Mean ocean salinity is 35‰ (35 grams of salt per litre). It ranges from <30‰ in rainy/ice-melt areas (Baltic, Arctic) to >40‰ in enclosed seas with high evaporation (Mediterranean: 38‰, Red Sea: 40‰).
Salinity and temperature determine water density, the driver of the thermohaline circulation. In the North Atlantic, water cools and becomes saltier (through evaporation and ice formation), becomes dense and sinks, feeding the deep current of the AMOC. Greenland ice melting injects fresh water that reduces salinity and could weaken this circulation.