A study on the sonic layer depth and the cutoff frequency in the East Sea/Japan Sea

The sonic layer depth (SLD) is the acoustical equivalent of the mixed layer depth (MLD), although they are defined differently. The SLD characterizes the potential of the upper ocean to trap acoustic energy in a surface duct while the MLD characterizes upper ocean mixing. The SLD and MLD often coincide because sound speed increases with depth down to the MLD, where (typically) a decrease in temperature occurs, resulting in a local maximum sound speed. However, the SLD and MLD are not always the same because sound speed is substantially more sensitive to temperature than to salinity compared to density. Using an available set of temperature-salinity observation profiles, the SLD and the MLD are analyzed and compared over the annual cycle in the East Sea/Japan Sea (East Sea). The both variability in the East Sea can be affected by basin-scale circulation patterns, boundary currents, meso-scale eddy activity, deep-water formation, and pronounced monsoons. In particular, geographic location and the high spatio-temporal variability of atmospheric forcing are most likely to control the distributions in the East Sea. A parabolic equation transmission model is used to evaluate the cutoff frequency trapped in surface duct.