Vertical Distribution of Salinity

Salinity is not uniform from the surface to the ocean floor. It changes with depth, but the pattern of that change is not the same everywhere — it depends on where you are in the ocean. The key idea is that vertical distribution is controlled largely by the balance between evaporation and precipitation at the surface, and by the mixing of water layers.

The most important generalisation is this: salinity generally increases with depth in the higher latitudes, where surface salinity is low. In the middle and lower latitudes, the opposite happens — salinity decreases with depth. So the surface is not always the saltiest layer; in cold regions, the surface is actually fresher than the water below.

The reason lies in the surface processes. At high latitudes, evaporation is weak and precipitation is relatively high, so the surface water gets diluted and has lower salinity. This lighter, fresher water sits on top of the denser, saltier water beneath, giving a profile where salinity rises as you go down. In contrast, in the tropics and subtropics, strong evaporation removes fresh water from the surface, leaving it saltier. That dense, salty surface water sinks or mixes downward, so salinity is highest near the top and declines with depth.

A useful way to picture the vertical structure is to think of three broad depth zones:

• The surface layer, where salinity is most variable because it is directly affected by evaporation, rainfall, and river inflow.
• The intermediate layer, where salinity changes are more gradual and reflect the mixing of surface and deep waters.
• The deep layer, below about 1,500 metres, where salinity is remarkably uniform and shows very little change with depth.

This deep uniformity is a striking feature of the oceans. Below roughly 1,500 metres, temperature and salinity both become nearly constant, and the water column is essentially homogeneous. The vertical variation that exists is confined mostly to the upper layers of the ocean. …