Rivers inside the ocean
The ocean is not a still body of water. Large, organized flows called currents move water across thousands of kilometers, sometimes at the surface and sometimes far below it. They matter for climate because they transport heat. The tropics receive more energy from the Sun than the poles, and both the atmosphere and the ocean carry some of the excess toward higher latitudes. Water absorbs and holds a great deal of heat compared with air, one of the useful properties of water, so the ocean is a major store as well as a mover of heat.
There are two broad systems: wind-driven surface currents, and the slower circulation of the deep ocean, which is driven mostly by differences in density.
Surface currents
Surface currents are pushed primarily by prevailing winds, such as the trade winds near the tropics and the westerlies at mid-latitudes. Because the Earth rotates, moving water is deflected, to the right in the Northern Hemisphere and to the left in the Southern Hemisphere (the Coriolis effect). The shapes of the continents also redirect flow.
The result in each major ocean basin is a large circular pattern called a gyre. In the North Atlantic, for example, the gyre rotates clockwise, while in the South Atlantic it rotates counterclockwise. Each gyre has a western edge where the current is narrow, fast and warm, and an eastern edge where it is broader, slower and often cooler.
- Warm currents carry tropical water toward the poles. The Gulf Stream, which flows north along the eastern coast of North America before heading across the North Atlantic, is a well-known example. It is often credited with helping keep parts of northwestern Europe milder than other places at similar latitudes, though the atmosphere and prevailing winds also play a large part, and how much credit the current deserves is debated.
- Cold currents bring cooler water toward the equator along the western coasts of continents. Examples include the California Current and the Humboldt Current off South America. Cool coastal water tends to cool the air above it, which can make coasts foggy and dry.
Currents also affect storms. Warm water is part of what fuels tropical cyclones, and the paths of those storms often track along warm currents, though many other factors decide where they go.
Upwelling
In some places, winds push surface water away from the coast, and cold, nutrient-rich water rises from below to replace it. This is called upwelling. These nutrients support plankton, which in turn supports fish, birds and marine mammals, so many of the world’s richest fishing areas are in upwelling zones.
The deep ocean and the “conveyor belt”
Below the surface, water moves mainly because of differences in density. Cold water is denser than warm water, and salty water is denser than fresh water. In certain high-latitude regions, such as parts of the North Atlantic and around Antarctica, surface water becomes cold and salty enough, partly because sea ice formation leaves salt behind, to sink. It then spreads slowly through the deep ocean basins.
This sinking is balanced by upwelling elsewhere, which links deep and surface flows into a single global pattern, often described as the ocean’s “conveyor belt” or overturning circulation. The circuit is very slow, taking many centuries for a parcel of water to complete a full loop. The conveyor belt image is a simplification: the real picture involves many separate flows, eddies and mixing.
A part of this system in the Atlantic carries warm surface water northward and returns cold deep water southward. Scientists study whether it could weaken as the climate changes, and how much that would affect regional weather. This is an active area of research, and projections carry real uncertainty, so be cautious about dramatic claims either way.
Year-to-year patterns: El Nino and La Nina
Currents are not constant. In the tropical Pacific, the winds and ocean temperatures shift in a loose cycle every few years. During El Nino, warm surface water spreads farther east than usual and the usual upwelling off South America weakens. During La Nina, the opposite tends to occur. These swings are linked to changes in rainfall, drought and temperature across many regions, and they can influence global average temperature from year to year. The effects differ from place to place, and each event is a bit different, which is why forecasts are expressed as probabilities.
Why this matters
Currents shape regional climate, marine ecosystems, shipping routes and the spread of heat through the climate system. The ocean has absorbed a large share of the extra heat from the greenhouse effect, which is one reason ocean temperature is closely watched. Changes in currents can alter fish populations, coastal weather and extreme weather patterns, though pinning specific events on specific currents usually requires careful analysis.
Satellites help track sea surface temperature and height, and floating instruments measure temperature and salinity at depth. Combined, these tools give a more detailed picture than was possible a few decades ago, but the deep ocean remains harder to observe than the surface.
Ocean circulation also changes over geological time, as continents drift and seaways open or close, and it is one of the factors scientists consider when they study how Earth has moved between warm periods and ice ages.