Ocean currents are continuous movements of seawater that flow through the global ocean. These flows are broadly categorized by the forces that drive them: wind or density. Surface currents are primarily driven by wind and the Coriolis effect, circulating in the upper layers of the ocean to depths of about 400 meters. Deep currents are fundamentally different because they are driven by the density of the water itself. These submerged, slow-moving streams distribute water masses across the major ocean basins far below the surface.
Characteristics of Deep Ocean Currents
Deep currents occur well below the ocean’s surface, extending from the base of the surface layer down to the abyssal plains. Their movement is governed by density gradients, a phenomenon known as pycnocline circulation. Seawater density is determined by its temperature and salinity; colder, saltier water is heavier than warmer, fresher water.
These currents reside primarily below the thermocline, the layer where temperature rapidly decreases with depth, usually around 200 meters. Compared to fast-moving surface currents, deep currents are incredibly slow, often moving at speeds of only a few centimeters per second. Despite this sluggish pace, they transport immense volumes of water, sometimes more than 100 times the flow of the Amazon River.
The Process of Thermohaline Circulation
The formation of deep currents is explained by Thermohaline Circulation, which derives its name from thermo (temperature) and haline (salinity). This process drives deep water to sink and begin its global journey, starting in high-latitude polar regions like the North Atlantic and the Southern Ocean.
Cold polar air rapidly cools the surface water, causing it to contract and become denser. Density also increases when sea ice forms on the surface, a process known as brine rejection. When seawater freezes, salt is largely excluded from the ice structure.
This rejected salt is released into the remaining unfrozen water nearby, significantly increasing its salinity. The combination of low temperature and high salinity creates water so dense it sinks rapidly toward the ocean floor, a process called downwelling. This cold, dense water then flows along the bottom of the ocean basin, initiating the deep current.
Specific areas such as the Greenland-Norwegian Sea and the Labrador Sea in the North Atlantic are major formation zones for this dense water mass. Surface water flows in to replace the sinking water, creating a continuous overturning motion. This sinking and replacement drives the circulation of water from the top to the bottom of the world ocean.
Mapping the Global Ocean Conveyor
Once the cold, dense water sinks, it begins its path along a vast, interconnected loop known as the Global Ocean Conveyor Belt. This system spans all major ocean basins, moving deep water from the Atlantic through the Southern Ocean and into the Pacific and Indian Oceans. For instance, deep water masses generated in the North Atlantic flow southward, eventually circulating around Antarctica.
This flow is incredibly slow; a single parcel of water may take hundreds to a thousand years to complete a full circuit. As the deep water travels, it slowly mixes with surrounding water and gradually becomes less dense. This less dense water eventually rises back toward the surface in a process known as upwelling.
Upwelling occurs broadly across the world ocean, but is particularly prominent in the Pacific and Indian Oceans. The surface currents then carry this warmer water back toward the North Atlantic to complete the cycle. This global circulation system involves the overturning of the entire world ocean.
Deep Currents and Planetary Influence
Deep currents regulate Earth’s climate and support marine ecosystems. They are essential to the global distribution of heat. The currents transport cold water away from the poles, allowing warmer surface water to flow back and moderate temperatures in higher latitudes.
The deep-water circulation also carries dissolved oxygen from the surface down to the deepest parts of the ocean, preventing the deep sea from becoming stagnant. Deep currents also influence nutrient cycling. As they travel, they accumulate nutrients from decaying organic matter that settles on the ocean floor.
When the deep water upwells, it brings these accumulated nutrients back to the surface. This influx supports the growth of phytoplankton, which form the base of the marine food web. The entire system plays a role in the long-term storage and cycling of carbon dioxide.