By Wanda Thibodeaux • Updated Mar 24, 2022
According to the National Oceanic and Atmospheric Administration (NOAA), roughly 71 % of Earth’s surface is ocean, and these vast waters contain 97 % of the planet’s freshwater. Ocean currents—natural conveyors of heat, salt, and nutrients—are profoundly shaped by the salinity of the water.
Physics tells us that less dense material rises while denser material sinks. In the ocean, denser, saltier water settles toward the seafloor, forcing lighter, fresher water to rise. This exchange generates the vertical component of a convection current, which drives horizontal circulation patterns.
Temperature reflects the kinetic energy of water molecules. Warm water expands, reducing its density; cold water contracts, becoming denser. Consequently, warmer surface water tends to rise, while colder, denser water sinks, further fueling convection.
Heating expands water, creating space that allows more salt and dissolved minerals—such as calcium—to remain dissolved. Thus, warmer water can exhibit higher salinity. When salinity and temperature combine to make a parcel of water denser than its surroundings, it sinks, initiating a convection cycle. This interplay can even reverse the expected vertical ordering—cold water may lie atop warm water if the latter’s salinity is sufficiently high.
Salt and other minerals enter the ocean through multiple pathways: riverine runoff erodes terrestrial salts, hydrothermal vents release minerals from the seafloor, and anthropogenic inputs (e.g., sewage, industrial effluents) contribute additional loads.
