Mass flows of water, or currents, are essential to understanding how heat energy moves between Earth’s water bodies, landmasses, and atmosphere. The ocean covers 71 percent of the planet and holds 97 percent of its water, making the ocean a key factor in the storage and transfer of heat energy across the globe. The movement of this heat through local and global ocean currents affects the regulation of local weather conditions and temperature extremes, stabilization of global climate patterns, cycling of gases, and delivery of nutrients and larva to marine ecosystems.

Ocean currents are located at the ocean surface and in deep water below 300 meters (984 feet). They can move water horizontally and vertically, which occurs on local and global scales. The ocean has an interconnected current, or circulation, system powered by wind, tides, Earth’s rotation (Coriolis effect), the sun (solar energy), and water density differences. The topography and shape of ocean basins and nearby landmasses also influence ocean currents. These forces and physical characteristics affect the size, shape, speed, and direction of ocean currents.

Surface ocean currents can occur on local and global scales and are typically wind-driven, resulting in horizontal and vertical water movement. Horizontal surface currents that are local and typically short term include rip currents, longshore currents, and tidal currents. In upwelling currents, vertical water movement and mixing brings cold, nutrient-rich water toward the surface while pushing warmer, less dense water downward, where it condenses and sinks. This creates a cycle of upwelling and downwelling. Prevailing winds, ocean-surface currents, and the associated mixing influence the physical, chemical, and biological characteristics of the ocean, as well as global climate.

Deep ocean currents are density-driven and differ from surface currents in scale, speed, and energy. Water density is affected by the temperature, salinity (saltiness), and depth of the water. The colder and saltier the ocean water, the denser it is. The greater the density differences between different layers in the water column, the greater the mixing and circulation. Density differences in ocean water contribute to a global-scale circulation system, also called the global conveyor belt.

The global conveyor belt includes both surface and deep ocean currents that circulate the globe in a 1,000-year cycle. The global conveyor belt’s circulation is the result of two simultaneous processes: warm surface currents carrying less dense water away from the Equator toward the poles, and cold deep ocean currents carrying denser water away from the poles toward the Equator. The ocean’s global circulation system plays a key role in distributing heat energy, regulating weather and climate, and cycling vital nutrients and gases.

Transcript (English)

- [Narrator] As Ballard knows from decades on the ocean, it's an immensely complex system.

- The ocean is anything but static. If you were a water molecule and wanted to travel around the planet, it would be like getting in this rollercoaster. Lots of ups, lots of downs, and it's a massive amount of water that's moving.

- [Narrator] The ride begins north of Iceland, where cooling water sinks. The drop is slow at first, but as the current builds, it gains momentum. Through ridges and trenches, over lava fields and endless plains, the water hurdles southward across the equator, down the coast of South America, like a giant deep-sea river.

- [Robert] By the time this current reaches the Southern Ocean, it gets massive. There's no landmass to block it, so it builds and builds, and ultimately flows with the volume of 100 Amazon Rivers.

- [Narrator] At the bottom of the world, the Southern Ocean acts like a giant blender, mixing the world's seawater.

- [Robert] Then it comes back north again, and when it gets to the tropics, it starts to warm up and begins to rise to the surface. It becomes the Gulf Stream now and starts heading back home. The whole circuit takes a thousand years.

- [Narrator] A thousand years, and yet the continual cycling of all that heat and energy affects everything from long-term climate change to daily weather forecasts.

- [Robert] We have water rising, falling, spinning, circulating, bumping into things, and we try to take all of those complex factors, put 'em into a global model, and come up with a circulation system, our best shot.

Transcripción (Español)

- [Narrador] Como Ballard sabe por décadas en el océano, es un sistema inmensamente complejo.

- El océano es cualquier cosa menos estático. Si fueras una molécula de agua y quisieras viajar alrededor del planeta, sería como subirse a esta montaña rusa. Muchos altibajos, muchas caídas, y es una cantidad masiva de agua que se mueve.

- [Narrador] El viaje comienza al norte de Islandia, donde el agua fría se hunde. La caída es lenta al principio, pero a medida que la corriente se forma, gana impulso. A través de crestas y trincheras, sobre campos de lava y llanuras interminables, el agua se precipita hacia el sur cruzando el ecuador, por la costa de Sudamérica, como un gran río de aguas profundas.

- [Robert] Para cuando esta corriente llega al Océano Austral, se vuelve masiva. No hay masas de tierra que la bloqueen, así que se acumula y acumula, y finalmente fluye con el volumen de 100 ríos Amazonas.

- [Narrador] En el fondo del mundo, el Océano Austral actúa como una licuadora gigante, mezclando el agua de mar del mundo.

- [Robert] Luego regresa hacia el norte, y cuando llega a los trópicos, comienza a calentarse y empieza a subir a la superficie. Ahora se convierte en la Corriente del Golfo y comienza a regresar a casa. Todo el circuito toma mil años.

- [Narrador] Mil años, y sin embargo, el ciclo continuo de todo ese calor y energía afecta todo, desde el cambio climático a largo plazo hasta los pronósticos del clima diario.

- [Robert] Tenemos agua subiendo, bajando, girando, circulando, chocando con cosas, y tratamos de tomar todos esos factores complejos, ponerlos en un modelo global, y crear un sistema de circulación, nuestro mejor intento.