A global tuning experiment for the semidiurnal tide is performed with a barotropic model. The model is forced with the M 2 equilibrium tide and accounts for the self-attraction and loading (SAL) term. In addition to a quadratic drag, various linear internal wave drag terms adjusted by a scale factor of Oð1Þ are applied. The drag terms include the original Nycander (2005) tensor scheme, the Nycander tensor scheme reduced at supercritical slopes, and their scalar sisters, a Nycander scalar scheme computed for additional abyssal hill roughness, and the Jayne and St. Laurent (2001) scalar scheme. The Nycander scheme does not have a tunable parameter, but to obtain the best tidal solutions, it is demonstrated that some tuning is unavoidable. It is shown that the scalar Nycander schemes yield slightly lower root-mean square (RMS) elevation errors vs. the data-assimilative TPXO tide model than the tensor schemes. Although the simulation with the optimally tuned original Nycander scalar yields dissipation rates close to TPXO, the RMS error is among the highest. The RMS error is lowered for the reduced schemes, which place relatively more dissipation in deeper water. The inclusion of abyssal hill roughness improves the regional agreement with TPXO dissipation rates, without changing the RMS errors. It is difficult to have each ocean basin optimally tuned with the application of a constant scale factor. The relatively high RMS error in the Atlantic Ocean is reduced with a spatially varying scale factor with a larger value in the Atlantic. Our best global mean RMS error of 4.4 cm for areas deeper than 1000 m and equatorward of 66°is among the lowest obtained in a forward barotropic tide model.
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Climatological fields of mixed layer depth (MLD) are presented over the Aegean, Marmara, Black and Azov Seas. Monthly fields of MLD are formed by historical individual temperature and salinity profiles from combination of various data sets with additional quality control procedures applied. Various definitions that are based solely on temperature (T ) or those that include the impact of salinity (S) are applied to investigate the robustness in the pattern and values of the MLDs. Interpolation of the MLD fields to a 0.25 • × 0.25 • regular grid over the region is accomplished using a combination of median filter and ordinary kriging. Strong seasonal variability is noted in all regions. Given a density-based MLD criterion that includes both T and S, deep mixed layers (> 200 m) are noted in the Aegean Sea, especially eastern part of the region during winter while MLDs are generally much shallower (< 60 m) in the Black Sea. A criterion based on curvature method, which determines MLD according to first maximum of curvature of T may fail in representing deep MLDs in the Aegean Sea when the water column is well mixed. MLD fields obtained from all definitions are found to be strongly correlated to each other over the seasonal cycle, confirming the strong seasonal cycle. While the curvature method gives shallow MLDs only during winter, it has relatively large skill in comparison to the density-based MLD criterion. In general, MLD fields suffer from lack of input T and S profiles in the Marmara and Azov Seas, thus they may not be representative. Monthly MLD fields presented in this paper are available for various applications, such as mixed layer studies, ocean biology, ocean modeling and acoustic propagation.
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