Mixed layer depth in the Aegean, Marmara, Black and Azov Seas: Part II: Relation to the sonic layer depth

This paper provides the first analysis of the seasonal evolution of the sonic layer depth (SLD) relative to the mixed layer depth (MLD) for the Aegean, Marmara, Black, and Azov Seas. SLD identifies the acoustic ducting capabilities of the upper ocean and is of interest to investigations of upper ocean acoustics. A monthly SLD climatology on a regular 0.25° × 0.25° grid is constructed from interpolation of available quality-controlled ocean temperature and salinity profiles using the kriging methodology. A four step pre-processing procedure is designed to reduce noise and the effects of sampling irregularities. Monthly SLD fields are then compared relative to the much more widely studied MLD as computed using four different methods from the recent scientific literature. The goals of this analysis are to characterize the SLD relative to the MLD and provide a means for computing SLD from limited hydrographic information and/or MLD estimates. Very deep SLD values are found during winter, in the Aegean and Black Seas, when the near surface temperature values become lower than the temperature below the permanent pycnocline. When this occurs, the SLD drops to the bottom while the MLD remains much shallower at the seasonal pycnocline. For the months of May through October the SLD tends to be less than 25 m for the entire region. It is demonstrated that MLD obtained from the four methodologies have high correlations with SLD over the annual cycle, indicating a robust relationship. As a result, SLD can be estimated using least squares regression coefficients when salinity is unavailable or when observation profiles do not extend to deeper levels.