How does salinity and density affects sea surface temperature?
A low salinity layer that is associated with low density (assuming the same temperature) will remain on top of the water column. As such the water will be able to get warmer and the difference in salinity and temperature will make it even more difficult to mix it down with the rest of the water column. So the combination of high temperature and low salinity results in even lower densities. You can play with the relationship using http://www.mt-oceanography.info/Utilities/density.html