Subsurface current

A subsurface current is an oceanic current that runs beneath surface currents. Examples include the Equatorial Undercurrents of the Pacific, Atlantic, and Indian Oceans, the California Undercurrent, and the Agulhas Undercurrent, the deep thermohaline circulation in the Atlantic, and bottom gravity currents near Antarctica. The forcing mechanisms vary for these different types of subsurface currents.

The most common of these is the density current, epitomized by the Thermohaline current. The density current works on a basic principle: the denser water sinks to the bottom, separating from the less dense water, and causing an opposite reaction from it. There are numerous factors controlling density.

One is the salinity of water, a prime example of this being the Mediterranean/Atlantic exchange. The saltier waters of the Mediterranean sink to the bottom and flow along there, until they reach the ledge between the two bodies of water. At this point, they rush over the ledge into the Atlantic, pushing the less saline surface water into the Mediterranean.

Another factor of density is temperature. Thermohaline (literally meaning heat-salty) currents are very influenced by heat. Cold water from glaciers, icebergs, etc. descends to join the ultra-deep, cold section of the worldwide Thermohaline current. After spending an exceptionally long time in the depths, it eventually heats up, rising to join the higher Thermohaline current section. Because of the temperature and expansiveness of the Thermohaline current, it is substantially slower, taking nearly 1000 years to run its worldwide circuit.

One factor of density is so unique that it warrants its own current type. This is the turbidity current. Turbidity current is caused when the density of water is increased by sediment. This current is the underwater equivalent of a landslide. When sediment increases the density of the water, it falls to the bottom, and then follows the form of the land. In doing so, the sediment inside the current gathers more from the ocean bed, which in turn gathers more, and so on. As a limited amount of sediment can be carried by a certain amount of water, more water must become laded with sediment, until a huge, destructive current is washing down some marine hillside. It is theorized that submarine depths, such as the Marianas Trench have been caused in part by this action. There is one additional effect of turbidity currents: upwelling. All of the water rushing into ocean valleys displaces a significant amount of water. This water literally has nowhere to go but up. The upwelling current goes almost straight up. This spreads the nutrient rich ocean life to the surface, feeding some of the world’s largest fisheries. This current also helps Thermohaline currents return to the surface.

...
Wikipedia