The Great Ocean Conveyor
Western Boundary Intensification, Gulf Stream Rings, and Thermohaline Overturning
Oceanographers measure currents in Sverdrups ($\text{Sv}$), where one Sverdrup equals one million cubic meters per second—roughly the flow of all Earth's rivers combined. Squeezing through the Florida Straits at 30 Sverdrups, the Gulf Stream swells to 150 Sverdrups off Cape Hatteras, moving more than one hundred times the flow of all world rivers.
Why western boundary currents like the Gulf Stream and Kuroshio are narrow, two-meter-per-second jets, while eastern return currents like the California Current are wide, slow drifts, was solved by Henry Stommel in 1948. Because Earth's Coriolis parameter increases toward the poles, wind-driven vorticity packs tightly against the western wall of every ocean basin.
Once the Gulf Stream leaves the shelf at Cape Hatteras, it meanders like an unheld firehose until loops pinch off into spinning eddies 200 kilometers wide. Northward loops trap warm Sargasso Sea water inside cold slope water (warm-core rings), while southward loops trap cold shelf plankton inside warm water (cold-core rings) for up to a year.
On January 10, 1992, a Pacific storm washed twelve containers off the ship Ever Laurel, releasing 28,800 plastic bath toys. Curtis Ebbesmeyer tracked their beachings across fifteen years. The drift data proved that the North Pacific Subpolar Gyre completes one circuit every three years, while a subset rode Arctic ice to reach Scotland by 2007.
While wind drives the upper ocean, the deep abyss is driven by thermohaline density differences controlled by heat and salt. As the North Atlantic Current carries salty tropical water into the Nordic and Labrador seas, winter winds strip out 1.2 petawatts of heat. Chilled to 2 degrees Celsius, this dense water plunges three kilometers to the abyssal floor at 18 Sverdrups.
Geochemist Wallace Broecker mapped the Great Ocean Conveyor Belt by measuring radioactive carbon-14 ($\text{}^{14}\text{C}$, half-life 5,730 years) in deep water worldwide. Once North Atlantic Deep Water sinks near Greenland, its radiocarbon clock ticks downward. The water creeps south to Antarctica and north into the Pacific, taking 1,000 to 1,500 years for one full circuit.
Dry winds evaporate far more water from the Mediterranean Sea than rivers replace, raising its salinity to 38.4 PSU. At the 300-meter sill of the Strait of Gibraltar, fresh Atlantic water flows inward on top. Underneath, dense Mediterranean water pours outward like a 1-Sverdrup waterfall, sinking to 1,000 meters and spreading across the Atlantic.
During World War II, German and Italian submarine commanders used the two-layer density current at Gibraltar to slip past British hydrophones. Trimming ballast to neutral buoyancy inside the upper Atlantic inflow, U-boats drifted into the Mediterranean with engines off. To exit, they dropped to 200 meters and rode the salty bottom outflow back out.
Flowing eastward through the Drake Passage around Antarctica without hitting land, the Antarctic Circumpolar Current is the largest current on Earth at 130 to 150 Sverdrups. Extending four kilometers to the seafloor, this unbroken ring acts as a thermal fence. It has kept Antarctica locked in ice since the passage opened 34 million years ago.
The only low-latitude passage linking tropical oceans is the Indonesian archipelago. There, Pacific trade winds pile sea level 20 centimeters higher than on the Indian Ocean side. That pressure head drives 15 Sverdrups of warm water through the Makassar and Lombok straits as the Indonesian Throughflow, the warm return valve of the global conveyor.
At the southern tip of Africa, the fast Agulhas Current overshoots the continent and makes a hairpin U-turn back into the Indian Ocean. At that turn, it sheds massive Agulhas Rings—300-kilometer-wide vortices reaching two kilometers deep. These eddies drift across the South Atlantic and inject the extra salt needed for North Atlantic sinking.
Henry Stommel proved in 1961 that thermohaline overturning has an on-off bistable switch because freshwater caps can halt deep sinking. Around 12,900 years ago, glacial Lake Agassiz burst into the North Atlantic and lowered surface salinity below the sinking threshold. That flood shut down the overturning current and plunged Europe into the 1,200-year Younger Dryas freeze.