The Great Ocean Conveyor (v2.0)
The plot on the left shows a schematic of the “ocean conveyor” which may be thought of as a circulation that connects the major ocean basins. Light waters (denoted by the red arrows encompassing waters warmer than 12C or so) are formed by heating in the tropics. In the North Atlantic, these waters penetrate to very high latitudes, and are transformed to dense, cold waters (with temperatures less than 4C shown in blue) in the Norwegian and Labrador Seas. Spilling over the ridges between Greenland and Europe, this North Atlantic Deep Water moves southward through the Atlantic and into the Southern Ocean.
In order for this circulation to be in balance, the transformation of light to dense water must be balanced by a corresponding transport of dense to light water. This is where our picture differs from what you will find on other websites and in many textbooks. In our picture, the dominant location where the densest water (shown in blue) becomes intermediate water (shown in purple with temperatures between 4C and 12C) is in the Southern Ocean rather than in the interior of the Pacific and Indian Ocean. This implies that the important processes for regulating this transformation are Southern Ocean winds (which draw up deep water from below) and eddies (which act to oppose this deep upwelling). For the classic picture to be true, the dominant process responsible for lightening the dense waters is downward diffusion of heat in tropical latitudes.