Thermohaline circulation is a global system of ocean currents driven by differences in water density. These density variations primarily arise from changes in ocean water temperature and salinity, which are the main factors influencing ocean water density and large-scale circulation patterns.
Understanding Global Ocean Circulation
Thermohaline circulation is a vast, interconnected system of ocean currents extending throughout the world’s oceans. It is often called the “global conveyor belt” due to its continuous, slow movement of water across ocean basins. This circulation plays a significant role in distributing heat from the equator to the poles, influencing global climate patterns. Beyond heat, these currents also transport vital nutrients, dissolved gases, and other materials, supporting marine ecosystems. Unlike wind-driven surface currents, thermohaline circulation involves the entire ocean depth, moving tremendous volumes of water over long timescales, sometimes taking around 1,000 years to complete a full cycle.
How Temperature Shapes Ocean Water
Ocean water density is significantly influenced by temperature. Generally, colder water is denser than warmer water. As water cools, its molecules slow down and pack more closely, occupying less space and increasing its density. This increased density causes colder water to sink.
Differences in temperature contribute to the layering, or stratification, of ocean water. Warmer, less dense water remains at the surface, while colder, denser water sinks below it. For example, surface waters in polar regions cool significantly, becoming denser and prone to sinking.
How Salinity Shapes Ocean Water
Salinity, the concentration of dissolved salts in water, also determines ocean water density. Water with higher salinity is denser than water with lower salinity because dissolved salts add mass without significantly increasing volume. Even small differences in salinity can lead to notable density variations.
Processes that increase ocean salinity include evaporation, where water leaves as vapor, leaving salts behind, and the formation of sea ice, which expels salt into the surrounding water. Conversely, freshwater input from rivers, precipitation, and melting ice can decrease salinity. Saltier, denser water tends to sink below less salty water.
The Combined Influence on Deep Currents
The combined effects of temperature and salinity drive the deep ocean currents of thermohaline circulation. Extremely dense water forms when water is both very cold and highly saline, causing it to sink to the ocean floor, initiating the global system of deep currents. These super-dense waters primarily form in high-latitude regions, such as the North Atlantic and around Antarctica.
In the North Atlantic, warm, salty surface waters from lower latitudes cool significantly as they move northward. As sea ice forms, salt is rejected into the remaining seawater (brine rejection), further increasing its salinity and density. This cold, highly saline water then sinks, forming masses like the North Atlantic Deep Water (NADW), which flows southward into the deep ocean basins.
Around Antarctica, intensely cold temperatures and sea ice formation create Antarctic Bottom Water, the densest water mass in the world’s oceans. This water also sinks and spreads across the seafloor. These deep currents move slowly, eventually resurfacing in other parts of the world’s oceans, completing the global circulation pattern.