Surface ocean currents, in the upper 1000 meters of the ocean, are driven by winds and are strongly influenced by change in density between layers of water. These ocean currents tend to follow the winds in the lower atmosphere, except where they are diverted by continents. Some wind-driven currents can even go upwind. An example is the North Equatorial Countercurrent.
Because temperature and salinity determine density, deep-water circulation is called thermohaline circulation.
Higher density water sinks into the deep ocean.
Mixing brings cold water up to the surface.
The thermohaline circulation is actually driven by mixing.
Wind force, friction, and Coriolis are three forces that act on water at the sea surface.
When the three forces balance, the current goes 45 degrees to the right looking downwind in the Northern Hemisphere, This is the Ekman current.
Current turns with depth. Each layer goes at an angle to the layer above it.
Each layer moves slower than the one above it.
Below the Ekman layer, friction is not important.
Ekman transport of surface layer water tends to produce sloping sea surfaces by "piling up" the water in some locations and "removing" surface water from others. This causes presssure gradients inside the ocean.
Inside the ocean the only important forces are pressure gradients and Coriolis force due to ocean currents.When the two forces balance, the current is a geostrophic current.
Upwelling occurs where Ekman transport moves surface waters away from a coast or away from nearby areas (a divergence zone). As the winds carry the surface waters away from a coast, cooler, often more nutrient-rich, water from below replaces the surface waters.
Downwelling occurs where Ekman transport pushes surface water together (a convergence zone).
The deep, thermohaline circulation connects all the oceans.
A "conveyor belt" model of this connection suggests that warm water from the Pacific and Indian Oceans is exported to the Atlantic Ocean along the surface, where it cools and sinks off Norway and Greenland, returning at depth back to the Pacific and Indian Oceans. As heat is lost to the atmosphere in the north, the colder water sinks below the warmer surface layer and migrates through the depths of the global oceans.
The entire circuit takes as long as 1,000 years to complete.
The heat lost in the far North Atlantic warms Europe. If the conveypor belt circulation weakens or stops, Europe and the Northern Hemisphere could be plunged into a new ice age.