By Emma Zimmerman
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June 11, 2026
The ocean, and all of the water in it, is constantly in motion. Though it may sometimes appear calm on the surface, enormous currents continuously move ocean water around the globe like a giant conveyor belt. These currents regulate weather, move nutrients across ecosystems, and shape the climates of entire continents. Currents on the surface of the ocean are controlled by the wind, but deeper in the ocean currents are controlled by water density in a process known as thermohaline circulation . Thermohaline refers to the temperature (thermo) and salinity, or saltiness, (haline) of the water. Circulation refers to how the water moves throughout the ocean from pole to pole. Though this is a complicated sounding word, the process is actually pretty simple. Here’s how it works: As ocean water travels toward the poles it becomes colder, and when it gets cold enough, it eventually begins to freeze and forms into sea ice. What is important about this process is that when seawater freezes, the salt is left behind in the surrounding water. As a result, the surrounding water becomes saltier, colder, and in turn becomes more dense than the water around it. Eventually, this water becomes dense enough to sink deep into the ocean, and warmer surface water moves in to replace it. This process of freezing, melting, and replacement continues on and on, resulting in a continuous cycle of moving water that forms deep ocean currents, often referred to as the “global ocean conveyor belt.”