wind stress, the magnitude of the current is proportional to the water depth. Viewed in a reference frame that is fixed in space rather than fixed to the earth's surface, the inertial waves are not wavelike at all, but are steady in time. Interpretation of the dynamics at frequencies other than inertial is also more straightforward in a fixed reference frame.

Peter D. Craig

Papers

Modeling Wave-Enhanced Turbulence in the Ocean Surface Layer

Marine Downscaling of a Future Climate Scenario for Australian Boundary Currents

Internal Tide Observations from the Australian North West Shelf in Summer 1995

Constant-eddy-viscosity models of vertical structure forced by periodic winds

A model of diurnally forced vertical current structure near 30° latitude