Types of Ocean Currents

Ocean currents are not a single, uniform movement of water. They can be sorted in two main ways: by how deep they sit in the ocean, and by the temperature of the water they carry. Each classification tells you something different about how the ocean works, and together they explain the global pattern of water movement.

Classification by depth

Look at the ocean as two stacked layers. The top layer, called surface currents, holds only about 10 per cent of all the water in the ocean. This is the upper 400 metres of the water column — the part that feels the wind and the sun. The remaining 90 per cent of ocean water sits below this, moving as deep water currents. These deep waters do not drift about randomly; they circulate around the ocean basins because of variations in density and gravity. The driving force is temperature: at high latitudes, the cold air chills the surface water enough to make it denser, and that dense water sinks deep into the ocean basins. Once down there, gravity pulls it along the seafloor, and density differences keep it moving from region to region. So while surface currents are largely wind-driven, deep currents are density-driven.

Classification by temperature

The second way to classify currents is by whether they carry warm or cold water.

Cold currents bring cold water into warm water areas. Their typical location follows a clear rule: they are found on the west coast of continents in the low and middle latitudes, and this holds true in both hemispheres. In the Northern Hemisphere, they also appear on the east coast in the higher latitudes. So a cold current is not defined by being cold in absolute terms — it is cold relative to the water it flows into.

Warm currents do the reverse: they bring warm water into cold water areas. They are usually observed on the east coast of continents in the low and middle latitudes, again true in both hemispheres. In the Northern Hemisphere, warm currents are also found on the west coasts of continents in high latitudes.