Ocean Observing and Climate
OVAC Theme 3 - Thermohaline Circulation
The Thermohaline Circulation (THC) is the part of the large-scale global circulation which is driven by horizontal density gradients (to be distinguished from the basin gyres which are driven by the wind). In a small number of regions of the ocean, notably the Labrador and Nordic Seas in the North Atlantic, and the Ross and Weddell Seas off Antarctica, wintertime cooling creates extremely dense waters, which sink to the ocean bottom; surface waters move poleward to replace them. Deep water is drawn gradually to the surface by the wind-forced Ekman suction in the tropics and in the Southern Ocean, as well as by mixing with less dense waters. These processes of sinking and rising are linked by the global circulation known as the Global Conveyor Belt.
The THC is crucial to the maintenance of Europe's temperate climate, as the North Atlantic Current transports large amounts of warm surface water northwards toward the convection regions. The marine geological record in the Atlantic suggests that the THC was disrupted about 8,000 years ago resulting in a sudden large cooling in Europe, and some climate models suggest similar behaviour as a possible consequence of global warming. Weakening without complete shutdown could still lead to significant changes for UK climate. To be able to predict the climate with any confidence we therefore need to understand firstly what sets the strength of the thermohaline circulation, and secondly how it varies. This involves both observations of the circulation at crucial locations around the Conveyor Belt and the use of numerical models.
