Ocean currents are essentially giant rivers of water within the ocean itself. Much like rivers, ocean currents can flow at varying speeds and differ in shape, volume, and even, temperature. These currents are driven by many factors, including wind, tides, changes in water density, salinity (salt content), temperature, and Earth's rotation.

Ocean currents fall into two primary categories - surface currents and deep currents. Surface currents are responsible for 10% of the ocean's volume whereas deep currents are responsible for 90%. We also classify ocean currents as warm currents and cold currents. Warm currents originate at the equator because these regions receive more direct sunlight (higher solar energy density). By contrast, cold currents originate near the poles. The warm currents transport warm water north or south to the lower latitudes and cold currents transport cold water towards the equator.

Global wind systems largely drive surface currents. For example, a prominent feature of ocean currents is big current loops called gyres. Gyres start in the tropics and move west, curve eastwards in the higher latitudes, and come back down to the equator, where they complete the loop. The gyres originate from powerful trade winds.

While most people would be alarmed to visit the beach and be met with an utterly still sea, we generally give little thought to how the oceans work. And yet, ocean currents play a critical role in maintaining the conditions of planet Earth as we know it.

Salinity can have a significant effect on deep ocean currents. We tend to think of ocean currents as rivers that move east or west around the planet (horizontal movement), but in some areas, the ocean currents also move up and down (vertical movement). For example, in the North Atlantic, which surrounds Iceland, Norway, and Greenland, the surface water sinks to form a "river" into the deep ocean. This happens because the surface water is denser than the adjacent layers down below. Denser means heavier, and heavier things sink. But why is the surface water denser in this area? Because it has a higher salt concentration. The water arriving in the North Atlantic originates from hotter climates where evaporation has taken place.

Now, let us dive deeper into how ocean currents affect the climate. As a definition, climate refers to the general atmospheric conditions in a location over a long period of time (typically 30 years or more). Several factors influence climate, including the distance from the equator, distance from the ocean, altitude, ocean currents, and geographic obstacles like mountains.

If you look at a map of ocean currents, you'll notice something striking. You can find deserts in almost all areas where cold currents flow. You can spot these on a satellite map because they will be brown or beige in colour, denoting an area of little vegetation. For example, if you look at southern Africa, you'll notice that the east coast is green and lush, leading to the tropical climate that we see in countries like Madagascar. However, the opposite is true on the west coast, with countries like Namibia and Angola largely consisting of deserts.

But why does this happen? The east is receiving a warm current from the Indian ocean, while the west coast is getting a cold current from the southern hemisphere. The temperature of the water directly impacts how much evaporation can happen. On the east coast, lots of evaporation occurs, promoting higher humidity in the coastal area. This humidity allows plant life to grow. However, on the west coast, barely any evaporation occurs, leaving the air dry and making it impossible for the coastal land to support a wide variety of life. 

Further Reading

https://www.nationalgeographic.org/topics/resource-library-ocean-currents/

https://www.youtube.com/watch?v=p4pWafuvdrY (Video)