Numerical modelling of the ocean: new scientific advances in ocean models to foster exchanges within NEMO community and contribute to future developments

NEMO (Nucleus for European Modelling of the Ocean) is a state-of-the-art modelling framework of the ocean that includes components for the ocean dynamics, the sea-ice and the biogeochemistry, so as a nesting package allowing to set up zooms and a versatile data assimilation interface (see https://www.nemo-ocean.eu/).
NEMO is used by a large community in Europe and world-wide (~200 projects, ~100 publications each year) covering a wide range of applications : oceanographic research, operational oceanography, seasonal forecast and climate projections.
NEMO is in particular used in 6 Earth System Models within CMIP6 and in Copernicus Marine Services (CMEMS) model-based products.

This session will provide a forum to properly address the new scientific advances in numerical modelling of the ocean and their implication for NEMO developments associated with:
• Ocean dynamics at large to coastal scales, up to 1km resolution ;
• Ocean biogeochemistry
• Sea-ice
• New numerical schemes associated to energy conservation constraints
• High performance computing challenges and techniques

The session will cover both research and operationnal activities contributing to new analysis, ideas and developments of ocean numerical models.
Presentations of results based on new NEMO functionalities and new NEMO model configurations are welcome.

Convener: Doroteaciro Iovino | Co-conveners: Claire Levy, Mike Bell, Jerome Chanut, Julien Le Sommer
| Wed, 25 May, 08:30–10:00 (CEST)
Room 1.15/16

Presentations: Wed, 25 May | Room 1.15/16

Chairperson: Doroteaciro Iovino
Virtual presentation
Francesca Mele, Italo Epicoco, Silvia Mocavero, Daley Calvert, and Mike Bell

Nowadays one of the main challenges in scientific computational field is developing the next
generation of HPC technologies, applications and systems towards exascale. This leads to focus the
efforts on the development of a new, efficient, stable and scalable NEMO reference code with
improved performances adapted to exploit future HPC technologies in the context of CMEMS systems.
On the main factors that limit the current scalability is an inefficient exploitation of the single node
performance. Different technical solutions have been tested to fully exploit memory hierarchies and
hardware peak performance. Between all, the fusion of DO loops together and by dividing the
computation over tiles are the two optimization strategies more efficiently take advantage of the
cache memory organization. This work focuses on the first one.
The loop fusion is a transformation which takes two adjacent loops that have the same iteration space
traversal and combines their bodies into a single loop. This optimization improves data locality so
giving a better exploitation of the cache memory and a reduction of the memory footprint because the
temporary arrays can be replaced with scalar values.
Performance tests have been executed on a domain size of 3002x2002x31 grid points running 1-year
GYRE_PISCES simulations with IO disabled on the Zeus Intel Xeon Gold 6154 machine, available at
CMCC. An increasing number of cores - from 504 to 2016 – have been used to test experiments with
the different HPC options.
The analysis focused on the routines where the optimizations have been applied. The use of the
extended halo introduces a penalty in the execution time that grows as the number of processes
increases and generally the use of loop fusion optimization slightly improves the performance. For
many routines, as subdomains get smaller, the improvements due to optimizations are less significant.
The simultaneous application of all optimizations leads to an improvement between 10% and 50%
(except for lateral diffusion). Looking at the total elapsed time, the new HPC optimizations speed up 

the elapsed time of a factor 1.25x. Unfortunately, non-optimized routines mitigate this improvement.

The same scalability test has been repeated running 1-month ORCA025 simulations with the output
set to be produced at the end of the run. The results show that the use of loop fusion optimization
slightly improves the performance. The use of tiling in ORCA025 introduces less benefits with
reference to GYRE. The simultaneous application of all optimizations doesn't lead many benefits in
ORCA025 since the improvement concerns only a subset of routines with the Tracer lateral diffusion
routine getting worse in all cases.
In conclusion the impact of the new optimized code behaves differently depending on the
configuration. The overhead introduced by the extended halo implies a computation time cost that the
proposed optimizations are able to regain difficultly. Tiling is the aspect with the highest impact in
these optimizations (especially in GYRE) and loop fusion has in general a low impact. The optimizations
should be applied to all the rest of the code to obtain more benefits.

How to cite: Mele, F., Epicoco, I., Mocavero, S., Calvert, D., and Bell, M.: Preparing NEMO4.2, the new NEMO modelling framework for the next generation HPCinfrastructures, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13484, https://doi.org/10.5194/egusphere-egu22-13484, 2022.

Virtual presentation
Daley Calvert, Mike Bell, Francesca Mele, Italo Epicoco, Sebastien Masson, Maff Glover, and Gurvan Madec

NEMO is an example of a memory-bound code, where computational performance is limited by memory bandwidth due to intensive main memory access. This can be mitigated by more efficient use of the memory hierarchy, ensuring that data is held in fast low-level cache for as long as possible. Loop blocking or “tiling” is one way of achieving this, by partitioning loops so that data access is divided into smaller blocks that fit into cache.

This optimisation technique has been applied to several numerical ocean models, including MITgcm, ROMS and CROCO. We describe an implementation of tiling in the NEMO 4.2 release candidate and its performance in realistic global configurations.

The MPI domain is partitioned horizontally (over latitude and longitude) by modifying DO loop bounds and local working array declarations. A DO loop at the time-stepping level iterates over the tiles asynchronously. Lateral boundaries use an extended halo (two points wide instead of one) so that MPI communications can be suppressed when the tiling is active. Tiling has so far been implemented in the active tracer, vertical diffusion and dynamical code sections of NEMO.

Tiling of the longitudinal axis always results in a loss of performance while tiling of the latitudinal axis can perform very well, reducing the execution time of individual code sections by up to 40% in ORCA2 simulations. However, this is offset against the cost of additional calculations due to the two-point halo, which increases the execution time of individual code sections by up to 15%. Additionally, the number of calculations is proportional to the number of tiles, further reducing potential performance gains from the tiling. This can be mitigated by removing halo calculations where possible.

Overall tiling performance in ORCA025 is significantly worse compared to ORCA2. This is because a greater proportion of time is spent in code that is not tiled (75% in ORCA025 vs 45% in ORCA2) and because the performance of some tiled code is worse. Tiling in the vertical, as well as the horizontal, not only recovers the performance of this tiled code but improves it beyond that in the ORCA2 simulations.

How to cite: Calvert, D., Bell, M., Mele, F., Epicoco, I., Masson, S., Glover, M., and Madec, G.: Loop blocking (“tiling”) in NEMO, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13438, https://doi.org/10.5194/egusphere-egu22-13438, 2022.

Virtual presentation
Florian Lemarie, Nicolas Ducousso, Laurent Debreu, and Gurvan Madec

Because of the Boussinesq assumption employed in the vast majority of oceanic models,
the acoustic waves are filtered and the fast dynamics corresponds to the external
gravity-wave propagation which is much faster than other (internal) processes.
The fast and slow dynamics are traditionally split into separate subproblems
where the fast motions are nearly independent of depth.  It is thus natural to
model these motions with a two-dimensional (barotropic) system of equations while
the slow processes are modeled with a three-dimensional (baroclinic) system.
However such splitting is inexact, the barotropic mode is not strictly depth-independent
meaning that the separation of slow and fast modes is non-orthogonal, even in the linear case.
A consequence is that there are some fast components contained in the slow motions which induce
instabilities controlled by time filtering of the fast mode.
In this talk we present an analysis of the stability and accuracy of the barotropic–baroclinic mode splitting
in the case where the baroclinic mode is integrated using a Runge-Kutta
scheme and the barotropic mode is integrated explicitly (i.e. the so-called split-explicit approach).
By referring to the theoretical framework developed by Demange et al. (2019),
the analysis is based on an eigenvector decomposition using the true
(depth-dependent) barotropic mode. We investigate several strategies to achieve stable
integrations whose performance is assessed first on a theoretical ground and then
by idealized linear and nonlinear numerical experiments.

How to cite: Lemarie, F., Ducousso, N., Debreu, L., and Madec, G.: Stability and accuracy of Runge-Kutta-based split-explicit time-stepping algorithms for free-surface ocean models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13278, https://doi.org/10.5194/egusphere-egu22-13278, 2022.

On-site presentation
Sibylle Téchené, Gurvan Madec, Jérôme Chanut, Andrew Coward, and Dave Storkey

As Ocean General Circulation Models (OGCMs) become more complex, their computational efficiency remains a crucial challenge for ocean simulations. By using smart numerical choices, however, it is possible to increase the time step of an OGCM for a fixed spatial resolution, thereby reducing the computational cost and improving efficiency. To do this we are revisiting the NEMO time-stepping scheme. Modified Leapfrog  (MLF) has been used so far. We implement Runge Kutta 3rd order (RK3) from Wicker and Skamarock (2002) as an intermediate step toward a coupled space-time scheme. RK3 time stepping implemented in NEMO is a single step iterative method using a predictor-corrector like formulation. It uses three stages to integrate the model from time step n to n+1. During the first stage ( from n to n+⅓ ) we build guesses of tracers and momentum using an euler time stepping : on the one hand tracers are estimated by integrating advection terms only, on the other hand momentum are estimated by integrating advection, pressure gradient and coriolis terms only. During the second stage ( from n to n+½ ) we use n+⅓ tracers and momentum fluxes from stage 1 to build tracers and momentum guesses at n+½ using the same equations. The last stage integrates full equations from n to n+1 with n+½ tracers and momentum flux guesses from stage 2. We integrate the barotropic mode from n to n+1 at the beginning of the time step and interpolating it linearly at n+⅓ and n+½ as proposed in the accompanying paper by Lemarie et al. (2022). The new RK3 implementation is now ready to be validated in NEMO. It has been tested on most of NEMO's components : to dynamics, active tracers, sea ice, passive tracers, ice shelves, open boundaries and AGRIF zoom. Global simulations with sea ice and biogeochemistry at low resolution show that the model using RK3 is stable when doubling the time step and is almost twice as fast as MLF ( 80% speed up ). Nevertheless, some work remains before considering RK3 as fully operational in NEMO.

How to cite: Téchené, S., Madec, G., Chanut, J., Coward, A., and Storkey, D.: Gain of efficiency with a new time scheme in NEMO : Runge Kutta 3rd order, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13426, https://doi.org/10.5194/egusphere-egu22-13426, 2022.

On-site presentation
Andrew Styles, Mike Bell, David Marshall, and David Storkey

Gyres are prominent surface structures in the global ocean circulation that often interact with the sea floor in a complex manner. Diagnostic methods, such as the depth-integrated vorticity budget, are needed to assess exactly how such model circulations interact with the bathymetry. Terms in the vorticity budget can be integrated over the area enclosed by streamlines to identify forces that spin gyres up and down. We diagnose the depth-integrated vorticity budgets of both idealized gyres and the Weddell Gyre in a realistic global model. It is shown that spurious forces play a significant role in the dynamics of all gyres presented and that they are a direct consequence of the Arakawa C-grid discretization and the z-coordinate representation of the sea floor. The spurious forces include a numerical beta effect and interactions with the sea floor which originate from the discrete Coriolis force when calculated with the following schemes: the energy conserving scheme (ENE); the enstrophy conserving scheme (ENS); and the energy and enstrophy conserving scheme (EEN). Previous studies have shown that bottom pressure torques provide the main interaction between the depth-integrated flow and the sea floor. Bottom pressure torques are significant, but spurious interactions with bottom topography are similar in size. Possible methods for reducing the identified spurious topographic forces are discussed. Spurious topographic forces can be alleviated by using either a B-grid in the horizontal plane or a terrain-following vertical coordinate.

How to cite: Styles, A., Bell, M., Marshall, D., and Storkey, D.: Spurious forces can dominate the vorticity budget of ocean gyres on the C-grid, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-792, https://doi.org/10.5194/egusphere-egu22-792, 2022.

Virtual presentation
Michael J Bell and Diego Bruciaferri

The NEMO ocean model has a fairly generic s-level vertical co-ordinate option that allows the model levels to follow either the ocean bathymetry resolved by the model or a smoothed "envelope'' version of it. Sufficiently accurate calculation of the net horizontal pressure forces on the resulting steeply sloping cells is well known to be difficult and many schemes have attempted to address this issue. We focus here on two types of schemes that calculate these forces by integrating around the cell perimeters (or faces). The first type calculates the pressure forces on the faces of each velocity cell first as proposed by Lin (1997). The second type is a re-arrangement of Song's density Jacobian scheme derived by Shchepetkin & McWilliams (2003). In these schemes the density and the height along coordinate surfaces are approximated by local piecewise continuous polynomial reconstructions and the integrals are calculated by direct integration of the polynomials or by quadrature methods. Higher order schemes are obtained by using higher order polynomials. In some cases these polynomials are constrained to avoid over-fitting of potentially noisy fields. We have implemented these schemes in NEMO treating model variables as point values rather than cell-mean values. The implementation for the second type follows previously published work closely whilst that for the first type differs somewhat from previous work. We have also implemented a ``pre-processing'' step that calculates cubic reconstructions of the density profile at the deepest point in the stencil used to calculate the net horizontal forces. The ``pre-processing'' step subtracts this interpolated profile from all the other profiles in the stencil. We present results for these schemes in a standard sea-mount test case with steep bathymetry and horizontal density surfaces in which the currents should be identically zero. We show that both types of schemes using appropriate quadratic or cubic reconstructions give currents that are one order of magnitude smaller than those given by second order accurate schemes. We also show that in this test case applying the ``pre-processing'' step with a second order accurate scheme provides results that are competitive with the higher order schemes. We also discuss some results for the “realistic” Atlantic Margin Model domain, priorities for further work and the development of schemes that treat model variables as grid-cell mean values.

How to cite: Bell, M. J. and Bruciaferri, D.: Accurate calculation of pressure forces on cells defined by steeply sloping coordinates, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5012, https://doi.org/10.5194/egusphere-egu22-5012, 2022.

Virtual presentation
Charles Pelletier, Romain Bourdallé-Badie, Hervé Giordani, Guillaume Samson, Gilles Garric, Coralie Perruche, and Élodie Gutknecht

Ocean convective events are triggered by vertical density instabilities, e.g., following significant surface heat loss at dusk or upon sea-ice formation. They have a significant impact on the vertical distribution of ocean heat, salt, and biogeochemical content, most frequently in its shallowest few hundreds of meters, but also occasionally much deeper. The representation of such events within ocean models requires a specific treatment adapted to vertically nonlocal processes, since most turbulent closure schemes are based upon Boussinesq-derived diffusivity coefficients and are thus fundamentally local. Consequently, different methods have been employed for making vertical diffusion schemes also accounting for convection. Past examples used in NEMO include, e.g., adding a nonlocal term in the diffusivity parameterization (e.g., K-profile) or simply enhancing the vertical diffusivity. 

Another more recent example is the mass-flux convective (MFC) scheme, which has recently been adapted for the NEMO ocean model and can be used as a convective complement to any diffusion-only scheme (e.g., scaled from turbulent kinetic energy). The MFC scheme allows generating subgrid-scale buoyancy-driven convective plumes which can reach down to 2000m depth in extreme cases. Within the plumes, physical quantities of interest (e.g., temperature, salinity) are vertically transported independently from the large-scale model dynamics, at velocities which can exceed by far anything permitting by it (up to 20 cm s-1) implying fast, deep-reaching vertical mixing.  Hence, in addition to local fluxes associated with small, diffusion-related eddies, the large-scale model solution can also be affected by nonlocal convective fluxes associated with large eddies produced by the MFC scheme. 

In this talk, we present new developments and tests related to the MFC parameterization. Namely, we introduce the application of this scheme to passive biogeochemical tracers, as well as turbulent kinetic energy and model dynamics (i.e., ocean velocities). Results over 1D configurations illustrating the impact of these novelties are presented. Finally, we also show first results over a NEMO global 1/4° configuration involving previously unexplored convection-prone conditions, such as sea-ice formation.

How to cite: Pelletier, C., Bourdallé-Badie, R., Giordani, H., Samson, G., Garric, G., Perruche, C., and Gutknecht, É.: New developments and diagnostics related to the mass flux convective parameterization in the NEMO ocean model , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7231, https://doi.org/10.5194/egusphere-egu22-7231, 2022.

On-site presentation
Remi Tailleux

The momentum balance underlying most numerical ocean models is one in which the primary forces driving fluid motions are rotation, the pressure anomaly gradient, and vertical buoyancy forces. While this form of momentum balance has the advantage of simplicity, it does not in itself reveal much about the dynamical control exerted by the near material invariance of potential temperature and salinity, whereby oceanic motions are expected to be preferentially stirred along `neutral’ directions, nor does it reveal much about the possible dynamical controls imposed by energetics constraints. The main aim of this work is to show that it is possible to rewrite the momentum balance in a way that is much more explicit about the role of the preferential directions for lateral stirring as well as about how energetics affect momentum. The new momentum balance is obtained by combining Crocco’s theorem with the theory of available potential energy. Its main properties is that the forces that make it up (save for friction) are all perpendicular or nearly perpendicular to the 3D velocity field. One force is a new form of Nycander’s P vector, which controls the local changes in available potential energy. The directions that it defines coincide with those defined by the standard neutral vector except in the Southern Ocean and Gulf Stream region, where it is hypothesised that neutral rotated diffusion must cause spurious diapycnal mixing. One another force is given by the gradient of a new form of Bernoulli function, which controls the local changes in total energy. As a result, both forces define two different set of neutral directions, one along which lateral stirring leaves the available potential energy unchanged, the other along which lateral stirring leaves the total energy unchanged. The newly obtained momentum balance can be Reynolds averaged, which reveals the role of the eddy kinetic energy, eddy available potential energy, as well as of the variances and co-variances of sub-grid scale variability of temperature and salinity on the momentum balance for resolved motions. More generally, a full energy cycle can be constructed, with exact equations describing the energy exchanges between resolved and unresolved energy reservoirs, that can form the basis for energetically and thermodynamically consistent parameterisations.

How to cite: Tailleux, R.: A new approach to the formulation of energetically and thermodynamically consistent ocean models and parameterisations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12192, https://doi.org/10.5194/egusphere-egu22-12192, 2022.

On-site presentation
Antoine-Alexis Nasser, Gurvan Madec, Laurent Debreu, and Casimir De Lavergne

Using a 1/4° shallow water model, Adcroft & Marshall (1998) show that the solution obtained on a rotated mesh depends on the angle between the numerical grid and the physical coastline. They conclude that piecewise constant coastline exert a spurious form stress on the model boundary current.

Here, we reproduce these results and test the combination of free-slip and no-slip boundary conditions with the stress tensors’ vorticity-divergence and symmetric forms. We show that, for all angles of rotation, the equilibrium solutions actually remain similar, provided that the system has numerically converged (i.e. higher resolution with physics unchanged). It appears that having at least 4 grid-points by internal radius of deformation is necessary to accurately represent flows along coastlines. Surprisingly, the symmetric tensor combined with the free-slip condition leads to dynamics that are insensitive to resolution and akin to no-slip. We argue that the way the free-slip condition is implemented with the symmetric tensor is not suitable. We propose an alternative implementation and discuss its advantages and limitations.

How to cite: Nasser, A.-A., Madec, G., Debreu, L., and De Lavergne, C.: Finding better numerical solutions for circulation along piecewise-constant coastlines in ocean models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10204, https://doi.org/10.5194/egusphere-egu22-10204, 2022.

Presentation form not yet defined
Assessment of the Finite VolumE Sea Ice Ocean Model (FESOM2.0), with respect to Partial bottom cells, embedded sea ice and vertical mixing library CVMix
Patrick Scholz, Dmitry Sidorenko, Sergey Danilov, Nikolay Koldunov, Qiang Wang, Dmitry Sein, and Thomas Jung
On-site presentation
Katherine Hutchinson, Julie Deshayes, Christian Éthé, Clement Rousset, Martin Vancoppenolle, Nicolas Jourdain, and Pierre Mathiot

None of the coupled climate models contributing to the DECK experiments of CMIP6 directly simulate ocean circulation under the ice-shelves of Antarctica, thereby omitting a potentially critical piece of the climate puzzle. Ocean-ice shelf interactions are vital to the production of Ice Shelf Water (ISW), a parent water mass of the globally important Antarctic Bottom Water (AABW). In NEMO these interactions can be explicitly represented or parameterised. For the parameterization, the cavities are left closed and the observed melt is injected along the depth interval between the ice shelf base and the bathymetry at the position of the ice shelf front. While this accounts for an input of freshwater along the Antarctic coastline, it does not allow for a change in melt rate in response to ocean conditions, nor does it adequately represent dense water production on the continental shelf. 

To address this, we explicitly simulate ocean circulation beneath three large Antarctic ice shelves thought to be responsible for the majority of dense water production feeding AABW, namely: Filchner-Ronne Ice Shelf (FRIS), Ross Ice Shelf and Larsen C Ice Shelf. All smaller cavities are provisionally left parameterized. We present a new NEMO configuration with ocean circulation under the large Antarctic ice shelves and explicit ISW formation. Results show that the grid resolution of the NEMO global 1° configuration, is sufficient to set-up sub-ice shelf circulation patterns that are in line with observations and produce melt rate patterns that agree well with both high resolution models and satellite measurements. The net melt of FRIS after 2 cycles of CORE forcing corresponding to the period 2005-2009 is 95 Gt/yr which is slightly lower than observed, while the net melt of Ross is overestimated at 125 Gt/yr (Rignot et al., 2013: FRIS = 155 Gt/yr, Ross = 48 Gt/yr). Work is in progress to address these opposing tendencies in melt rates of the two major Antarctic ice shelves, by including the effect of tides within the cavities. We propose that the next step in climate modelling be to open the ice shelf cavities where the global model grid resolution is sufficient to do so, and employ a parameterization for the small ice shelves and for the inlets of large cavities that are too small to be resolved. 

How to cite: Hutchinson, K., Deshayes, J., Éthé, C., Rousset, C., Vancoppenolle, M., Jourdain, N., and Mathiot, P.: Increasing complexity of NEMO for climate applications by explicitly simulating the large sub-ice shelf seas, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2683, https://doi.org/10.5194/egusphere-egu22-2683, 2022.

Virtual presentation
Aimie Moulin and Emanuela Clementi

The latest NEMO version (v4.2-RC, Release Candidate) has been updated to include new processes related to wave-current interactions. This study is assessing the impact of those new developments, especially the effect of the wave-induced mixing in the Mediterranean sea dynamics. A set of sensitivity experiments are performed using the hydrodynamic model NEMO v4.2-RC coupled with the spectral wave model WaveWatchIII (WW3) v6.07 through the OASIS library. The configuration is based on the operational Copernicus Marine Service Mediterranean forecasting physical system (MedFS). Both models are implemented at 1/24° resolution and are forced by ECMWF 1/10° horizontal resolution atmospheric fields. The models are one-way coupled therefore the wave model is sending fields every hour to the hydrodynamic model. Two-year (2019–2020) numerical experiments are carried out in both uncoupled and coupled mode. In order to validate the system, numerical results are compared with in-situ and satellite data. This study is focused on the impact of the coupling on upper-ocean properties (such as temperature, salinity and surface currents) and mixed layer depth, at mesoscale. The sensitivity of the ocean dynamic to the wave-current interaction is also evaluated during a specific extreme event. Numerical simulations show a global decrease of the wind stress in the Mediterranean Sea due to the interaction with waves. The wave-induced drag coefficient leads only to minor improvements in the circulation fields. The shear of the current in the upper meters is almost due to the Stokes-drift as the mixing by waves is reducing the shear of the mean current. The modifications of the Turbulent Kinetic Energy vertical closure scheme and the inclusion of the Langmuir turbulence lead to an increase in the mixing in specific areas, thus helping to deepen the Mixed Layer Depth.

How to cite: Moulin, A. and Clementi, E.: Assessment of wave-current interactions on the Mediterranean Sea dynamics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7348, https://doi.org/10.5194/egusphere-egu22-7348, 2022.

Virtual presentation
Shiming Xu

We propose a new ocean modeling framework (OMARE) which supports multi-scale simulation of the ocean through adaptive mesh refinement (AMR). It is based on refactoring NEMO model onto a modern software middleware of JASMIN, which enables, besides AMR, parallel computing platforms (with MPI), dynamic computational load balancing, transparent parallel I/O, etc. We demonstrate the three-level dynamical refinement which span the climate-centric resolution (0.5-deg) and submesoscale-resolving resolution (0.02-deg). The idealized double-gyre test case simulates realistic western boundary current system, and captures the dynamically changing, mesoscale- and submesoscale-rich regions with AMR. We also evaluate and analyze the computational performance of OMARE on typical high-performance computing platforms.

How to cite: Xu, S.: Ocean Modeling with Adaptive REsolution (OMARE) – A new multi-scale modeling framework based on NEMO, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1094, https://doi.org/10.5194/egusphere-egu22-1094, 2022.

Virtual presentation
Théo Brivoal, Jérome Chanut, and Mathieu Hamon

In the context of the IMMERSE project, we present here a new high-resolution configuration over the North-East Atlantic based on the NEMO model in its version 4.2. The configuration is based on already existing eNEATL36 (extended North – East ATLantic) configuration, which covers the Iberian – Biscay – Ireland area and the western Mediterranean Sea at a 1/36° resolution. It incorporates a kilometric resolution (1/108°) AGRIF zoom that covers the Atlantic and Mediterranean French and Spanish coasts and includes the Gibraltar Strait, Corsica and Sardinia. Two-way exchanges are enabled between the nest and the parent configuration. We will present a description of the configuration, alongside some initial results. First, we compare the performance of the NEMO 4.2 version with the pre-IMMERSE version (NEMO 4.0) on our configuration. The configuration is then validated over the 1.5 year target period from January 2017 to June 2018 against satellite data and in-situ observations. Finally, the impact of the high-resolution nest is evaluated by comparing the simulation with a twin experiment over the same period but without nest.

How to cite: Brivoal, T., Chanut, J., and Hamon, M.: A new high-resolution zoom over the North-East Atlantic based on NEMO 4.2 (IMMERSE) version, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2925, https://doi.org/10.5194/egusphere-egu22-2925, 2022.

On-site presentation
Corentin Clerc, Olivier Aumont, and Laurent Bopp

Filter-feeding gelatinous zooplankton (FFGZ), namely pelagic tunicates (salps, dolioids, appendicularians and pyrosomes), are increasingly recognized as an essential component of the marine ecosystem.  Long ignored because they are difficult to sample due to their fragility, these organisms play a poorly understood but essential role in the food web. First, they are prey for many organisms of interest (e.g. fish and turtles). Second, unlike other zooplankton (e.g., crustaceans) that feed on preys that are about an order of magnitude smaller, filter feeding gives them access to a much wider range of organisms, and induces a much lower prey-to-predator ratio. Moreover, salps, appendicularians and pyrosomes produce carcasses and fecal pellets that sink at extremely fast speeds (up to 1500 m/d, 10 times higher than copepods). This implies a rapid and efficient transfer of organic matter to depth. Although these organisms represent only a small proportion of the overall biomass, the induced flux of organic matter could be substantial. Current estimates, based on a very limited amount of observations, range between 0.01 PgC/year and 1 PgC/year from 100 to 1000m and are thus very uncertain. Here, we present an estimate of the influence of FFGZ on global marine biogeochemistry using the marine biogeochemical model PISCES. In our modeling framework, two processes characterize FFGZ: a preference for small prey organisms due to filter-feeding and the rapid sinking of carcasses and fecal pellets. Our simulated contribution of FFGZ to the total living marine biomass is less than 3%, but FFGZ-induced organic matter sinking at 1000m depth reaches 20%. Finally, we explore the impact of climate change on the role played by FFGZ in the marine ecosystem using 21st century climate change simulations.  As climate change is expected to benefit small-size phytoplankton in the coming decades, FFGZ could be favored over other groups of the same size range and hence would have an increasing role in sequestering carbon in the deeper ocean.

How to cite: Clerc, C., Aumont, O., and Bopp, L.: Current and future contribution of filter-feeding gelatinous zooplankton to global marine biogeochemistry, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8584, https://doi.org/10.5194/egusphere-egu22-8584, 2022.