Differences in salinity are a primary driver of ocean currents, particularly the large, slow-moving currents that circulate in the deep ocean. These movements are caused by changes in the water’s density, a property fundamentally controlled by both temperature and salt content. This vast, density-driven circulation system moves water masses across the globe, affecting ocean layers from the surface down to the seafloor.
How Salinity and Temperature Create Density Differences
The movement of deep ocean water is initiated by gravity acting upon water masses of different densities. Seawater density is determined by temperature and salinity; colder water is denser than warmer water, and saltier water is denser than less salty water. These properties combine to form a density gradient that dictates where water masses settle in the water column.
When water is cooled or its salt content increases, its density rises, causing it to sink beneath less dense water. In polar regions, this density increase is often caused by intense cooling and the formation of sea ice. As sea ice freezes, it expels salt, leaving the surrounding unfrozen water colder and significantly saltier, making it dense enough to sink to the deep ocean floor.
This sinking action, known as downwelling, drives a powerful, large-scale system of deep-water flow. Once a water mass sinks, it flows horizontally along the ocean bottom until its density matches the surrounding water. This mechanism provides the energy for the continuous movement of roughly 90% of the world’s ocean water.
The Global Deep Ocean Circulation System
The deep-water flow started by the sinking of cold, dense water in polar regions establishes a vast, interconnected current system spanning all major ocean basins. This global-scale circulation is a continuous, three-dimensional loop often described as the “global conveyor belt.”
The deep, dense water masses that form in the North Atlantic and around Antarctica move slowly along the ocean floor, filling the deep basins. These currents move at an extremely slow pace, typically measured in centimeters per second. A single parcel of water can take hundreds to over a thousand years to complete one full circuit of this deep-ocean path.
As the deep water travels, it gradually mixes with surrounding layers and is slowly warmed, causing it to become less dense. This less dense water eventually rises back to the surface, a process called upwelling, completing the global circulation loop.
Distinguishing Density-Driven Currents from Surface Currents
Ocean circulation is broadly categorized into two distinct systems: surface currents and deep currents. Surface currents are primarily driven by wind patterns, which exert friction on the upper layer of the water.
These wind-driven flows are energetic and relatively fast, affecting only the upper 10% of the ocean, typically extending to a depth of a few hundred meters. The Coriolis effect also influences the direction of surface currents, causing them to deflect.
In contrast, deep currents are driven solely by density differences and gravity, operating below the surface layer and extending down to the abyssal plains. While surface currents can move at speeds of several kilometers per hour, deep currents move at a sluggish pace.
Influence of Deep Currents on Global Climate
The slow, deep circulation system plays a significant part in regulating the Earth’s climate by distributing energy and substances around the globe. This vast water movement acts as a global heat distribution system, transporting warm water from the equator toward the poles. This movement of heat moderates temperatures, giving regions like Western Europe a much milder climate than other areas at similar latitudes.
Biological Role
The deep currents also perform a vital biological role by cycling nutrients and dissolved gases. As the deep water travels, it carries dissolved oxygen down to the ocean floor, sustaining deep-sea life. When this water rises back to the surface through upwelling, it brings accumulated nutrients necessary for the growth of phytoplankton, the foundation of the marine food web.