Empirical relationships between salinity and temperature, which vary with longitude, latitude, and depth, have been identified from past data from conductivity-temperature-pressure (CTD) probes and from Argo profiling floats. These data have been interpolated to standard pressure levels and organized spatially so that regression models can be fitted to them at each level within sub-regions of the South Atlantic. A variety of models have been examined, with all combinations of temperature, temperature squared, longitude, and latitude used as regressors. Because of the sparsity of the data, evaluating which of these models should give the most accurate estimates of salinity is difficult.
Here are some links describing the steps followed in estimating salinity: