Deep ocean currents represent large-scale movements of water far below the ocean’s surface. Understanding these deep currents is important for comprehending global processes, including the regulation of climate and the support of diverse marine ecosystems. These submerged rivers of water play a significant role in distributing energy and matter throughout the world’s oceans.
The Dominant Mechanism: Thermohaline Circulation
The primary force driving deep ocean currents is a process known as thermohaline circulation. This term combines “thermo,” referring to temperature, and “haline,” referring to salt. Thermohaline circulation is fundamentally driven by differences in seawater density. These density variations arise mainly from changes in temperature and salinity across different parts of the ocean. Denser water tends to sink, while less dense water rises, creating a continuous, interconnected flow.
How Temperature and Salinity Drive Density
Temperature and salinity are the two main properties that determine the density of seawater. As ocean water cools, its molecules become more tightly packed, which increases its density. This is particularly pronounced in polar regions, where surface waters chill significantly.
An increase in salinity also makes water denser because dissolved salts add mass. This occurs through evaporation in warmer regions, which leaves salt behind, or through sea ice formation. When sea ice forms, salt is expelled into the surrounding water (brine rejection), increasing the salinity and density of the residual seawater. These dense, cold, and salty waters then sink from the surface to the deep ocean, initiating the current flow.
The Global Conveyor Belt
The sinking of dense water in specific regions, such as the North Atlantic and around Antarctica, initiates a vast, interconnected global circulation pattern. Once this dense water sinks, it travels slowly across ocean basins at great depths, forming a continuous loop. This deep water eventually rises to the surface in other areas through a process called upwelling, completing its journey. This entire system is often visualized as the “global conveyor belt” due to its continuous, circulating nature. A full cycle of this deep ocean circulation can take hundreds to thousands of years to complete, highlighting its immense scale and slow pace.
Impacts of Deep Ocean Circulation
Deep ocean circulation plays a significant role in redistributing heat around the globe. By transporting cold water from polar regions towards the equator and warmer water in return, it influences regional and global climate patterns. The process also has a substantial impact on marine ecosystems. Upwelling zones, where nutrient-rich deep waters rise to the surface, support highly productive fisheries by providing essential nutrients for marine life. Deep ocean currents also contribute to the ocean’s capacity for carbon sequestration, helping to regulate atmospheric carbon dioxide levels over long timescales.