Deep ocean currents are slow, subsurface movements of water that circulate heat, oxygen, and nutrients throughout the global ocean system. This continuous circulation is often called thermohaline circulation, a term highlighting the physical properties of seawater that drive these deep flows. The movement of these currents is fundamentally controlled by differences in water density, which is governed by three primary factors: temperature, salinity, and the resulting density. This global system redistributes heat across the planet and influences global climate patterns.
The Fundamental Driver of Deep Currents
Deep ocean currents are driven by buoyancy, where differences in water density cause heavier water masses to sink and lighter water masses to rise. Density is the mass of water contained within a specific volume, and it initiates deep circulation. Even minor variations in density cause massive volumes of water to move across the ocean basin. The denser a volume of water is, the more it is affected by gravity, causing it to sink through the water column.
The sinking of dense water, known as downwelling, powers the deep ocean currents. Once a water mass sinks, it pushes the water below it, initiating a slow, deep-water flow along the ocean floor. This flow results from gravity acting on the density differences created at the ocean surface. The rate and direction of these deep currents are determined by where and how water masses acquire high density through changes in temperature and salinity.
How Temperature Controls Ocean Density
The “thermo” component of thermohaline circulation refers to temperature, which has an inverse effect on water density. As surface water cools, molecules pack more closely together, causing the water to become denser. The colder the water becomes, the denser it gets, up to its freezing point. This process is most pronounced in high-latitude, polar regions, such as the North Atlantic and the Southern Ocean.
In these frigid environments, surface water loses heat to the atmosphere, cooling the water mass to near-freezing temperatures. This intense cooling increases the water’s density until it becomes heavier than the water layers below it. Once the surface water reaches this density threshold, it sinks rapidly into the deep ocean basin, initiating the formation of deep water masses like North Atlantic Deep Water. Without this cold-induced sinking in the polar regions, the deep ocean would remain largely stagnant.
The Impact of Salinity on Water Movement
Salinity, the “haline” component, is the second major factor that controls water density and influences deep current formation. Salinity is the measure of dissolved salts in seawater, and its relationship with density is direct: higher salinity results in higher density. This occurs because dissolved salts add mass to the water without significantly increasing its volume. Two primary processes at the ocean surface increase salinity and drive density changes.
One process is evaporation, which occurs in warm, arid regions like the subtropics. Water vapor leaves the surface, but dissolved salts are left behind. This concentration of salt increases the density of the remaining surface water, causing it to sink. The second, more intense process occurs in polar regions through brine exclusion. When sea ice forms, salt is largely rejected from the ice structure and forced into the surrounding liquid water.
Brine exclusion significantly increases the salinity and density of the water just beneath the newly formed ice. The resulting water mass is both extremely cold and highly saline, making it among the densest water found in the global ocean. This super-dense water sinks to the deepest parts of the ocean, providing a push for the entire deep ocean circulation system.
Mapping the Global Ocean Conveyor
The combined action of density, temperature, and salinity creates the continuous flow known as the Global Ocean Conveyor Belt, or the Meridional Overturning Circulation (MOC). This circulation system operates like an interconnected loop, driven by the sinking of dense water in the North Atlantic and around Antarctica. Warm, less-dense surface currents, such as the Gulf Stream, flow poleward, where they cool and become saltier, eventually sinking to form the deep, cold return flow.
The deep water masses travel slowly along the ocean floor, moving southward from the North Atlantic and eastward around Antarctica. This deep flow is slow; a single parcel of water takes an estimated 1,000 years to complete the global circuit. Eventually, this cold, deep water gradually rises back toward the surface in a process called upwelling, primarily in the Southern Ocean and parts of the Pacific. This continuous movement distributes heat globally, moderating climate, transports oxygen to the deep ocean, and brings nutrient-rich water back to the surface.