Thermohaline (temperature- and salinity-controlled density) circulation of the oceans can be simplistically defined by a great conveyor belt. In this model, warm, salty surface water is chilled and sinks in the North Atlantic to flow south towards Antarctica.

There, it is cooled further to flow outward at the bottom of the oceans into the Atlantic, Indian, and Pacific basins. After upwelling primarily in the Pacific and Indian Oceans, the water returns as surface flow to the North Atlantic. While traveling deep in the ocean the originally nutrient-depleted water becomes increasingly enriched by organic matter decomposition in important nutrients (e.g., phosphate, nitrate, silicate) and dissolved CO2. Figure courtesy of Jim Kennett and Jeff Johnson, University of California Santa Barbara.

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