There are warm surface currents and cold surface currents. The warm currents are driven from the tropics by winds and the rotation of the Earth to places of greater latitude away from the equator. The Western Boundary Current is an example of a warm current. Cold surface currents flow from the poles towards the equator bringing cooler surface water to the warm tropical waters.
Deep-ocean currents are driven by water density, salinity and temperature levels. The deep-ocean currents form after evaporation and the salinity levels become denser, forcing the surface water to the bottom of the ocean and thus moving bodies of deep-ocean water in deep under ocean-river formations.
Perhaps the most famous warm-water-surface current is the Gulf Stream, which flows like a river in the ocean traveling thousands of miles separating the vast body of the open ocean from the coastal waters. Ships can travel in the strong current of the Gulf Stream using less fuel as they ride the current. The Gulf Stream is vast; it carries 4,500 times more water than the Mississippi river.
The strong surface current driven by the wind and atmospheric pressure bring the nutrient-rich cold waters to the surface of the ocean, providing nutrition like plankton in plentiful amounts. For this reason whales follow the upwelling currents in search of plankton and other small fish that they eat. Fishermen also know where to look for large shoals of fish by locating an upwelling that becomes a feeding ground for many species, arriving in large shoals.
All ocean currents travel in either a clockwise or counterclockwise direction, depending which hemisphere they are in. This is because of the Coriolis Effect and is the same thing that makes hurricanes turn in one direction or the other. The ocean currents, likewise, form large circular patterns called gyres, which means swirling vortex. Ocean gyres can be thousands of kilometers across and each major ocean on the planet has a large gyre named after that ocean.
