The life of almost all marine animals relies on the 'polar pumps' caused by rising and falling water. Philip Sexton explains how they work.
Episode 2 of Blue Planet II is centred around ‘The Deep’, a largely unknown world of brutal cold and utter darkness. The waters that fill the deep ocean form in restricted small areas of the ocean near the Poles, where surface waters become dense enough (via cooling and higher salinity) to sink into the abyss. These sinking waters thereby form the downwelling limbs of the deep ocean circulation system that flows throughout the ocean interior. The sinking happens in specific, restricted areas of the polar North Atlantic and the Southern Ocean. The deep water that is formed in the polar North Atlantic is referred to as North Atlantic Deep Water (NADW) whereas that in the Southern Ocean is referred to as Antarctic Bottom Water (AABW). These water masses are commonly distinguished by their contrasting temperatures and salinities, as you can see from the figure below of the distribution of salinity throughout the Atlantic Ocean.
The sinking of these polar surface waters into the abyss is essential for almost all marine animals, because these freshly formed deep waters are responsible for supplying the abyss with the oxygen required for animals to breathe. Without this regular re-supply of oxygen, the ocean interior would quickly become oxygen deficient (or even anoxic), posing a major threat to most marine life.
Deep water produced in the North Atlantic (NADW) is somewhat warmer and somewhat saltier than that produced in the Southern Ocean (AABW). Warmth reduces water density and salt increases it. So it appears that in the modern ocean, there must be a fine balance in the temperatures and salinities of the high latitude North Atlantic and Southern Ocean source waters that hence allow both NADW and AABW to occupy the modern abyss (but with NADW filling slightly more of the ocean interior than AABW, and thus being slightly denser than AABW).
With ongoing global warming it is predicted that upper ocean waters of both of these polar regions will become less dense (more ‘buoyant’) and thus less likely to sink, thereby weakening or collapsing the polar pumps that re-supply oxygen to the abyss. This increased surface buoyancy will arise from the warming of surface waters in a warmer climate, and from a stronger hydrological cycle and ice sheet melting, both of which serve to lower the salinity (and thus density too) of polar surface waters.