Observations on the relationship between scattering layer and mixed layer

High-resolution (stations at every 3 nautical mile interval) physical oceanographic measurements were carried out in the shallow waters (<150 m depth) of northeastern Arabian Sea during December 2003. Temperature and salinity data showed tri-layer structure, i.e. a strong thermocline sandwiched between an upper and a bottom homogeneous layer. Thickness of the surface homogeneous layer varied between 20 and 60 m, while that of the bottom layer varied from 23 to 85 m. Thermocline thickness varied from 7 to 20 m and the respective gradients from 0.12 to 0.4°C/m, with maximum gradient coinciding with minimum thickness. The temperature and salinity structure revealed two distinct water-masses in the surface and bottom homogeneous layers, with a strong thermocline in the transition zone. The dynamically unstable conditions in the upper and lower homogeneous layers and highly stable conditions in the thermocline result in the formation of the tri-layer structure in this region. When the thermocline was strong, the scattering layer was well demarked, whereas the layer was not well demarked when the thermocline was weak. Gradient in the thermocline is found to restrict the downward transfer of scatterers across the thermocline. This information can be utilized to estimate the depth of the surface mixed layer from echograms with a good degree of accuracy.