The ocean waters move about the globe in great “rivers” called ocean currents. Surface currents have a big effect on the world’s climates. They make polar regions milder and the tropics cooler. There are two kinds of ocean currents. Surface currents are driven by the winds and move in roughly circular patterns, called gyres. These go round in a clockwise direction in the Northern Hemisphere and anticlockwise in the Southern Hemisphere. The second kind, deepwater currents, are driven by differences in density (the colder and saltier the water is, the greater its density).

In the Northern Hemisphere, the oceanic gyres rotate in a clockwise direction, causing the Gulf Stream–North Atlantic–Norway Current in the Atlantic Ocean and the Kuroshio–North Pacific Current in the Pacific Ocean. In the Southern Hemisphere the anticlockwise circulation of the gyres creates the Peru (Humboldt) Current off South America and the Benguela Current off southwestern Africa. Southern Hemisphere currents are also influenced by the powerful eastwards-flowing Antarctic Circumpolar Current: both the Humboldt and Benguela currents draw water from it.
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Influence on climate

Warmed by the Sun, surface waters flow in currents from the tropics towards the poles, while cold currents move in to take their place. In this way, ocean currents influence climate zones and weather patterns around the world. For example, warm currents travelling along more temperate coasts increase temperatures of coastal areas—sometimes reaching some distance inland—by warming the sea breezes that blow over them. Examples of this are the Gulf Stream (see below) and the Humboldt Current, which brings a cooler subtropical climate to Lima, Peru, in contrast with that of its surrounding tropical latitudes.
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The Gulf Stream is a warm Atlantic Ocean surface current. It originates in the Gulf of Mexico and flows northeastwards, following the eastern coastlines of the United States and Newfoundland before heading out across the Atlantic Ocean. There it splits in two: the Canary Current turns south while the North Atlantic Current (or Drift) streams towards Europe. The current splits again, with one branch, the Norwegian Current, flowing past Scotland and up the coast of Norway.

​​​​​​​The Gulf Stream brings mild, wet weather to northwestern Europe and makes this region much more temperate for its high latitude than other areas at the same latitude (for example, Labrador in Canada).
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In the Earth's polar regions, ocean water becomes very cold, forming sea ice. As a result, the surrounding seawater gets saltier. This is because when ice forms, the salt is left behind in the liquid. As the seawater gets both colder and saltier, its density increases, causing it to sink. Warmer water flows in to balance out the missing surface water. This, in turn, becomes colder and saltier enough to sink.
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The force of the sinking water pushes the North Atlantic Deep Water southwards towards Antarctica in a deepwater current. From here, some of it flows north into the Indian and Pacific Oceans. Here the water eventually becomes warm enough to rise.

​​​​​​​The warm surface waters are then driven by winds. Some travels back northwards through the Atlantic Ocean to the North Atlantic Deep Water, completing a global loop. The rest joins the Antarctic Circumpolar Current, a surface current travelling clockwise around Antarctica driven by powerful westerly winds.

The Gulf Stream and its northern extensions form part of the Atlantic Meridional Overturning Circulation (AMOC), a link in the global thermohaline circulation. The AMOC carries water northwards into the North Atlantic Ocean and back again. When the warm water reaches Arctic regions, it cools, becomes denser and sinks before flowing southwards as a deepwater current.

The cooling process has, in recent years, become severely hampered by global warming, which has caused water temperatures in the Arctic to rise. At the same time, melting ice from the Greenland ice sheet tips fresh water into the sea. This is less dense than salty seawater, so the disruption to the AMOC is even greater.
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