In case the seas are partially or wholly enclosed and the exchange of water with Open Ocean is limited, salinity is affected to a great extent. It is interesting to note that if such a sea is located in a humid region, salinity is bound to be low, rather very low, because of abundant rainfall and large inflow of river water.
The Baltic Sea receives a lot of fresh water from the rivers falling into it. It is also characterised by low evaporation on its surface due to cool temperatures. Due to all these factors, it has a seasonal salinity variation from 1 to 2.5 percent.
On the other hand, where such seas with restricted exchange with the open ocean are situated in dry, hot regions, the rapidity of evaporation will naturally be high. Besides, there will be scanty rainfall and little stream flow.
So the salinity will be high, rather higher. As noted earlier, the Red Sea has an average salinity exceeding 40%o and the Mediterranean Sea has an average salinity ranging from 18 to 39%o subject to its proximity to the discharge of fresh water.