"Surface salinity" refers to the amount of salt or saltiness present in the top layer or uppermost part of a body of water like the ocean or a lake. Full definition
The system is vulnerable because even a relatively small decrease in surface salinity prevents water - no matter how cold it is - from sinking. (usask.ca)
The ocean surface salinity shown is much more variable than expected. (earthtimes.org)
Linsley's analysis resulted in the first multi-century record of sea surface salinity for the Makassar Strait. (ldeo.columbia.edu)