Ocean currents explained: what an ocean currents map shows, the two types, and the climate link

Any ocean currents map you have seen is doing something slightly dishonest. It draws arrows on a flat surface, which suggests that the ocean moves the way a river does. It does not. There are at least two circulation systems stacked on top of each other, running on completely different timescales, and a single map cannot show both.

An ocean current is a continuous, directed movement of seawater. The forces behind it include wind, the Coriolis effect, breaking waves, cabbeling, and differences in temperature and salinity. On top of those, the shape of the seafloor, the configuration of coastlines and interactions between currents all bend the result. Movement happens horizontally on scales that span entire oceans, and vertically as upwelling and downwelling, which is how nutrients and dissolved gases including carbon dioxide travel between the surface and the deep.

What an ocean currents map actually shows

Almost always, the surface layer only. That is the part driven by wind, and it is the part that satellites can see, so it is the part that gets drawn.

The vocabulary on those maps is more precise than it looks. Currents get classified by temperature as warm or cold, and separately by velocity, size and direction as drifts, currents or streams. A drift, such as the North Atlantic Drift Current, is surface water moving forward under a prevailing wind. A current, such as the Labrador Current, has a more definite direction. The distinction is not decorative; it tells you how much of the motion is weather and how much is structure.

Flow is measured in sverdrups. One sverdrup, 1 Sv, is a million cubic metres per second. Keeping that unit in mind is the fastest way to stop reading an ocean currents map as a diagram of surface streaks and start reading it as a budget of volume.

The two types, and what drives each

There are two types of current, and the split is clean.

Surface currents are wind-driven. Because of the Coriolis effect they do not run with the wind but at an angle to it: the Ekman spiral produces the characteristic clockwise rotation in the Northern Hemisphere and the anticlockwise rotation in the Southern. The resulting gyres are not symmetrical. The branch flowing towards the equator on the eastern side is broad and diffuse, while the branch flowing towards the pole on the western side is narrow and fast. That asymmetry, called western intensification, is why the Gulf Stream exists as a distinct feature at all: a warm, swift current that leaves the Gulf of Mexico, passes through the Straits of Florida, runs up the eastern seaboard of the United States, then turns east near 36°N off North Carolina and heads for northwestern Europe as the North Atlantic Current.

Deep-water currents are density-driven. Water density depends on temperature and salinity together, and gradients in density set large volumes of water in motion without any wind involved.

Deep ocean currents, explained

Follow the Gulf Stream past the point where the maps usually stop. It carries warm water poleward, cools on the way, and eventually becomes dense enough to sink at high latitudes, forming North Atlantic Deep Water. That sinking is the entry point to the second system.

This is thermohaline circulation, from thermo for heat and haline for salt. It is popularly called the ocean conveyor belt or the global conveyor belt, a term coined by the climate scientist Wallace Smith Broecker. Specialists more often say meridional overturning circulation, or MOC, and the reason for the switch matters: the conveyor-belt picture implies one continuous global loop, whereas the circulation patterns produced by temperature and salinity are not necessarily a single circuit.

Deep ocean currents are also where the measurement problem becomes obvious. Continuous, full-depth, basin-wide observation of the overturning circulation has only been running since 2004, and the circulation itself turns over on a timescale measured in centuries. Two decades of data against a several-hundred-year cycle is a short record, and any statement about long-term trends in the deep has to be read with that in mind.

How do ocean currents affect climate?

The short answer is that they move heat, and heat is what climate is made of. Ask how do ocean currents affect climate and the mechanism is transport: a warm current running along a coast raises temperatures there, a cold one lowers them, and the effect can be large enough to decide what grows in a region.

Thermohaline circulation adds a second, slower channel by supplying heat to the polar regions, which in turn influences how much sea ice forms and persists. Because that limb runs on centuries rather than seasons, it is the part of the system where a change would be both slow to detect and slow to reverse.

There is a third contribution that has nothing to do with temperature at all. The vertical component, upwelling and downwelling, carries dissolved carbon dioxide between the surface and the deep. That makes circulation part of the carbon budget as well as the heat budget, and it is another reason why the question of how do ocean currents affect climate does not have a single tidy answer.

Why the biggest current is the one nobody draws

The largest ocean current on the planet is not the Gulf Stream. It is the Antarctic Circumpolar Current, also called the West Wind Drift, which runs clockwise as seen from the South Pole, west to east, all the way around Antarctica. Its mean transport is estimated at 137 ± 7 sverdrups, possibly more.

It exists in that form because there is no landmass connecting to Antarctica to interrupt it, and the consequence is not subtle: an unbroken circumpolar current keeps warm water away from the continent, which is a precondition for Antarctica holding the ice sheet it has. A single current is part of the reason the planet has the climate it does, and on most maps it is drawn as a few thin arrows at the bottom edge where the projection is worst.