Past exam of the geophysical fluid dynamics course of the University of Oxford / 2025 / 2 / c / i / Solution

Northern high-latitude cooling removes heat from surface water and raises its density. If the surface water becomes denser than the water below, convection mixes it downward and forms dense water that can be exported in the deep southward branch of the Atlantic meridional overturning circulation. This water-mass transformation helps maintain the contrast represented by $\alpha \Delta T$ in 2(a). Freshening can oppose it by reducing density and increasing stable stratification. Local convection and the basin-integrated sinking volume flux are related but are not the same process: export and the return path also depend on the large-scale dynamics.