Chapter 11 - The global circulation and transformation of water masses

Abstract

Surface and deep circulation patterns connect at high latitudes, where deep waters form or rise back to the surface. Deep water formation is associated with buoyancy loss at the surface, primarily through wintertime heat loss to the atmosphere. In returning to the surface layer, deep waters re-gain potential energy through a combination diapycnal mixing and wind-driven upwelling. These horizontal and vertical movements of water combine as the Meridional Overturning Circulation (MOC). Incorporating pathways between oceans, the global pattern of currents is known as the thermohaline circulation (THC). Following the THC, water parcels are subject to surface and interior fluxes of heat and salt that transform water mass properties. Associating the 3D circulation and water mass transformation, redefinition of the MOC in latitude–density coordinates confirms the dominant role of surface heat and freshwater forcing in the North Atlantic, while the global THC is most naturally defined and understood in temperature–salinity space.