The Blue Machine

To the north, on the right side of Erik’s boat, lay the cold Arctic Ocean. To the south, on his left, lay the North Atlantic, a region of fierce storms and warmer water. And underneath him was a ledge, a reasonably flat region of shallower water that divided the deep Arctic Ocean and the deep Atlantic Ocean. That ledge sat 100–300 metres below the sea surface, invisible to Erik but still high enough to act as a barrier between the oceans on either side, each 2–3 kilometres deep in that region. But the water is not the same on either side of the ledge. The basin on the north side is full of dense, salty, cold Arctic water, and like water overflowing from a bath this dense water slithers across the ledge to meet the Atlantic. There it encounters warmer water which is less dense, and so the huge cold overflow slides underneath the Atlantic waters, hugging the slope and tumbling downwards underneath the rest of the ocean until it reaches the bottom, 2.5 kilometres below the ledge. This is the Denmark Strait Overflow, the largest waterfall in the world, plunging down a long underwater mountainside to join an underwater deep pool at the bottom. It’s estimated that 3 million cubic metres of water flood down this cataract every single second, more than one thousand times the flow of Niagara Falls. The flood isn’t smooth: as the cold water slides downwards, it drags along some of the warmer water above it, creating a huge plume of turbulence and warming slightly as it goes. But it is continuously supplying a huge quantity of cold polar water to the bottom of the Atlantic. This is why the deep ocean is cold–wherever dense water forms it will sink, and cold water is dense so it ends up at the bottom.