Using a global set of in situ temperature and salinity profile observations, the sonic layer depth (SLD) and the mixed layer depth (MLD) are analyzed and compared over the annual cycle. The SLD characterizes the potential of the upper ocean to trap acoustic energy in a surface duct while MLD characterizes upper ocean mixing. The SLD is computed from temperature and salinity profile pairs using a new tunable method while MLD is computed using recently developed methods and either temperature only profiles or temperature and salinity profile pairs. Both SLD and MLD estimates provide information on different and important aspects of the upper ocean. 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. The depth of this maximum sound speed is the SLD. The SLD and MLD are not always the same because sound speed is substantially more sensitive to temperature than to salinity compared to density. Since MLD is a commonly known and studied parameter, MLD is often used as a proxy for SLD in scientific and operational applications. In the boreal spring when fresh restratification events occur, the SLD is 10 m deeper (shallower) than the MLD in 39% (7%) of the observed profiles. A parabolic equation acoustic transmission model is used to evaluate the relative skill of the SLD and MLD estimates to predict surface acoustic trapping as measured by a simple metric.
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