You might be familiar with some version of the seven seas—early Greeks used the term to encompass the Aegean, Adriatic, Mediterranean, Black, Red, and Caspian Seas (with the Persian Gulf); and later the phrase was used by Medieval European literature to describe the North, Baltic, Mediterranean, Black, Red, and Arabian Seas, as well as the Atlantic Ocean.
More modernly, the seven seas have been used to describe regions of the five oceans—the Arctic, North Atlantic, South Atlantic, North Pacific, South Pacific, Indian, and Southern Oceans. Throughout history, terminology for what was viewed as distinct regions of the charted world has adapted to embrace the expanding edges of those explorers' maps and nautical charts.
Today, however, we know that these regions of the watery world are not as disparate as early explorers may have once thought—each of these bodies of water, from the frigid reaches of the Southern Ocean around Antarctica, to the balmy stretches of the Caribbean Sea, to the icy waters surrounding the Aleutian Islands, are connected through a deep-ocean current called the global ocean conveyor belt. This global circulation pattern helps to cycle nutrients and energy across the planet, supporting the world’s food chain and creating a dynamic marine environment.
This global current is powered by changes in ocean chemistry in different parts of the world. Local differences in seawater temperature and levels of salinity give different parcels of water varying densities, causing them to sink or rise in the water column.
Very cold, salty water, such as you would find in the Arctic Ocean where the formation of sea ice excludes salt and increases the salinity of the surrounding waters, is very dense, and thus sinks thousands of meters down to the ocean floor.
Once at the bottom of the water column, this cold, dense water spreads out to make room for incoming water that is continuing to chill and sink from the surface. This sinking motion pulls in more water from the surrounding surface, creating a current.
As it spreads at depth, the dense, cold water has nowhere to go but south. It moves across the floor of the Atlantic Ocean, past the equator, and on towards the Antarctic continent, where it’s pushed around the southern landmass and fed more cold, salty water sinking from the surface.
From here, the waters split—some of it is pushed back north towards the Indian subcontinent, and the rest of it moves up towards the North Pacific. On this journey north, the waters are warmed by the sun, becoming less dense and rising up in the water column.
Once the water reaches its new, higher position in the water column as a result of surface winds, equatorial heat influx, and salinity reduction, it once again spreads to make room for more rising water parcels, creating the second half of the current of the global ocean conveyor belt.
If you’re heading out to the beach this summer, just ask yourself—where else in the world has this water been?
Republished with permission from NEEF: https://www.neefusa.org/nature/water/stirring-seven-seas
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