Mixed layer depth in the Aegean, Marmara, Black and Azov Seas: Part I: General features

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.