Oceans, currents, and climate

Oceans cover about 71% of the Earth's surface. They are not still pools of water — they are constantly moving, shifting huge amounts of heat from the warm tropics toward the cold poles. This great circulation of seawater shapes the climate of every continent. Without it, much of the world would be a far less liveable place.

The five oceans

• Pacific Ocean — the largest and deepest ocean, covering about a third of the planet. Stretches from Asia and Australia to the Americas.
• Atlantic Ocean — the second-largest. Separates the Americas from Europe and Africa.
• Indian Ocean — bounded by Africa, southern Asia, Australia, and Antarctica.
• Arctic Ocean — the smallest and shallowest, covered with sea ice for much of the year.
• Southern Ocean (or Antarctic Ocean) — surrounds Antarctica; officially recognized as a separate ocean since 2000.

Ocean currents

An ocean current is a continuous flow of water in a particular direction — like a river inside the sea. Surface currents are driven by the wind. Deep-ocean currents are driven by differences in temperature and saltiness — warm, salty water sinks and slides along the ocean floor, while colder lighter water rises elsewhere. Together, these surface and deep currents form one giant loop circling the planet, sometimes called the global ocean conveyor belt or the thermohaline circulation.

The Gulf Stream

The Gulf Stream is one of the most famous warm currents in the world. It carries enormous amounts of warm tropical water from the Gulf of Mexico north-east across the Atlantic toward Europe. The heat it releases keeps western Europe — Britain, Ireland, France, and Scandinavia — much milder than other places at the same latitude. London is at roughly the same latitude as cold Canadian provinces, but its winters are far gentler, thanks to the Gulf Stream.

El Niño and La Niña

Every few years, surface waters of the tropical Pacific become unusually warm. This is called El Niño ("the little boy" in Spanish, named because it often peaks around Christmas). El Niño weakens the trade winds, shifts rainfall patterns, and disrupts weather around the world — causing droughts in Australia and Indonesia, heavier rain in South America, and warmer winters in North America. The opposite phase, La Niña, brings unusually cold tropical Pacific water and roughly opposite effects.

Why this matters

Ocean currents distribute heat, set the climate of coastal regions, control rainfall patterns, and even influence where storms form. Warm currents flowing past cool coasts produce mild, rainy climates (like the British Isles). Cold currents flowing past warm coasts produce dry, foggy climates (like the Atacama Desert in Chile, beside a cold ocean current). Understanding currents means understanding why each part of the world has the climate it has — and why climate change matters so much when currents start to slow down.