 Solution

Meridional overturning circulation links relatively warm upper-ocean transport to a colder return flow at depth, with exchanges among basins and vertical branches. Surface heat loss and salinity changes alter water density; sufficiently dense water can sink and spread along deep pathways. Freshwater input lowers salinity and can inhibit dense-water formation. Winds, particularly in the Southern Ocean, and energy supplied to mixing are also important to the global return and upwelling pathways. The circulation is not powered by surface density forcing alone in an otherwise closed, unmixed ocean.

The Atlantic meridional overturning circulation carries heat northward, helping regulate ocean-atmosphere heat exchange and regional climate. Overturning also ventilates the deep ocean and redistributes carbon, oxygen and nutrients. Changes in its strength can alter regional sea-surface temperatures, air-sea heat release and atmospheric circulation. They need not eliminate every western-boundary current or every source of European warmth: wind-driven ocean circulation and atmospheric heat transport are distinct, interacting contributions. The two-box model below isolates a density/salinity feedback rather than representing all these mechanisms.