"Barrier conditions the water", this is my own statement, as I understood the claim has to be the case. since two different liquids that have different densities, do not mix right away in the normal circumstances. so I looked at it this way.
A-Sea one
B- small sea; barrier; different density from A and C, both is feeding it with the different salinities
C-Sea two; has different density then A.
So B will always be in equilibrium. making the water coming from the Sea with less salinity slightly more dense, and the water coming from the sea with high salinity slightly less dense. ( This is what I mean by barrier conditions the water).
if you look at it this way, then the Sea A actually is not mixing with sea C, but Both A and C are mixing with B; which has different body of water; different characteristic of salinity, density, etc. . This way both Sea A and C, keep their own characteristic. yet you see them meet, due to the barrier.
if I look at the mediterranean Sea and the Atlantic Ocean, this is the only way I can think of why is the Atlantic Ocean not changing the characteristic of The Mediterranean Sea. of course, this is my own understanding.