The ocean is always moving and this mass movement of water is referred to as a current. Some might think of a current as a river within the ocean. Solar heating, winds, gravity, and the Coriolis Effect influence Ocean currents. Within the ocean, there are surface currents, deep water currents, upwellings, and density currents.
Surface currents move water horizontally. Surface currents are powered by the wind creating large circular patterns. The winds that drive the surface currents are the Westerlies and the Trade Winds. The Westerlies blow west to east (40-50o latitudes). The Trade Winds blow east to west (20o latitudes). Surface currents only move the surface water, which is only a few hundred meters of the seawater. The lower layers of the water move slower due to the friction between each layer of water. The Gulf Stream is an example of a surface current that is used by ships to quickly travel over the oceans.
Deep water currents move 90% of the ocean water. Deep ocean currents are driven by density and temperature. These currents flow under the surface of the ocean, and are hidden from immediate detection. These are currently being researched by floating devices, which maintain their depth according to slightly differing densities of waters.
Upwelling and downwelling areas in the oceans are areas where the water moves vertically. Upwelling is the movement of deep cold water to the surface of the ocean. The movement of this water brings up high concentrations of nutrients from organisms from deep water and allows fish to flourish. Along the coast of Oregon, Washington and Peru, upwellings bring fish to make for good fishing areas.
Density currents occur deep in the ocean between different masses of seawater. When dense seawater sinks, the less dense seawater mixes. Density currents occur when dense sea water sinks under less dense sea water, mixing surface water further into the ocean.