Deep Ocean currents

Ocean surface currents affect only the top 100 m or so of the ocean. Deep currents involve the whole ocean. Deep currents are set in motion by differences in the density of sea water. They move only a few metres a day.

Most deep currents are called thermohaline circulations because they depend on the water's temperature ("thermo") and salt content ("haline"). If seawater is cold and salty, it is dense and sinks.

Typically, dense water forms in the polar regions. Here the water is cold and weighed down by salt left behind when sea ice forms. Dense polar water sinks and spreads out towards the Equator deep below the surface.

Oceanographers call dense water that sinks and starts deep ocean currents "deep water". In the Northern Hemisphere the main area for the formation of deep water is the North Atlantic.

Dense salty water from the Mediterranean pours deep down very fast - 1m/sec - through the Straits of Gibraltar to add to the North Atlantic deep water.

There are three levels in the ocean:

• the "epilimnion" - the surface waters warmed by sunlight, up to 100-300 m down;
• the "thermocline" - where it becomes colder quickly with depth;
• the "hypolimnion" - the bulk of deep, cold ocean water.