Assessment of the z ∼ time‐filtered arbitrary Lagrangian‐Eulerian coordinate in a global eddy‐permitting ocean model

A recognized deficiency of ocean models with a constant-depth vertical coordinate is for truncation errors in the vertical advection scheme to result in spurious numerical mixing of tracers, which can be substantially stronger than that prescribed by the model's mixing scheme. The z∼ vertical coordinate allows vertical levels to displace in a Lagrangian fashion on time scales shorter than a few days, but reverts to fixed levels on longer timescales, and is intended to reduce numerical mixing from transient vertical motions such as internal waves and tides. An assessment of z∼ in a 1/4° global implementation of the Nucleus for European Modeling of the Ocean model is presented. It is shown that, in the presence of near-inertial gravity waves in the North Atlantic, z∼ significantly reduces Eulerian vertical velocities with respect to those in a simulation with the default z* vertical coordinate; that the vertical coordinate approaches an isopycnal, or adiabatic, surface on short timescales; and that both tendencies are enhanced when the z∼ timescale parameters are lengthened with respect to the default settings. Analysis based on density transformation rates shows that numerical mixing is consistently reduced by of order 10% as the z∼ timescales are lengthened. The realism of the model simulation with different timescale parameters is assessed in the global domain, and it is shown that drifts in temperature and salinity, and the spindown in z* of the Antarctic Circumpolar Current, are reduced with z∼, without incurring significant drawbacks in other metrics such as the strength of the overturning circulation or sea ice cover

The INALT family – a set of high-resolution nests for the Agulhas Current system within global NEMO ocean/sea-ice configurations

The Agulhas Current, the western boundary current of the South Indian Ocean, has been shown to play an important role in the connectivity between the Indian and Atlantic oceans. The greater Agulhas Current system is highly dominated by mesoscale dynamics. To investigate their influence on the regional and global circulations, a family of high-resolution ocean general circulation model configurations based on the NEMO code has been developed. Horizontal resolution refinement is achieved by embedding “nests” covering the South Atlantic and the western Indian oceans at 1/10∘ (INALT10) and 1/20∘ (INALT20) within global hosts with coarser resolutions. Nests and hosts are connected through two-way interaction, allowing the nests not only to receive boundary conditions from their respective host but also to feed back the impact of regional dynamics onto the global ocean. A double-nested configuration at 1/60∘ resolution (INALT60) has been developed to gain insights into submesoscale processes within the Agulhas Current system. Large-scale measures such as the Drake Passage transport and the strength of the Atlantic meridional overturning circulation are rather robust among the different configurations, indicating the important role of the hosts in providing a consistent embedment of the regionally refined grids into the global circulation. The dynamics of the Agulhas Current system strongly depend on the representation of mesoscale processes. Both the southward-flowing Agulhas Current and the northward-flowing Agulhas Undercurrent increase in strength with increasing resolution towards more realistic values, which suggests the importance of improving mesoscale dynamics as well as bathymetric slopes along this narrow western boundary current regime. The exploration of numerical choices such as lateral boundary conditions and details of the implementation of surface wind stress forcing demonstrates the range of solutions within any given configuration

Paramétrisation de la restratification après convection profonde en mer du Labrador

GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

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

Impact of spatial resolution on simulated surface water mass transformations in the Atlantic

International audienceWe analyze the water mass transformation in coarse (1°) and high (1/6°) resolution ocean simulations with the identical configuration of the CLIPPER model and interannual ERA15 forcing function. Climatological characteristics of surface water mass transformation in the two experiments are quite different. The high resolution experiment exhibits a stronger surface transformation in equatorial and tropical regions, in the Gulf Stream area and in the location of the formation of Subtropical Mode Water (STMW), associated with high levels of eddy kinetic energy. The coarse resolution experiment shows a better representation of the transformation rates corresponding to the densest subpolar mode waters and Labrador Sea Water (LSW). This is explained by the differences in lateral mixing procedures between high and coarse resolution experiments. The high resolution 1/6° run is eddy-resolving only in the tropics and mid-latitudes. In these areas eddies are found to enhance the process of water mass transformation compared to the isopycnal diffusion used to parameterized the eddies in the 1° model. Despite its 1/6° resolution, the high resolution model does not adequately represent eddies in the subpolar gyre and Labrador Sea. In these areas the high resolution model fails to correctly simulate water mass transformation because the lateral mixing (provided through the bi-harmonic sub-gridscale parameterization) of newly ventilated waters with surrounding waters is not efficient enough. In contrast in the coarse 1° resolution model, the strong lateral mixing and the unrealistically broad boundary currents imposed by the high diffusivity required for numerical stability mixes newly formed LSW waters with the warmer and saltier waters of the rim current. Finally, it results in a more effective representation of the surface water mass transformation in high latitudes in the 1° model. A possible impact of the increased lateral diffusion in high resolution experiment on the representation of re-stratification in the Labrador Sea was studied in sensitivity experiments with different lateral diffusion coefficients compared to the regional eddy-resolving 1/15° simulation in the subpolar North Atlantic. If the eddies are not resolved in subpolar latitudes (as in the case of 1/6° model), the GM90 parameterization with the coefficient close to 800 m2 s−1 provides the closest agreement with the solution of eddy-resolving 1/15° model

Mesoscale Eddies in the Labrador Sea and Their Contribution to Convection and Restratification

International audienceThe cycle of open ocean deep convection in the Labrador Sea is studied in a realistic, high-resolution (4 km) regional model, embedded in a coarser (1⁄3°) North Atlantic setup. This configuration allows the simultaneous generation and evolution of three different eddy types that are distinguished by their source region, generation mechanism, and dynamics. Very energetic Irminger Rings (IRs) are generated by barotropic instability of the West Greenland and Irminger Currents (WGC/IC) off Cape Desolation and are characterized by a warm, salty subsurface core. They densely populate the basin north of 58°N, where their eddy kinetic energy (EKE) matches the signal observed by satellite altimetry. Significant levels of EKE are also found offshore of the West Greenland and Labrador coasts, where boundary current eddies (BCEs) are spawned by weakly energetic instabilities all along the boundary current system (BCS). Baroclinic instability of the steep isopycnal slopes that result from a deep convective overturning event produces convective eddies (CEs) of 20-30 km in diameter, as observed and produced in more idealized models, with a distinct seasonal cycle of EKE peaking in April. Sensitivity experiments show that each of these eddy types plays a distinct role in the heat budget of the central Labrador Sea, hence in the convection cycle. As observed in nature, deep convective mixing is limited to areas where adequate preconditioning can occur, that is, to a small region in the southwestern quadrant of the central basin. To the east, west, and south, BCEs flux heat from the BCS at a rate sufficient to counteract air-sea buoyancy loss. To the north, this eddy flux alone is not enough, but when combined with the effects of Irminger Rings, preconditioning is effectively inhibited here too. Following a deep convective mixing event, the homogeneous convection patch reaches as deep as 2000 m and a horizontal scale on the order of 200 km, as has been observed. Both CEs and BCEs are found to play critical roles in the lateral mixing phase, when the patch restratifies and transforms into Labrador Sea Water (LSW). BCEs extract the necessary heat from the BCS and transport it to the deep convection site, where it fluxed into convective patches by CEs during the initial phase. Later in the phase, BCE heat flux maintains and strengthens the restratification throughout the column, while solar heating establishes a near-surface seasonal stratification. In contrast, IRs appear to rarely enter the deep convection region. However, by virtue of their control on the surface area preconditioned for deep convection and the interannual variability of the associated barotropic instability, they could have an important role in the variability of LSW

Multi-grid algorithm for passive tracer transport in the NEMO ocean circulation model: a case study with the NEMO OGCM (version 3.6)

International audienceOcean biogeochemical models are key tools for both scientific and operational applications. Nevertheless the cost of these models is often expensive because of the large number of biogeochemical tracers. This has motivated the development of multi-grid approaches where ocean dynamics and tracer transport are computed on grids of different spatial resolution. However, existing multi-grid approaches to tracer transport in ocean modelling do not allow the computation of ocean dynamics and tracer transport simultaneously. This paper describes a new multi-grid approach developed for accelerating the computation of passive tracer transport in the Nucleus for European Modelling of the Ocean (NEMO) ocean circulation model. In practice, passive tracer transport is computed at runtime on a grid with coarser spatial resolution than the hydrodynamics, which reduces the CPU cost of computing the evolution of tracers. We describe the multi-grid algorithm, its practical implementation in the NEMO ocean model, and discuss its performance on the basis of a series of sensitivity experiments with global ocean model configurations. Our experiments confirm that the spatial resolution of hydrodynamical fields can be coarsened by a factor of 3 in both horizontal directions without significantly affecting the resolved passive tracer fields. Overall, the proposed algorithm yields a reduction by a factor of 7 of the overhead associated with running a full biogeochemical model like PISCES (with 24 passive tracers). Propositions for further reducing this cost without affecting the resolved solution are discussed

Simulating or prescribing the influence of tides on the Amundsen Sea ice shelves

International audienceThe representation of tides in regional ocean simulations of the Amundsen Sea enhances ice-shelf melting, with weakest effects for Pine Island and Thwaites ( +30%). Tides increase vertical mixing throughout the water column along the continental shelf break. Diurnal tides induce topographically trapped vorticity waves along the continental shelf break, likely underpinning the tidal rectification (residual circulation) simulated in the Dotson-Getz Trough. However, the primary effect by which tides affect ice-shelf melting is the increase of ice/ocean exchanges, rather than the modification of water masses on the continental shelf. Tide-induced velocities strengthen turbulent heat fluxes at the ice/ocean interface, thereby increasing melt rates. Approximately a third of this effect is counterbalanced by the resulting release of cold melt water that reduces melt downstream along the meltwater flow. The relatively weak tideinduced melting underneath Pine Island and Thwaites could be partly related to their particularly thick water column, which limits the presence of quarter wavelength tidal resonance. No sensitivity to the position of Pine Island and Thwaites with respect to the M2 critical latitude is found. We refine and evaluate existing methodologies to prescribe the effect of tides on ice-shelf melt rates in ocean models that do not explicitely include tidal forcing. The best results are obtained by prescribing spatially-dependent tidal top-boundary-layer velocities in the melt equations. These velocities can be approximated as a linear function of existing barotropic tidal solutions. A correction factor needs to be applied to account for the additional melt-induced circulation associated with tides and to reproduce the relative importance of dynamical and thermodynamical processes

Internal tides off the Amazon shelf Part I : importance for the structuring of ocean temperature during two contrasted seasons

Tides and internal tides (IT) in the ocean can significantly affect local to regional ocean temperature, including sea surface temperature (SST), via physical processes such as diffusion (vertical mixing) and advection (vertical and horizontal) of water masses. Offshore of the Amazon River, strong IT have been detected by satellite observations and well modelled ; however, their impact on temperature, SST and the identification of the associated processes have not been studied so far. In this work, we use high resolution (1/36°) numerical simulations with and without the tides from an ocean circulation model (NEMO). This model explicitly resolves the internal tides (IT) and is therefore suitable to assess how they can affect ocean temperature in the studied area. We distinguish the analysis for two contrasted seasons, from April to June (AMJ) and from August to October (ASO), since the seasonal stratification off the Amazon River modulates the IT’s response and their influence in temperature. The generation and the propagation of the IT in the model are in good agreement with observations. The SST reproduced by the simulation including tides is in better agreement with satellite SST data compared to the simulation without tides. During ASO season, stronger meso-scale currents, deeper and weaker pycnocline are observed in contrast to the AMJ season. The observed coastal upwelling during ASO season is better reproduced by the model including tides, whereas the no-tide simulation is too warm by +0.3 °C for the SST. In the subsurface above the thermocline, the tide simulation is cooler by −1.2 °C, and warmer below the thermocline by +1.2 °C compared to the simulation without the tides. The IT induce vertical mixing on their generation site along the shelf break and on their propagation pathways towards the open ocean. This process mainly explains the cooler temperature at the ocean surface and is combined with vertical and horizontal advection to explain the cooling in the subsurface water above the thermocline and a warming in the deeper layers below the thermocline. The surface cooling induced in turn an increase of the net heat flux from the atmosphere to the ocean surface, which could induce significant changes in the local and even for the regional tropical Atlantic atmospheric circulation and precipitation. We therefore demonstrate that IT, via vertical mixing and advection along their propagation pathways, and tides over the continental shelf, can play a role on the temperature structure off the Amazon River mouth, particularly in the coastal cooling enhanced by the IT. Keywords: internal tides, Amazon continental shelf and slope, temperature, modeling, satellite data, mixing, heat flux