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Abstract. We examine the past and projected changes in Arctic sea ice properties in 6 climate models participating in the High Resolution Model Intercomparison Project (HighResMIP) in the Coupled Model Intercomparison Project Phase 6 (CMIP6). Within HighResMIP each of the experiments are run using a reference resolution configuration (consistent with typical CMIP6 runs) and higher resolution configurations. The role of horizontal grid resolution in both the atmosphere and ocean model components in reproducing past and future changes in the Arctic sea ice cover is analysed. Model outputs from the coupled historical (hist-1950) and future (highres-future) runs are used to describe the multi-model, multi-resolution representation of the Arctic sea ice and to evaluate the systematic differences (if any) that resolution enhancement causes. Our results indicate that there is not a strong relationship between the representation of sea ice cover and the ocean/atmosphere grid: the impact of horizontal resolution depends rather on the examined sea ice characteristic and the model used. However, the refinement of the ocean grid has a more prominent effect compared to the atmosphere: eddy-permitting ocean configurations provide more realistic representations of sea ice area and sea ice edge. All models project substantial sea ice shrinking: the Arctic loses nearly 95 % of sea ice volume from 1950 to 2050. The model selection based on historical performance potentially improves the accuracy of the model projections and predicts the Arctic to turn ice-free as early as in 2047. Along with the overall sea ice loss, changes in the spatial structure of the total sea ice and its partition in ice classes are noticed: the marginal ice zone (MIZ) dominates the ice cover by 2050 suggesting a shift to a new sea ice regime much closer to the current Antarctic sea ice conditions. The MIZ-dominated Arctic might drive developments and modifications of model physics and parameterizations in the new generation of GCMs.

<p>New and updated physics and parameterizations implemented in the NEMO ocean model from version 4 onwards are tested in a global eddying ocean/sea ice configuration, specifically the GLOB16 system. Such configuration is at the base of the operational short-term Global Ocean Forecast System (GOFS) adopted at the Euro-Mediterranean Center on Climate Change (CMCC) and uses a nonuniform tripolar grid with 1/16° horizontal resolution (corresponding to 6.9 km at the Equator) and 98 vertical levels. We performed a set of short-term simulations forced by the ECMWF operational atmospheric fields at 1/10° spatial resolution.</p> <p>Among all the recent functionalities of the NEMO model, this work focuses on the new features that could impact the ocean energy budget. The new formulation of tides, the parameterization of the mixing induced by breaking internal waves and the formulation of the surface wave-induced mixing are selected. Test simulations are compared against a control run employing a set of metrics computed on the global domain and regional ocean sectors. Additionally, model results are evaluated against available satellite estimates to provide a first validation of the variability of upper ocean energy budget.</p> <p>In the simulation in which the surface wave-induced mixing is included, external input forcings are needed to provide an accurate representation of the surface wave processes. Here, integrated wave parameters from WAVEWATCH III model feed the NEMO ocean model, in the forced mode.</p> <p>Our analysis shows that all new ocean implementations impact global and regional patterns of sea surface salinity and sea surface height; conversely, only enhanced surface mixing affects the sea surface temperature and the mixed layer depth. However, all experiments showed the tendency to reduce the surface and basin-averaged ocean energy with updated mixing processes.</p>

Flows past two spheres immersed in a horizontally moving, linearly-stratified fluid are investigated at a moderate Reynolds number of 300. Characterisation of flow patterns considers representative geometrical configurations defined by varying both the distance between the spheres and their relative orientation to the free stream direction. Simulations are performed on unstructured meshes chosen to accurately resolve the dynamics of fluids in regions close to the spheres for Froude numbers Fr ∈ [0.25, ∞]. Results illustrate the evolution of boundary layers, separation, and the wakes interaction under the influence of a gravity induced buoyancy force. Computations utilise a limited area, nonhydrostatic model employing Non-oscillatory Forward-in-Time (NFT) integration based on the Multidimensional Positive Definite Advection Transport Algorithm (MPDATA). The model solves the Navier-Stokes equations in the incompressible Boussinnesq limit, suitable for describing a range of mesoscale atmospheric flows. Results demonstrate that stratification progressively dominates the flow patterns as the Froude number decreases and that the interactions between the two spheres' wakes bear a resemblance to atmospheric flows past hills.

A numerical study of stably stratified flows past spheres at Reynolds numbers Re = 200 and Re = 300 is reported. In these flow regimes, a neutrally stratified laminar flow induces distinctly different near-wake features. However, the flow behaviour changes significantly as the stratification increases and suppresses the scale of vertical displacements of fluid parcels. Computations for a range of Froude numbers F r ∈ [0.1, ∞] show that as Froude number decreases, the flow patterns for both Reynolds numbers become similar. The representative simulations of the lee-wave instability at F r = 0.625 and the two-dimensional vortex shedding at F r = 0.25 regimes are illustrated for flows past single and tandem spheres, thereby providing further insight into the dynamics of stratified flows past bluff bodies. In particular, the reported study examines the relative influence of viscosity and stratification on the dividing streamline elevation, wake structure and flow separation. The solutions of the Navier-Stokes equations in the incompressible Boussinesq limit are obtained on unstructured meshes suitable for simulations involving multiple bodies. Computations are accomplished using the finite volume, non-oscillatory forward-in-time (NFT) Multidimensional Positive Definite Transport Algorithm (MPDATA) based solver. The impact and validity of the numerical approximations, especially for the cases exhibiting strong stratification, are also discussed. Qualitative and quantitative comparisons with available laboratory experiments and prior numerical studies confirm the validity of the numerical approach.