Relative contribution of temperature and salinity to ocean acoustic reflectivity

Marine seismic data display laterally coherent reflectivity from the water column that is attributed to fine-scale oceanic layering. The amplitude of the different reflections is the expression of acoustic impedance contrasts between neighbouring water masses, and therefore water reflectivity maps the ocean's vertical sound speed and density (i.e., temperature and salinity) variations. Here we determine the relative contribution of each parameter by computing the temperature and salinity partial derivatives of sound speed and density, and using them to estimate reflection coefficients from a real oceanographic dataset. The results show that the mean contribution of density variations is 5-10%, while 90-95% is due to sound speed variations. On average, 80% of reflectivity comes from temperature contrasts. Salinity contribution averages 20%, but it is highly variable and reaches up to 40% in regions prone to diffusive convection such as the top of the Mediterranean Undercurrent in the Gulf of Cadiz.