The global conveyor connecting all oceans
The thermohaline circulation is a slow, global ocean conveyor that transports water and heat across all ocean basins. Warm surface water flows northward in the Atlantic, Mediterranean, and other basins. In high northern latitudes, this water cools and its salinity increases through evaporation. In the Nordic Seas and Labrador Sea, the water becomes dense enough to sink to the bottom of the ocean and begin a journey lasting centuries.
This cold deep water flows southward through the Atlantic, around Africa, and into the Indian and Pacific Oceans. Along the way, it mixes gradually with warmer water above, gaining heat and losing density. Eventually it rises in upwelling regions, primarily in the Southern Ocean and Pacific, completing the loop. The entire circuit takes approximately 1000 years, but the residence time for water in the deep ocean is much longer, making deep water an effective long-term carbon storage medium.
Heat transport and climate sensitivity
The thermohaline circulation transports as much heat toward the poles as the entire atmospheric system. This heat transport is especially important for the Northern Hemisphere, where the Atlantic conveyor brings tropical warmth to high northern latitudes. The consequence is that Western Europe is 5-10 degrees Celsius warmer than other regions at the same latitude.
Changes to this circulation can produce rapid climate shifts. During ice ages, the conveyor weakens when freshwater from ice sheet melt disrupts the sinking mechanism in the North Atlantic. This produces millennial-scale oscillations in climate called Dansgaard-Oeschger events. Modern concern focuses on whether anthropogenic warming and increased ice melt could weaken the Atlantic Meridional Overturning Circulation, potentially causing abrupt regional cooling even as the globe warms. Monitoring deep-water formation rates is a priority for climate early warning systems.