The ocean’s water column is in constant motion. Surface currents, which generally extend to about 400 meters, are largely the result of global wind patterns pushing water across the sea surface. These currents, like the Gulf Stream, move quickly, distributing heat across the globe. Below this wind-driven layer, deep ocean currents circulate water throughout the rest of the ocean basin. Unlike surface movements, deep currents are powered by differences in the water’s density. This density-driven circulation regulates global climate over long timescales.
Density as the Foundation of Deep Water Movement
The circulation of the deep ocean is governed by density stratification. Water masses naturally arrange themselves in layers, with denser water sinking toward the ocean floor and less dense water remaining closer to the surface. The entire process is referred to as Thermohaline Circulation (THC), a name that describes the two variables controlling seawater density: temperature (“thermo”) and salt content (“haline”).
Seawater density is inversely related to temperature, meaning colder water is denser than warmer water. Similarly, the concentration of dissolved salts affects density, so saltier water is denser than fresher water. When a parcel of ocean water becomes both sufficiently cold and sufficiently salty, its density increases enough to become unstable relative to the water beneath it. This denser water mass then begins to sink, initiating the downward movement that drives the deep ocean currents.
How Temperature and Salinity Create Deep Water
The formation of the densest water masses occurs predominantly in the polar regions. Temperature plays a direct role as warm surface water flows poleward and encounters frigid polar air. The intense cooling of the surface layer increases density. However, temperature alone is often not enough to cause water to sink thousands of meters to the abyssal plain.
The second factor, salinity, is amplified through a process known as brine rejection. When seawater freezes to form sea ice, the salt content is expelled into the remaining liquid water immediately below the ice. This creates a residual water mass that is near-freezing and hypersaline, or exceptionally salty.
The combination of extreme cold and heightened salinity results in a water mass denser than any other in the ocean. This dense water then sinks rapidly in deep convective plumes. The two most important deep water masses formed this way are the North Atlantic Deep Water (NADW) and the Antarctic Bottom Water (AABW).
Mapping the Global Deep Circulation (The Conveyor Belt)
The deep water masses formed by temperature and salinity gradients are organized into a system often referred to as the Global Conveyor Belt. This global circuit is also known as the Meridional Overturning Circulation (MOC). The circulation begins in high-latitude sinking zones, such as the North Atlantic and the Weddell Sea off Antarctica.
Once the dense water sinks, it flows slowly along the ocean floor, spreading across the ocean basins. Water from the North Atlantic, for example, flows southward, eventually joining the flow around Antarctica and branching into the deep Indian and Pacific Oceans. A single water parcel may take approximately 1,000 years to complete its journey through this deep-sea pathway.
To maintain this circulation, the cold, dense water that sinks must eventually return to the surface. This return flow is accomplished through upwelling, a process where deep water rises. While some upwelling occurs globally through gradual mixing, large-scale upwelling is often concentrated in areas far from the sinking sites, particularly in the Southern Ocean. Here, strong winds interact with the ocean’s layers to bring the nutrient-rich deep water back toward the surface, completing the global loop.