International audienceWe consider air–sea interaction at the (atmospheric) synoptic and the mesoscaledue to momentum transfer only. Two superposed one-layer fine-resolution shallow-watermodels are numerically integrated, where the upper layer represents the atmosphere and thelower layer the ocean. The frictional force between the two layers is implemented using aquadratic drag law and experiments with different values of the surface drag coefficient areperformed. The actual energy loss of the atmosphere and the energy gain by the ocean, dueto the interfacial shear, is determined and compared to estimates based on average speeds.The correlation between the vorticity in the atmosphere and the ocean is determined. Resultsdiffer from previous investigations where the exchange of momentum was considered atbasin scale. It is shown that the ocean has a passive role, absorbing kinetic energy at nearlyall times and locations, results showing that the energy input to the ocean increases almostquadratically with the value of the drag coefficient. Due to the feeble velocities in the ocean,the energy transfer depends only weakly on the oceanic velocity. The ocean dynamics leavenevertheless their imprint on atmospheric dynamics, leading to a quenched disordered stateof the atmosphere–ocean system for the highest value of the drag coefficient considered.This finding questions the ergodic hypothesis for the idealized configuration studied here.The ergodic hypothesis is at the basis of a large number of experimental, observational andnumerical results in ocean, atmosphere and climate dynamics
A new mechanism that induces barotropic instability in the ocean is discussed. It is due to the air-sea interaction with a quadratic drag law and horizontal viscous dissipation in the atmosphere. The authors show that the instability spreads to the atmosphere. The preferred spatial scale of the instability is that of the oceanic baroclinic Rossby radius of deformation. It can only be represented in numerical models, when the dynamics at this scale is resolved in the atmosphere and ocean. The dynamics are studied using two superposed shallow water layers: one for the ocean and one for the atmosphere. The interaction is due to the shear between the two layers. The shear applied to the ocean is calculated using the velocity difference between the ocean and the atmosphere and the quadratic drag law. In one-way interaction, the shear applied to the atmosphere neglects the ocean dynamics; it is calculated using the atmospheric wind only. In two-way interaction, it is opposite to the shear applied to the ocean. In one-way interaction, the atmospheric shear leads to a barotropic instability in the ocean. The instability in the ocean is amplified, in amplitude and scale, in twoway interaction and also triggers an instability in the atmosphere.
<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&#176; 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&#176; 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>
<p>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&#176; resolution and are forced by ECMWF 1/10&#176; 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&#8211;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.</p>
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