Tropical Channel NEMO-OASIS-WRF Coupled Simulations at Very High Resolution

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Extensive validation and diagnostics for the tangent and adjoint models - Application to NEMO

International audienceThe Tangent and Adjoint Models (TAM) are very effective tools for ocean data assimilation, sensitivity and stability analysis, and uncertainty propagations. However the development of such tools can be tedious and error prone and validation is therefore an important aspect. Fortunately there exist many ways to evaluate the validity of the TAM, from very basic to sophisticated. Some of these validation tools can also be used to diagnose the degree of non-linearity of the direct model. We will present a review of existing diagnostics for TAM and propose several new ones, and apply them to the recently released TAM for the European ocean community model NEMO

NEMO Tangent & Adjoint Models (NemoTam) Reference Manual & User's Guide

The development of the tangent linear and adjoint models (TAM in the following) of the dynamical core of the NEMO ocean engine (NEMOTAM) is a key objective of the VODA project. TAM are widely used for variational assimilation applications, but they are also powerful tools for the analysis of physical processes, since they can be used for sensitivity analysis, parameter identification and for the computation of characteristic vectors (singular vectors, Liapunov vectors, etc.). In the framework of VODA, a work package has been set-up in order to develop a comprehensive NEMOTAM package, and to define an effective long-term development strategy for ensuring synchronisation of NEMOTAM with future NEMO releases. This is a heavy task, but it is worth the effort since NEMOTAM will benefit all NEMO users for the wide range of applications described above. Ideally, this strategy should be defined to allow NEMOTAM to adapt to future NEMO developments as quickly and as efficiently as possible, so that new releases of NEMOTAM can be made soon after new releases of NEMO. This will require careful coordination between the main development teams of NEMO, NEMOTAM and possibly NEMOVAR (INRIA, NEMO Team, CERFACS, ECMWF)

Les modèles climatiques gagnent en précision

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Role of the Tide on the Structure of the Amazon Plume: A Numerical Modeling Approach

International audienceThe dynamical balance on the Amazon shelf and its implication on the properties of the Amazon River plume is not fully understood and poorly represented in global- and basin-scale ocean models. In this study, the sensitivity of the Amazon shelf dynamics to tidal forcing is explored with a set of high-resolution numerical simulations (1/36°) with and without the tide. A comparison of the simulations with sea surface salinity in situ measurements at 5°N (a location where the plume seasonally detaches from the coast and retroflects toward the east) revealed that the explicit resolution of the tide significantly improves the representation of the offshore spread of the river plume. This study further highlights the finding that tidal currents affect the properties of the whole Amazon plume. This sensitivity is explained by a near total collapse of the northwestward alongshore mean flow located near the river mouth, once the tidal forcing is included. This weakening of the ambient flow reduces (i) the dilution ratio between the ambient salty shelf waters and the riverine freshwaters and (ii) the constraint on the cross-shore extension of the low-salinity bulge. With tides, the plume is fresher near the river mouth (by up to 5 units), more extended in the cross-shore direction, and more easily exported offshore by the North Brazil Current at the shelf break

An “objective” definition of potential vorticity. Generalized evolution equation and application to the study of coastal upwelling instability

International audienceIn this paper, we propose a form for potential vorticity (PV), rescaled using the Lorenz's rearranged density profile, the novelty being that we here take into account its time evolution. We argue this rescaled PV is more representative of the dynamics, in particular to evaluate the respective impact of mixing and friction on the generation of geostrophic circulation. The impact of mixing at global scale, which only modifies the global stratification at rest, is taken into account in the evolution equation of this "objective" definition of PV, in the sense that it scales the PV changes with respect to its effect on the circulation. Numerically, we show that all terms can be calculated coherently using a single computation cell. We illustrate our purpose by studying the instability of coastal upwelling currents, using a numerical model at high resolution. The configuration is a periodic flat channel on the f-plane with vertical walls at the southern and northern boundaries. A constant wind is applied over a fluid at rest with an initial linear stratification. An upwelling current forms at the northern coast. After a few days, instabilities develop and vortices eventually emerge with surface intensified cyclones and subsurface anticyclones. We show that these instabilities and eddies are associated with (rescaled) PV anomalies

An “objective” definition of potential vorticity. Generalized evolution equation and application to the study of coastal upwelling instability

In this paper, we propose a form for potential vorticity (PV), rescaled using the Lorenz’s rearranged density profile, the novelty being that we here take into account its time evolution. We argue this rescaled PV is more representative of the dynamics, in particular to evaluate the respective impact of mixing and friction on the generation of geostrophic circulation. The impact of mixing at global scale, which only modifies the global stratification at rest, is taken into account in the evolution equation of this “objective” definition of PV, in the sense that it scales the PV changes with respect to its effect on the circulation. Numerically, we show that all terms can be calculated coherently using a single computation cell. We illustrate our purpose by studying the instability of coastal upwelling currents, using a numerical model at high resolution. The configuration is a periodic flat channel on the f-plane with vertical walls at the southern and northern boundaries. A constant wind is applied over a fluid at rest with an initial linear stratification. An upwelling current forms at the northern coast. After a few days, instabilities develop and vortices eventually emerge with surface intensified cyclones and subsurface anticyclones. We show that these instabilities and eddies are associated with (rescaled) PV anomalies, triggered by mixing and friction. We describe rescaled PV budgets in a layer bounded between the surface and an isopycnal level. Eulerian and Lagrangian diagnostics allow to analyze irreversible PV production terms, distinguishing the influence of advection, friction (associated with wind stress) and mixing. We find that friction plays the main role, generating negative PV anomalies, while mixing acts to dampen this negative PV production. The association of this negative PV anomaly with the outcropping front leads to the baroclinic destabilization of the upwelling front, creating subsurface anticyclonic vortices and surface intensified cyclonic vortices. Varying the strength of the wind forcing shows that mixing is the most sensitive process, with a net effect that is strongly reduced or even reversed with moderate to weak winds. When the dynamics is fully turbulent, with filaments and vortices of small sizes, the PV production by mixing and friction is enhanced but the Lagrangian diagnostics are more difficult to analyze, since fluctuations at grid scale become significant and numerical effects – associated with imperfections of the numerical schemes – spoil the PV budget calculation

3D wave-resolving simulation of sandbar migration

The problem of sandbar migration on the storm timescale is revisited with a 3D waveresolving hydro-sedimentary model. The model accurately simulates the successive offshore and onshore bar migration observed in a large-scale flume experiment (LIP11D) in response to wave forcing representing storm and post-storm (recovery) conditions. The diagnosis of sand transport and the analysis of a composite asymmetric wave cycle reveal the migration mechanisms in each phase. In all cases, sediment resuspension is dominated by breakinginduced turbulence, while net sediment transport and bed profile evolution are primarily the result of undertow distribution across the sandbar, rather than a trade-off between onshore and offshore fluxes. In the erosion phase, a strong undertow carries the mobilized sediment seaward of the bar crest. In the accretion phase, the sandbar becomes the breaking point to more moderate waves and the undertow is limited to the lee-side of the bar, causing an counterflow migration of the bar crest. The contribution of wave-related onshore fluxes is significant in this case-although secondary in magnitude-and coincide with higher mobilization and currents during the wave crest period. We conclude that computationally efficient 3D wave-resolving models (including morphological acceleration) can be used to improve our understanding of nearshore morphodynamic problems in realistic applications

On effective resolution in ocean models

International audienceThe increase of model resolution naturally leads to the representation of a wider energy spectrum. As a result, in recent years, the understanding of oceanic submesoscale dynamics has significantly improved. However, dissipation in submesoscale models remains dominated by numerical constraints rather than physical ones. Effective resolution is limited by the numerical dissipation range, which is a function of the model numerical filters (assuming that dispersive numerical modes are efficiently removed). We present a Baroclinic Jet test case set in a zonally reentrant channel that provides a controllable test of a model capacity at resolving submesoscale dynamics. We compare simulations from two models, ROMS and NEMO, at different mesh sizes (from 20 to 2 km). Through a spectral decomposition of kinetic energy and its budget terms, we identify the characteristics of numerical dissipation and effective resolution. It shows that numerical dissipation appears in different parts of a model, especially in spatial advection-diffusion schemes for momentum equations (KE dissipation) and tracer equations (APE dissipation) and in the time stepping algorithms. Effective resolution, defined by scale-selective dissipation, is inadequate to qualify traditional ocean models with low-order spatial and temporal filters, even at high grid resolution. High-order methods are better suited to the concept and probably unavoidable. Fourth-order filters are suited only for grid resolutions less than a few kilometers and momentum advection schemes of even higher-order may be justified. The upgrade of time stepping algorithms (from filtered Leapfrog), a cumbersome task in a model, appears critical from our results, not just as a matter of model solution quality but also of computational efficiency (extended stability range of predictor-corrector schemes). Effective resolution is also shaken by the need for non scale-selective barotropic mode filters and requires carefully addressing the issue of mode splitting errors. Possibly the most surprising result is that submesoscale energy production is largely affected by spurious diapycnal mixing (APE dissipation). This result justifies renewed efforts in reducing tracer mixing errors and poses again the question of how much vertical diffusion is at work in the real ocean