= 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.
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