It leads to a very thorough mixing of the surface water; and hence determines salinity.
Among these movements are, first, the tidal currents, which everywhere surge backwards and forwards in the neighbourhood of the land; secondly, currents and drifts, which actually carry warm and saline waters from the equator to the polar region, from the western coasts to the eastern coasts, from open seas to enclosed seas, water from the bottom to the surface in some regions and causes surface water to sink to the bottom elsewhere.
This mixing, when it is permanent and long continued, would lead to an absolute uniformity of composition at the surface.
These factors thus cause mixing of water and changes in salinity distribution. However, in the open sea, this mixing is comparatively easier than in the enclosed seas.
The enclosed sea latter have lesser communication with the open sea and, therefore, salinity increases, e.g., the higher salinity of the Mediterranean, which is separated from the Atlantic Ocean by narrow Gibraltar strait.
On the other hand, those enclosed seas, which have a wide opening to connect them with the open sea, have variations in salinity depending on the currents.
For example, the waters of the Norwegian Sea and the North Sea are comparatively less saline due to the influx of the waters of the Gulf Stream.
Similarly, the Gulf of Mexico also has water of higher salinity, as warm and considerably saline equatorial water enters the region of sub-tropical dry lat itudes in the enclosed boundary.
The subsurface high "saline water outflowing from Mediterranean Sea also increases the salinity of the Bay of Biscay and the English Channel.