Displays

Vertical mixing is often regarded as the Achilles’ heel of ocean models. In particular, few models include a comprehensive and energy-constrained parameterization of mixing by internal ocean tides. Here, we present an energy-conserving mixing scheme which accounts for the local breaking of high-mode internal tides and the distant dissipation of low-mode internal tides. The scheme relies on four static two-dimensional maps of internal tide dissipation, constructed using mode-by-mode Lagrangian tracking of energy beams from sources to sinks. Each map is associated with a distinct dissipative process and a corresponding vertical structure. Applied to an observational climatology of stratification, the scheme produces a global three-dimensional map of dissipation which compares well with available microstructure observations and with upper-ocean finestructure mixing estimates. Implemented in the NEMO global ocean model, the scheme improves the representation of deep water-mass transformation and obviates the need for a constant background diffusivity.

How to cite: de Lavergne, C., Vic, C., Madec, G., Roquet, F., Waterhouse, A., Whalen, C., Cuypers, Y., Bouruet-Aubertot, P., and Hibiya, T.: A parameterization of local and remote tidal mixing, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3390, https://doi.org/10.5194/egusphere-egu2020-3390, 2020.

Mesoscale eddies stir along the neutral plane, and the resulting neutral diffusion is a fundamental aspect of subgrid-scale tracer transport in ocean models. Calculating neutral diffusion traditionally involves calculating neutral slopes and three-dimensional tracer gradients. The calculation of the neutral slope traditionally occurs by computing the ratio of the horizontal to vertical locally referenced potential density derivative. However, this approach is problematic in regions of weak vertical stratification, prompting the use of a variety of ad hoc regularization methods that can lead to rather nonphysical dependencies for the resulting neutral tracer gradients.

Here we introduce VENM; a search algorithm that requires no ad hoc regularization and significantly improves the numerical accuracy of calculating neutral slopes, neutral tracer gradients, and associated neutral diffusive fluxes. We compare and contrast VENM against a more traditional method, using an independent objective neutrality condition combined with estimates of spurious diffusion, heat transport, and water mass transformation rates. VENM is more accurate, both physically and numerically, and should form the basis for future efforts involving neutral diffusion calculations from observations and possibly numerical model simulations.

How to cite: Groeskamp, S., Barker, P., McDougall, T., Abernathey, R., and Griffies, S.: VENM: An Algorithm to Accurately Calculate Neutral Slopes and Gradients, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5618, https://doi.org/10.5194/egusphere-egu2020-5618, 2020.

Organic particle populations in the mesopelagic layer span a wide range of sizes and sinking speeds. A long-standing paradigm in ocean biogeochemistry posits that large, fast-sinking detrital particles contribute to most of the vertical export flux of particulate organic carbon (POC), whereas small, slow-sinking or suspended particles comprise most (>90%) of the stock. Over the last decades, most studies have placed emphasis on understanding and predicting the vertical fluxes driven by large particles owing to their influence on ocean carbon storage. Yet, there are compelling reasons to study the dynamics of suspended and slow-sinking small POC (sPOC) in greater detail. First, sPOC likely supports most of the mesopelagic respiration. Second, recent studies have shown that a number of mechanisms can inject large amounts of sPOC into the mesopelagic layer, to the point that the sPOC fraction may seasonally dominate total vertical POC fluxes. Thus, better accounting for sPOC fluxes might allow us overcome historical difficulties in balancing mesopelagic carbon budgets.

In the last decade, hundreds of bio-optical sensors deployed on autonomous profiling robots (bgc-Argo floats) have enabled observation of small particle stocks between the sea surface and 1000 m depth with a profiling frequency of 1-10 days. These observations are showing that mesopelagic sPOC follows distinct seasonal cycles in different oceanic areas, and allow identification of sPOC supply events (e.g., caused by vertical mixing or by disaggregation or fragmentation of larger particles) and of net sPOC removal. Regarding ocean biogeochemistry models, they have traditionally been tuned to estimate sinking POC fluxes but failed to capture POC stocks. Recently, the formulation of POC degradation rates in the model PISCESv2 was changed and a POC reactivity continuum approach was adopted, which greatly improved the representation of small and big POC stocks. These parallel developments now enable the quantitative assessment of mesopelagic POC dynamics.

Here we analyze the annual cycles of mesopelagic sPOC in the subpolar North Atlantic, as seen by biogeochemical Argo floats, and compare them to their PISCESv2-simulated counterparts. We then discuss the processes that drive mesopelagic sPOC seasonality in the observations and in 1D model simulations. Finally, we present a genetic algorithm approach that uses biogeochemical Argo float observations to optimize the PISCESv2 parameters that influence sPOC, focusing on the interplay between particle degradation rate and sinking speed.

How to cite: Galí, M., Falls, M., and Bernardello, R.: Seasonal dynamics of mesopelagic organic particles in the subpolar North Atlantic. Learning from the crosstalk between biogeochemical Argo float measurements and PISCESv2 simulations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16602, https://doi.org/10.5194/egusphere-egu2020-16602, 2020.

High-resolution ocean-atmosphere coupled models are able to simulate realistically air-sea interactions taking place at mesoscale between ocean eddies and fronts, and the lower atmosphere. These coupled processes have the potential to improve oceanic simulations by modulating wind work input and turbulent heat fluxes. However, the computational cost and the complexity of such coupled models appear prohibitive and inadequate in the context of global eddying oceanic simulations.

We propose here an alternative approach based on a one-dimensional vertical atmospheric boundary layer (ABL) model driven by large-scale atmospheric data (forecasts or reanalysis). Its intermediate complexity between a bulk parameterization and a full atmospheric model associated with a limited computational cost makes this approach well suited for applications ranging from process studies to global operational oceanography.

First, the ABL model is validated against a set of classic atmospheric testcases such as a SST front. The comparison with analytical and LES solutions indicates a good agreement with the ABL model results.

Then, two realistic configurations based on NEMO ocean model are presented to assess air-sea interactions: a global 1/4° configuration including sea-ice and a regional 1/36° configuration covering western Europe.

We show that the ocean-ABL coupled model produces negative correlations between surface current and wind stress mesoscale curl anomalies (oceanic eddy damping effect), and positive correlations between surface current and wind speed mesoscale curl anomalies (wind adjustment and ocean re-energization effects) in good agreement with literature. We also show that the simulated wind speed positively correlates with SST mesoscale anomalies, as observed with satellite data and full coupled models.

To summarize, the ocean-ABL coupled model is able to realistically represent mesoscale dynamical and thermal feedbacks while keeping a good consistency with the atmospheric forcing, and with a very limited computational cost (10% of the ocean model). The ABL model will be released with the next NEMO version.

How to cite: Samson, G., Lemarié, F., Brivoal, T., Bourdallé-Badie, R., Giordani, H., Redelsperger, J.-L., and Madec, G.: An atmospheric boundary layer model to improve air-sea interactions in eddying ocean models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18680, https://doi.org/10.5194/egusphere-egu2020-18680, 2020.

Kinetic energy backscatter (KEB) parameterizations of subgrid 2d turbulence have shown their efficiency in ocean models as they restore activity of mesoscale eddies. Modern KEBs utilize only two properties of badly resolved inverse energy cascade: KEB tendency should be larger than turbulent viscosity in spatial scale and amount of returning energy should compensate energy loss due to eddy viscosity. Typical operators for KEB tendency are Laplace operator with negative viscosity coefficient and stochastic process. Application of artificial neural networks (ANN) to approximate subgrid forces may give rise to new KEB models. The main challenge in this direction is to preprocess subgrid forces in such a way to reveal a part corresponding to returning of energy from subgrid scales. In this work, we propose to define subgrid forces as a term nudging a coarse-resolution model toward high-resolution model. This force is energy-generating and may be approximated with ANN. Conventional KEBs and ANN model are compared in Double-Gyre configuration of NEMO ocean model.

How to cite: Perezhogin, P.: A new data-driven subgrid 2d turbulence parameterization and comparison with conventional kinetic energy backscatter parameterizations in NEMO ocean model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19890, https://doi.org/10.5194/egusphere-egu2020-19890, 2020.

The ‘German Strategy for Adaptation to Climate Change’ (DAS) provides the political framework to climate change mitigation and adaptation in Germany. The associated ‘Adaption Action Plan’ envisages the establishment of an operational forecasting and projection service for climate, extreme weather and coastal and inland waterbodies. This service is intended to make use of a regional climate modeling framework, with NEMO v4.0.(1) as the ocean component. The atmospheric component will be provided by the German Weather Service (either the current weather forecasting model ICON or COSMO will be used) and will be coupled to NEMO after testing and calibration of NEMO on the regional scale.

The area of interest includes besides the North Sea and the Baltic Sea the entire North-West-Shelf to take into account cross-shelf transport, the water exchange between North Sea and Baltic Sea and the impact of North Atlantic weather systems on the internal dynamics of the seas. One focus area will be German Bight, well known for its large tidal flats, which make wetting & drying a desirable model feature, which will be tested in future. The used/implemented bathymetry includes the up to date measurements of the sea floor from the EMODNET network.

To achieve a proper description of the dynamics in this region the model has to be calibrated with regard to the timing and amplitude of the water levels in the coastal waters, the water inflow through the Danish straits, the thermal stratification as well as the seasonality and thickness of the sea ice in the Northern Baltic Sea.

These efforts are carried out in the pilot project ‘Projection Service for Waterways and Shipping’ (ProWaS).

How to cite: Abalichin, J., Ehlers, B.-M., and Janssen, F.: Implementation of NEMO v4.0.1 as ocean component for the regional climate modeling framework, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21486, https://doi.org/10.5194/egusphere-egu2020-21486, 2020.

One of the requirements to keep improving the science produced using NEMO is to enhance its computational performance. The interest in improving its capability to efficiently use the computational infrastructure its two-fold: on one side there are experiments that would only be possible if a certain threshold of throughput is achieved, on the other side any development that achieves an increase in efficiency would help saving resources while reducing the environmental impact of our experiments. One of the opportunities that raised interest in the last few years is the optimization of the numerical precision. Historical reasons brought many computational models to over-engineer the numerical precision: solving this miss-adjustment can payback in terms of efficiency and throughput. In this direction, a research was carried out in order to safely reduce the numerical precision in NEMO which led to a mixed-precision version of the model. The implementation has been developed following the approach proposed by Tintó et al. 2019, in which the variables that require double precision are identified automatically and the remaining ones are switched to use single-precision. The implementation will be released in 2020 and this work presents its evaluation in terms of both performance and scientific results.

How to cite: Tintó, O., Paronuzzi Ticco, S. V., Acosta, M. C., Castrillo, M., Serradell, K., and Doblas-Reyes, F. J.: An evaluation of the mixed precision version of NEMO 4.0.1, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16204, https://doi.org/10.5194/egusphere-egu2020-16204, 2020.

Wetting and drying processes in shallow water systems by surges, tides and seiches have important societal, physical and biological impacts. Operational regional models are now of sufficient resolution, O(1 km), that the processes of wetting and drying need to be included. Here we describe a flux limiter based approach that allows a numerical ocean model with a flux formulation of tracer advection to wet and dry. Following Warner et al. (2013), the flux limiter approach limits the outflow from a cell whose depth is below a critical value defined by the user. The limiter can be a step function or a smooth function of the water depth flux limiter, the latter increases model stability and avoids rapid alternation between dry and wet states on long slopes as the critical depth is approached. Furthermore, the user may proportionally limit the baroclinic fluxes as a cell transitions from wet to dry over the course of the large baroclinic time step. The simplicity of the flux limiter approach lends itself to its application within existing numerical models without significant intrusion into the code base. Here we explore the scheme's effectiveness, sensitivities and limitations within the 3D NEMO ocean model by assessing it using test cases of increasing complexity. It is shown to perform well in classic channel test cases and 2D parabolic test cases with analytic solutions. Its performance against analytical 1D dam break experiments is explored and used to interpret its performance against laboratory measurements of a 2D dam break. The scheme is also shown to run stably for a realistic 3D regional domain of the North West European shelf and to improve some aspects of the model's performance against tide gauges.

How to cite: O'Dea, E., Bell, M., Coward, A., and Holt, J.: Implementation and assessment of a flux-limiter based wetting and drying scheme, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7225, https://doi.org/10.5194/egusphere-egu2020-7225, 2020.

Ocean tides play a major role in ocean mixing, and setting up water properties, both in the deep and the shallow ocean. Whether parameterized or explicitly simulated, tides can not be ignored in modern ocean prediction models. Representing them explicitly, the approach followed here to prepare the upcoming CMEMS (Copernicus Marine Environment Monitoring Service) global prediction system, allows for the generation of a large quantity of internal waves propagating at great distances. This is a useful information for future high resolution wide swath altimetry missions but also for forcing regional systems, enabling remotely generated internal waves to enter the user domain, providing in some places an important part of the high frequency
energy.

In the present work, we review numerical aspects in the NEMO ocean model influencing the explicit representation of both external and internal tidal waves in a global 1/12° configuration. The numerical core of NEMO has indeed largely evolved recently to simulate tides, by now including the lunisolar tidal potential, Self Attraction and Loading effects, a split-explicit barotropic solver but also Lagrangian vertical coordinates to limit the spurious numerical diffusion. We report here the effect of various parameters, using data assimilative tidal models and altimetry data as references. Semi-diurnal energy budgets are also computed. Throughout this work, a systematic comparison to HYCOM results (Ansong et al. 2015 and Buijsman et al. 2016) is performed.

How to cite: Chanut, J., Bourdallé-Badie, R., Tranchant, B., Allain, D., Carrère, L., Koch-Larrouy, A., Lyard, F., and Morel, Y.: Explicit modelling of external and internal tidal waves in the global 1/12° NEMO ocean model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22157, https://doi.org/10.5194/egusphere-egu2020-22157, 2020.

A parameterisation scheme for restratification of the mixed layer by submesoscale mixed layer eddies is implemented in the NEMO ocean model. Its impact on the mixed layer depth (MLD) is examined in 30-year integrations of "uncoupled" ocean-ice and "coupled" atmosphere-ocean-ice-land global climate configurations used by the Met Office Hadley Centre. The specification of the mixed-layer Rossby radius in the scheme is shown to affect its impact on the MLD in the 1/4 degree uncoupled configuration by up to a factor of 2 in subtropical and mid-latitudes. This factor has been limited in the extent to which small mixed-layer Rossby radii are utilised to guard against CFL-type instabilities in the scheme, but such a limit was not found to be necessary for this implementation. An alternative form of the scheme is described that approximates the mixed-layer Rossby radius as a function only of latitude. This form is shown to yield similar results to the original formulation for an appropriate choice of parameters. The global mean impact of the scheme on the MLD is found to be almost twice as large in the 1 degree and 2 degree uncoupled configurations as it is in the 1/4 degree configuration, although the parameterised vertical buoyancy fluxes have closer agreement. This is shown to be the result of the scheme overcompensating for the decay in strength of resolved mixed layer density fronts in this model with decreasing horizontal grid resolution. The MLD criterion defining the depth scale of the scheme is shown to affect its global mean impact on the MLD by nearly a factor of 3 in the 1/4 degree uncoupled and coupled configurations, depending on whether the criterion is chosen to capture the actively mixing layer or well-mixed layer. Climatological MLD biases are improved overall in both cases, substantively reducing deep winter biases whilst slightly increasing shallow summer biases.

How to cite: Calvert, D., Nurser, G., Bell, M., and Fox-Kemper, B.: The impact of a parameterisation of submesoscale mixed layer eddies on mixed layer depths in the NEMO ocean model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22177, https://doi.org/10.5194/egusphere-egu2020-22177, 2020.

In the roadmap of modern parallel architectures development, the computing power of a node grows much more quickly than main memory performance (capacity, bandwidth). This leads to an even much higher gap between computing and memory resources. An efficient use of the cache memory is becoming ever more essential as optimization technique.
The NEMO model uses a finite difference integration method and a regular cartesian grid for space discretization. The NEMO code reflects this choice: a generic field is represented in memory as a 3D array; and the code is mainly composed of three-level nested loops. These loops often include only a few operations in the body; the results are stored in a temporary 3D array and then used in subsequent loops until the final calculation.
The aim of this work is to make better use of the cache memory by fusing DO loops together. The loop fusion is a transformation which takes two or more adjacent loops that have the same iteration space traversal and combines their bodies into a single loop.
The fusion of the loops is not trivial, and it could require introducing additional redundant operations to solve data dependencies. Unfortunately, this leads to a drawback of the overall performance. To avoid the redundant operation, we can adopt pointers to arrays and implement a pointer rotation at each loop iteration.
We have developed the loop fusion transformation in an advection kernel extracted from the NEMO oceanic model. We have compared 3 different versions of the optimized advection kernel, with 3 different levels of loop fusion.
The first prototype refers to the implementation where the extreme fusion is applied, and all loops in the routine have been fused. In this version, the operations are replicated up to 3 times. In the second prototype the buffer rotation has been applied only in the outermost loop. In the third prototype, the buffer rotation has also been implemented for the second dimension, and this version introduces only a limited amount of redundant operations.

The tests have been performed on the Athena cluster located at the CMCC supercomputing center. The supercomputing infrastructure is based on the Intel Xeon E5-2670 processors. The memory hierarchy is composed of 32KB of L1 cache, 256KB of L2 and 20MB L3 cache shared among the cores. The results clearly proved the effectiveness of the loop fusion approach that reaches a speedup of 2x with a high number of cores. The third prototype has proven to be the most promising solution. Prototypes 1 and 2 provide a good improvement up to 256 cores then the redundant operations lead to a loss of performance.
A deeper analysis measuring the Last Level Cache misses also showed how the loop transformation significantly reduced the number of cache misses.
Despite the good results achieved with the loop fusion optimization, we can remark that this optimization is strictly linked to the computing architecture. A fully portable performance improvement can be ensured by the adoption of a DSL (Domain Specific Language).

How to cite: Epicoco, I., Mele, F., Mocavero, S., Chiarelli, M., D'Anca, A., and Aloisio, G.: Refactoring the Memory Access Pattern to Improve Computational Performance in NEMO, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9732, https://doi.org/10.5194/egusphere-egu2020-9732, 2020.

In recent years, coastal disasters have been frequently caused by typhoons and storm surges accompanied by high waves due to global warming and the changing marine environment. In addition, the development of coastal areas in Korea has also led to suffering great damage to society every year. 

To cope with this issue, we have developed a new storm-surge prediction system based on the NEMO model for improving the predictability both the tide and the surge. This new regional tide-surge prediction system (RTSM) is constructed with a two-dimensional barotropic sigma coordinates and has a 1/12 degrees horizontal resolution. To find optimal coefficients of this model, several sensitivity experiments were conducted and verified with tide gauge measurements from the KHOA (Korea Hydrographic and Oceanographic Agency). Finally, we selected a bathymetry from SRTM (Shuttle Radar Topography Mission), Charnock coefficient as a constant value of 0.275 and the reference pressure for the inverse barometric effect as the domain mean. As the result of comparing surge-height predictions with the currently operating model (OPER-RTSM), the new system (RTSM) showed roughly 30% higher in forecast accuracy than the previous OPER-RTSM.

How to cite: La, N., An, B. W., Kang, K., Oh, S. M., and Kim, Y.: Verification of the Storm surge Forecasting System for the Korean Coastal Area, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15049, https://doi.org/10.5194/egusphere-egu2020-15049, 2020.

Hydrodynamic models (HDM) provide a reasonable estimate of the sea conditions. Thus making them a vital tool for climate change, engineering, and marine ecosystems. One of the parameters often derived from HDM is the Sea Surface Height (SSH). There exists however a very important hidden characteristic with respect to SSH derived from HDM. For instance, the modelled sea level may have a bias relative to a geodetic reference system datum. In many cases, this bias can change both spatially and temporally. This study now examines this bias by comparison of HDM modelled SSH with tide gauges derived SSH that are geodetically referenced to a more stable vertical reference frame such as the marine geoid (equipotential surface of the earth i.e. is the shape of the ocean surface under the influence of the gravity and rotation of Earth alone).

In this study, the performance of two HDM is analysed for the period 2014‒2015: the Nemo-Nordic (utilised for the Baltic and the North Sea) and the HIROMB-BOOS (used for operational sea forecast in Estonia). In these models, the derived SSH is compared to the fourteen tide gauges (TG) located along the Estonian coastal zone of the Baltic Sea. The vertical reference frame for these tide gauges is fitted to that of a regional high-resolution geoid model, thus deriving the Dynamic Topography. The methodology consisted of: (i) determining the offshore points that are closest to the tide gauge location, (ii) filtering and averaging of the data sets to remove outliers and high-frequency fluctuations (iii) calculation of the SSH bias between TG and HDM (iv) calculation of the standard deviation and root mean square error (RMSE).

In general, results show that both models conform to a similar trend as tide gauge. The bias however between tide gauge and models varied randomly in magnitude (both spatially and temporally) between both models. The maximum bias for the HIROMB was calculated to be an overestimation of 57 cm and for the Nemo an underestimation of 64 cm. These results hint of possible improvement that can be made in HDM by utilizing a high resolution geoid model that can assist in accurate engineering and scientific studies.

How to cite: Jahanmard, V., Delpeche-Ellmann, N., and Ellmann, A.: Comparison of Dynamic Topography Bias in HIROMB and NEMO-Nordic Model by Utilizing Marine Geoid, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20134, https://doi.org/10.5194/egusphere-egu2020-20134, 2020.

Accurate forecasts of storm surges caused by winds and atmospheric pressure are important for the protection of life and property in coastal regions and also for safe navigation. Therefore, Environment and Climate Change Canada (ECCC) maintains and develops surge prediction systems. This study focuses on the assessment of the timing and amplitude of predicted surges at selected locations during the passage of hurricane Dorian. The systems use barotropic ocean models to simulate water levels and currents at 1/30 and 1/12 degree horizontal resolutions.

The relatively low tidal range at the time of Dorian’s landfall helped prevent catastrophic flooding. However, with a closer superposition of the peak surge and high tide, the damage could have been more significant. Reasonably well predicted timing of the highest surge by the system helped prevent the over-issuance of warnings. Sensitivity of the forecast quality to the lead time, resolution and atmospheric forcing for the event will be presented.

How to cite: Huziy, O., Bernier, N., Pouliot, B., Timko, P., Wang, P., and Telford, D.: Performance of ECCC surge forecasting systems near Canada's East Coast during the passage of hurricane Dorian, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22097, https://doi.org/10.5194/egusphere-egu2020-22097, 2020.

How to cite: Bricaud, C. and Castrillo, M.: Overview of the first year of the NEMO global 1/36° configuration (ORCA36) development, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22147, https://doi.org/10.5194/egusphere-egu2020-22147, 2020.

The tuning phase of IPSL-CM6A-LR, a climate model of CMIP6 using NEMO as ocean component, lasted for 4 years, during which we explored different numerical recipes controlling ocean vertical mixing, among others. Analysis of all simulations is still ongoing, but two lessons can be learned so far. [1] After more than 2,000 yr of integration (using pre-industrial external forcings), the deep ocean has not reached an equilibrium, yet. [2] Sensitivity experiments exploring structural and parametric uncertainties indicate that some intrinsic climatic features of this model are quite robust. Overall, this suggests that we currently have little control on the backbone of numerical oceans.

How to cite: Deshayes, J.: Challenges raised by global ocean configurations in the context of climate modelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22151, https://doi.org/10.5194/egusphere-egu2020-22151, 2020.

The Nucleus for European Modelling of the Ocean (NEMO) is a state-of-the art modelling platform for oceanographic research, operational oceanography, sesonnal forecasts and climate studies. NEMO includes three major components; the blue ocean (dynamics), the white ocean (sea-ice), the green ocean (ocean biogeochemistry). It also allows coupling through interfaces with atmosphere (through OASIS software), waves, ice-shelves, so as nesting through the adaptive mesh refinement software AGRIF. Some reference configurations and test cases allowing to explore, to set-up and to validate the applications, and a set of tools to use the platform are also available to the community. The whole platform and its documentation are available under free licence.

The evolution and reliability of NEMO are organised and controlled by a European Consortium between CMCC (Italy), CNRS (France), MOI France), NOC (UK), UKMO (UK).

Consortium members agree on long term strategy and yearly plans, sharing expertise and efforts within the NEMO System Team: the core team of NEMO developers in order to ensure the successful and sustainable development of the NEMO System as a well-organised, state-of-the-art ocean model code system suitable for both research and operational work

How to cite: Samson, G., Levy, C., and System Team, N.: The NEMO (Nucleus for European Modelling of the Ocean) numerical ocean platform, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22213, https://doi.org/10.5194/egusphere-egu2020-22213, 2020.

Surface wind stress and heat fluxes (e.g. sensible and latent) are the major driving forces that modify the ocean dynamics and thermodynamics. Therefore, in a modelling framework, realistic momentum and heat fluxes are essential for simulating the global ocean. In the NEMO ocean general circulation model, these air-sea fluxes, which are difficult to be measured directly, are derived using bulk formulas. A large set of bulk formulas exist and this work tries to quantify the ocean response to the different bulk formulations implemented in the newest version of NEMO global ocean model (version 4.0.1). A set of experiments based on the CMCC ORCA025 configuration (1/4° of horizontal resolution) is run and examined. The numerical experiments differ for the bulk formula used  to estimate air-sea fluxes (ECMWF, NCAR, COARE and COARE3.5) and they consist of 2-years simulations forced by the recent JRA55-do-v1.3 (∼ 55km resolution). Preliminary results on the ocean sensitivity in terms of sea surface temperature, sea surface salinity and ocean currents, also in comparison with available observations, will be shown.

How to cite: Bonino, G., Iovino, D., and Masina, S.: Ocean sensitivity to bulk formulae parameterization: a NEMO-ORCA025 model study, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9591, https://doi.org/10.5194/egusphere-egu2020-9591, 2020.

The Southern Ocean is a crucial part of the global ocean circulation. The unique bathymetry and lack of meridional boundary in conjunction with an equator to pole temperature gradient and strong westerly winds results in an eastward flowing Antarctic Circumpolar Current (ACC). The ACC is the strongest ocean current in the world (173.3 ± 10.7Sv), vital in transporting heat, carbon and nutrients between the major ocean basins. 

Using prototype UK CMIP6 (HadGEM3-GC3.1) simulations at 1°, 1/4° and 1/12° spatial resolutions we illustrate the strong resolution dependence of the strength of the ACC through the Drake Passage. All three model resolutions exhibit a weak ACC compared to observations. The 1/4° and 1/12° models show a significant weakening over the first 50 years, stabilizing at 60Sv and 120Sv respectively.

We analyse the source of the weaker volume transport by decomposing the ACC transport into components arising due to northern and southern boundary density profiles (relative to the bottom density), Ekman transport and depth-independent flow. We attribute the weaker ACC in the 1/4° model to a lightening of the southern density profile and the formation of a reverse flow along the coast of Antarctica.

Our decomposition highlights the significant contribution to the ACC’s volume transport and variability made by both northern and southern density profiles, as well as the depth-independent component of the flow.

How to cite: Jonathan, T., Johnson, H., Marshall, D., Bell, M., and Hyder, P.: Antarctic Circumpolar Current Biases in a hierarchy of HadGEM3-GC3.1 Models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11444, https://doi.org/10.5194/egusphere-egu2020-11444, 2020.

How will the Gulf of Bothnia be impacted by future climate change? A changing climate will, in addition to warming, reduce the ice-season, change the salinity, sea level, and wave and current conditions. This will in turn have implications for eco systems, habitats, biodiversity, as well as human activities such as fishing, aquaculture, and wind parks. The SmartSea project aims to estimate the climate change impacts in this area. This study, which is part of the project SmartSea, assesses the changing physical and biogeochemical properties up to year 2059 using numerical experiments with forcing from two different Representative Concentration Pathways (RCP 4.5 and RCP 8.5) and four different global climate models. Here NEMO3.6 with LIM3 sea ice model is coupled to the biogeochemical model SCOBI. The model comprises the Gulf of Bothnia with a horizontal resolution of approximately one nautical mile.

The preliminary results comparing periods 1975-2005 and 2030-2059 and the pathway RCP4.5 and RCP8.5 show significant changes in sea ice conditions including a decrease of the ice season length, annual maximum ice volume, and extent of ice cover. In addition, the annual maximum ice volume is seen to arise earlier in the season. The temperature increases consistently, although the actual increase between the different simulations varies considerably. A general trend of decreasing salinity can also be seen. This is, however, less systematic than for ice conditions and temperature. The simulations indicate that the changes in both temperature and salinity are not spread evenly, but some areas will be affected more than others. The flow speed trends have been studied by comparing simulations for the period 1980-2005 and the pathway RCP4.5 and RCP8.5 for 2040-2059. The simulations indicate a rise in both local maximum flow speeds, as in average flow speeds, both in surface currents and depth averaged currents (barotropic currents).

How to cite: Fredriksson, S., Siiriä, S., Oikkonen, A., Roiha, P., Särkkä, J., Hordoir, R., Höglund, A., Hieronymus, J., Eilola, K., Ruvalcaba Baroni, I., and Arneborg, L.: Climate scenarios of the Gulf of Bothnia using a high-resolution regional ocean model (NEMO-SCOBI), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18908, https://doi.org/10.5194/egusphere-egu2020-18908, 2020.

A new one-dimensional parameterization of penetrative deep convection
has been developed for ocean general circulation models (OGCMs). This work is
motivated by the necessity for OGCMs to better represent the vertical mixing,
the production and properties of deep and intermediate waters, the overflows
and the transport of biogeochemical materials. Our approach is inspired from
atmospheric parameterizations of shallow convection which assumes that in the
convective boundary layer, the subgrid-scale fluxes result from two different
mixing scales : small eddies which are represented by an eddy-diffusivity
contribution, and large eddies associated with thermals which are represented
by a mass-flux contribution. The local (small eddies) and non-local
(large eddies) contributions are unified into an Eddy-Diffusivity-Mass-Flux
(EDMF) parameterization which treats simultaneously the whole vertical mixing.
EDMF is implemented and tested into the communautary ocean model NEMO. Deepening of dense water in 1D analytic cases, sucessfully reproduced in LES simulations, is significantly better captured with EDMF than with standard diffusion parameterizations. Also the convective events observed during winter 2013 in the western Mediterranean at the Lion station are more realistic in terms of sequencing and intensity with EDMF. We show that the best performances of EDMF are due to a best representation of the non-local entrainment fluxes which are counter-gradient in the stratified zone of the thermocline.

How to cite: Bourdallé-Badie, R., Giordani, H., and Madec, G.: Eddy-Diffusivity Mass-Flux Parameterization: A Approach to unify Diffusion and Convection in Ocean Models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22192, https://doi.org/10.5194/egusphere-egu2020-22192, 2020.