= 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.
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