Density currents represent the flow of one fluid driven by a difference in mass compared to the fluid it moves through. This movement occurs when water that is denser than its surroundings sinks and then spreads horizontally along the bottom surface. This circulation is fundamentally driven by gravity acting on these mass differences.
The Physics of Density-Driven Flow
Denser water carries more mass per unit volume and therefore sinks beneath less dense water, similar to how oil floats on water. This difference in density creates potential energy within the water column.
Gravity converts this potential energy into kinetic energy, resulting in a horizontal flow along the bottom boundary. The sinking motion, often called downwelling, feeds the horizontal current. Buoyancy acts as the counteracting force, pushing less dense water upward to replace the sinking mass.
The flow maintains its integrity because the density difference slows mixing with the surrounding ambient water. This allows the denser water to travel significant distances as a coherent current along the bottom. The flow is primarily horizontal, constrained by topography as the denser fluid seeks the lowest elevation.
Primary Variables That Alter Water Density
The density of water is determined by three main physical factors that create the necessary mass differences to drive these currents. Even small changes in density, often measured in hundredths of a gram per cubic centimeter, are enough to initiate large-scale circulation.
Temperature (Thermal Effects)
Temperature is a primary controller of water density, as colder water is significantly denser than warmer water because its molecules are packed more tightly together. When surface water cools, such as in polar regions during winter, its density increases, causing it to become unstable and sink. This process of cooling and sinking is one of the most powerful drivers of deep ocean circulation. Conversely, heating water causes it to expand and become less dense, encouraging it to remain near the surface layer.
Salinity (Salt Concentration)
Salinity, the concentration of dissolved salts, is a major factor influencing water density; water with higher salt content is denser than fresher water. Processes like evaporation remove pure water, leaving the salt behind and increasing the salinity, which makes the remaining surface water denser. When sea ice forms in polar regions, the salt is “rejected” and concentrated in the surrounding water, making it dense and causing it to sink rapidly. An increase of one part per thousand of salinity can have a density effect comparable to a temperature decrease of several degrees Celsius.
Suspended Sediment
The concentration of suspended particles, such as sand, silt, and clay, also increases water density, particularly in coastal environments. When water is heavily laden with sediment, it becomes heavier than the clear water around it. This effect is often seen near river mouths or on continental slopes where large amounts of material are introduced. This dense, turbid water mass then flows downslope under gravity, creating a sediment-driven current.
Major Types of Density Currents
These underlying variables combine to create two major types of density currents that operate in different environments and at vastly different speeds.
Thermohaline Circulation (Global Conveyor Belt)
Thermohaline Circulation is a slow-moving system of deep ocean currents driven by the combined effects of temperature and salinity. The process begins primarily in high-latitude regions, such as the North Atlantic and around Antarctica, where surface water is intensely cooled and concentrated with salt from ice formation. This water becomes so dense it sinks thousands of meters to the ocean floor, forming deep water masses like the North Atlantic Deep Water.
Once submerged, this dense water mass begins its slow journey across the global ocean basins, effectively acting as the “Global Conveyor Belt.” This deep flow is extremely slow, moving at speeds of only a few centimeters per second, and a full circuit can take about 1,000 years to complete. The circulation is crucial for distributing heat and nutrients around the planet, moderating global climate by transporting warm surface water poleward before it cools and sinks.
Turbidity Currents
Turbidity currents are fast, gravity-driven flows where the density difference is caused predominantly by a high load of suspended sediment. These currents are often described as underwater landslides, triggered by events like earthquakes, slope failures, or rapid sediment accumulation near the edge of a continental shelf. The high concentration of sediment gives the water enough density to accelerate rapidly downslope.
These currents can reach speeds of tens of kilometers per hour, making them far faster and more energetic than thermohaline flows. As they move, they possess enough erosive power to carve out large submarine canyons on continental slopes and deposit vast layers of sediment, known as turbidites, on the deep ocean floor. Turbidity currents are episodic and short-lived mechanisms of sediment transport in the ocean.