We can almost see it whole, the round-the-world journey that seawater takes. We can imagine taking the trip ourselves.
It begins north of Iceland, a hundred miles off the coast of Greenland, say, and on a black winter’s night. The west wind has been screaming off the ice cap for days now, driving us to ferocious foaming breakers, sucking every last ounce of heat from us, stealing it for Scandinavia. We are freezing now and spent, and burdened by the only memory we still have of our northward passage through the tropics: a heavy load of salt. It weighs on us now, tempts us to give up, as the harsh cold itself does. Finally comes that night when, so dense and cold we are almost ready to flash into ice, we can no longer resist: we start to sink. Slowly at first, but with gathering speed as more of us join in, and as it becomes clear that there is nothing to catch us--no water below that is denser than we are. We fall freely through the tranquil dark until we hit bottom, more than a mile and a half down.
There we join a pool of other cold, salty water parcels that fills the Greenland and Norwegian basins. From time to time the pool overflows the sill of the basins, an undersea ridge that stretches between Greenland and Iceland and Scotland. Then the falling starts again. Now it is not a parachute drop but a headlong rush, downslope and tumbling like a mountain stream, but more powerful even than Niagara: a giant underwater waterfall, cascading into the Atlantic abyss. Falling, we pull shallower water in behind us. From our right flank, as we reach the latitude of Newfoundland, we are joined by a cohort from the Labrador Sea; not quite as dense as we are, this water settles in above us, headed south along the slope of North America. Near Bermuda our ranks are swelled on the left by spinning blobs of warm Mediterranean water, even saltier than we are; they sail like Frisbees out of the Strait of Gibraltar and cross the ocean to join us. Greenland water, Labrador water, Med water--we all fall in together, and gradually we mingle: we are North Atlantic Deep Water now. Mediterranean salt seeps through us like a dye. Though at every step on the road some of us lose heart and turn back north, still our mighty host advances, 80 Amazon Rivers marching along the ocean floor, toward the equator and across it.
All through the South Atlantic our army remains intact, hugging the western slope of the ocean basin. But that reassuring guide ends where South America does, and in the stormy Southern Ocean we are scattered by the great centrifuge, the Mixmaster, the buzz saw--what metaphor can do justice to the Antarctic Circumpolar Current? Sweeping around the frozen continent from west to east, with no land to stop it, it carries now some 800 Amazons of water. It blends the waters of the world, obscuring their regional roots. The fierce winds drag us--ever so briefly--to the surface off Antarctica, where we absorb a blast of cold and quickly sink again. We spread north now into all the oceans, mostly at a depth of half a mile or so, some back into the Atlantic, some into the Indian Ocean, many of us into the Pacific. In that vast and empty basin we drift northward until we reach the equator; there the trade winds part the waters, and tropical heat mixes down into us, buoying us to the surface. It is time to head for home.
Blasting and wending our way through the confusion of Indonesia, with its near-impenetrable wall of islands, we cross the Indian Ocean, collecting salt from the hot shallows of the Arabian Sea. Southward then down the coast of Mozambique, and we are picking up speed, in preparation for our triumphant return--but rounding the Cape of Good Hope is not easy. Again and again we are beaten back. Only by detaching ourselves in spinning eddies from the main current do some of us manage to sneak into the South Atlantic. There we are joined by water that never bothered with Indonesia and Africa but instead took the colder shortcut around South America, through the Drake Passage.
