OS4.6

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 21^st 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.