Ocean on the Move: Thermohaline Circulation

The currents flowing through the ocean, a process called thermohaline circulation, can have an impact on climate.  

What is thermohaline circulation?

Cold water, in general, is denser than warm water. Likewise, water with a high salinity is denser than water that contains less salt. Surface ocean currents are primarily driven by winds. Deep ocean currents, on the other hand, are mainly a result of density differences. The thermohaline circulation, often referred to as the ocean's "conveyor belt", links major surface and deep water currents in the Atlantic, Indian, Pacific, and Southern Oceans. Multiple mechanisms conspire to increase the density of surface waters at high latitudes. Cold winds blowing over the oceans chill the waters beneath them. These winds also increase evaporation rates, further removing heat from the water. These chilled waters have increased densities, and thus tend to sink. Formation of sea ice also helps to increase the density of water near Earth's poles. As seawater freezes, salt is forced out of the ice in a process called "brine exclusion". The ice is essentially not salty. The excluded salt increases the salinity of the cold water immediately below the ice, making it denser still. The salty, cold water near the poles sinks toward the ocean floor.

Just as rivers on land flow downhill towards the sea, deep density-driven currents in the oceans move along submarine valleys towards the deepest parts of the ocean. The cold, salty waters that drive the thermohaline circulation form in the Arctic Ocean, the North Atlantic, and the Southern Ocean. The shallow ocean floor along the Bering Straight prevents deep currents from flowing out of the Arctic Ocean into the Pacific. Dense water on the floor of the North Atlantic moves southward, eventually joining the sinking waters of Southern Ocean in the far South Atlantic. Once again, a shallow section of the ocean floor blocks the flow from moving into the Pacific. In this case the Drake Passage, between the Antarctic Peninsula and the southern tip of South America, prevents the current from flowing westward. So the thermohaline circulation turns to the east. Here the current splits; some flows northward along the east coast of Africa into the Indian Ocean, while the rest continues eastward along the southern coast of Australia and finally, veering northward, makes it into the vast Pacific basin. 

At this point the two branches of the thermohaline circulation finally begin to mix with the lighter, warmer waters above and work their way back to the surface. Scientists estimate that the trip from the North Atlantic to the deep water upwelling sites in the Pacific takes about 1,600 years. To balance the flow of deep water into the Indian and Pacific basins, surface water must flow back out. Warm surface waters from the Pacific flow through the Indonesian Archipelago into the Indian Ocean, where they join with other currents that have risen from the depths. This combined flow works its way westward around the southern tip of Africa into the South Atlantic. Next, the surface flow moves northward through the Atlantic. Aided by a nudge from the warm Gulf Stream surface current, this water makes its way once again to the extreme North Atlantic, where the cycle begins again. This global circulation pattern mixes the waters of the world's oceans, turning the ocean reservoirs into a single, vast, interconnected system. 

Thermohaline circulation plays an important role in supplying heat to the polar regions. Therefore, it influences the rate of sea ice formation near the poles, which in turn affects other aspects of the climate system (such as the albedo, and thus solar heating, at high latitudes).

Water's long trip through the ocean's depths on the great ocean conveyor belt, far from surface water influences and contact with the atmosphere, contributes to the a lag time between climate forcings and our planet's reactions to them. Heat and dissolved carbon dioxide, carried to the oceans depths by the thermohaline circulation, may remain "buried" in the abyss for centuries. This "burial" may forestall initial effects of global climate change; but like zombies in a horror flick, may come back to haunt us much later when they arise from the depths.