Abstract
Abstract. A twin numerical experiment was conducted in the seas around the island of Sardinia (Western Mediterranean) to assess the impact, at regional and coastal scales, of the use of relative winds (i.e., taking into account ocean surface currents) in the computation of heat and momentum fluxes through standard (Fairall et al., 2003) bulk formulas. The Regional Ocean Modelling System (ROMS) was implemented at 3 km resolution in order to well resolve mesoscale processes, which are known to have a large influence in the dynamics of the area. Small changes (few percent points) in terms of spatially averaged fluxes correspond to quite large differences of such quantities (about 15 %) in spatial terms and in terms of kinetics (more than 20 %). As a consequence, wind power input P is also reduced by ~ 14 % on average. Quantitative validation with satellite SST suggests that such a modification of the fluxes improves the model solution especially in the western side of the domain, where mesoscale activity (as suggested by eddy kinetic energy) is stronger. Surface currents change both in their stable and fluctuating part. In particular, the path and intensity of the Algerian Current and of the Western Sardinia Current (WSC) are impacted by the modification in fluxes. Both total and eddy kinetic energies of the surface current field are reduced in the experiment where fluxes took into account the surface currents. The main dynamical correction is observed in the SW area, where the different location and strength of the eddies influence the path and intensity of the WSC. Our results suggest that, even at local scales and in temperate regions, it would be preferable to take into account such a contribution in flux computations. The modification of the original code, substantially cost-less in terms of numerical computation, improves the model response in terms of surface fluxes (SST validated) and it also likely improves the dynamics as suggested by qualitative comparison with satellite data.
Highlights
The assessment of the fluxes at the air/sea interface is an issue of crucial relevance for many topics in geophysics
The wind stress (τ ) in ocean models, when not directly provided by atmospheric forecasts, is usually computed through the so-called bulk formulas as described by Fairall et al (1996), where τ is equal to the square of the wind speed at 10 m times the air density multiplied by a dimensionless drag coefficient
Bulk Fluxes with Currents (BFC) setup shows, as the simulation progresses, slightly better performances than Bulk Fluxes (BF) in terms of BIAS and Root Mean Square Error (RMSE) metrics, while almost no differences are recorded for Anomaly Correlation Coefficient (ACC)
Summary
The assessment of the fluxes at the air/sea interface is an issue of crucial relevance for many topics in geophysics. The wind stress (τ ) in ocean models, when not directly provided by atmospheric forecasts, is usually computed through the so-called bulk formulas as described by Fairall et al (1996), where τ is equal to the square of the wind speed at 10 m times the air density multiplied by a dimensionless drag coefficient (usually function of wind speed). The contribution of the ocean currents in the computation of the wind stress has been neglected in ocean modelling for many years. The fastest ocean current is 1–2 order of magnitude smaller than the stronger wind: for this reason the surface current contribution was often neglected in applying bulk formulas, even if an estimation of surface currents is often available from the ocean model itself. Considering that the computation of the wind stress contains a squared velocity term, it can be un-
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