21.13 Role of Salt and Density in Ocean Circulation
Ocean water contains a variety of dissolved substances, with salt being the most abundant. The amount of salt in ocean water is called salinity. When ocean water has more salt, it becomes heavier or denser. This happens because the dissolved salt increases the mass of the water without significantly increasing its volume. As a result, higher salinity leads to higher water density. In the ocean, density differences between water masses are one of the key forces driving ocean circulation, especially in deep waters. These density differences arise mainly due to variations in temperature and salinity. Cold water is denser than warm water, and salty water is denser than less salty water. When surface waters become colder and saltier often due to evaporation or the formation of sea ice they become denser and sink to deeper layers. As denser water sinks, it pushes other water out of the way, creating slow-moving but large-scale currents that circulate throughout the global oceans. This deep circulation helps transport heat, and gases (such as oxygen and carbon dioxide) around the globe. It connects surface and deep water systems and plays a critical role in regulating the Earth's climate.