As we have seen, surface ocean currents are the dominant sources of deep water masses. In fact, it is a little more complicated than this as other deep water masses also feed one another. However, in a generalized sense, the surface and deep ocean currents can be viewed as an integrated system known as the Global Conveyor Belt, a concept conceived by the brilliant Geoscientist Wally Broecker of Columbia University. Diagrams of the Global Conveyor Belt (GCB) are two dimensional and therefore simplified and do not, for example, include all of the intermediate water masses or surface water currents. However, the key of the Global Conveyor Belt concept is that it explains the general systems of heat transport as well as bottom water aging and nutrient supply in the oceans.
Global Conveyor Belt
Video: Coral Growth (00:58)
The polyp is the major life form of coral. It can reproduce in two different ways. In sexual reproduction, the polyp releases eggs and sperm which fuse to form a zygote. The zygote develops into a planula and the planula lives in the surface part of the oceans, and spreads out with ocean currents before it falls to the bottom of the ocean and colonizes as a hard substrate, forming a new coral colony. The second type of reproduction by the polyp is called asexual reproduction. And in asexual reproduction, the polyp itself either buds, forms a new bud, or divides, forms two separate polyps. And in this type of reproduction, this colony will then grow itself as opposed to forming a new colony via sexual reproduction.
The following animation traces the path of water through the surface and deep ocean showing the dominant features of the GCB including formation of NADW in the North Atlantic.
Video: The Thermohaline Circulation - The Great Ocean Conveyer Belt (2:46) This video is not narrated.
The GCB shows the dominant source of deep water in the oceans as North Atlantic Deep Water and how this splits in two to flow into the Indian and Pacific Oceans. In these locations, upwelling of the deep water mass produces surface water currents that generally flow back towards the original source of deep water in the North Atlantic. For heat supply, the conveyor belt involves the transport of heat and moisture to northwest Europe by the Gulf Stream; this accounts for about 30% of the heat budget for the Arctic region, making the GCB extremely important for climate in the Arctic.
Because deep-water masses circulate very slowly, the GCB takes about 1500 years to complete, meaning that the oldest water in the oceans is about this age. In addition, because oxygen is gradually depleted in deep waters as they age, and because CO2 contents and nutrients conversely increase, the oldest water masses of the ocean in the North Pacific are among the most nutrient-rich, CO2 rich, and oxygen-depleted waters in the ocean. Conversely, the newly produced NADW waters are among the most nutrient depleted, CO2 depleted, and well-oxygenated waters in the world.
As it turns out, recent research on the detailed configuration of surface and deep currents shows that circulation is much more complex than the GCB. Floats deployed in the ocean don’t always follow expected pathways in the GCB model. Wind actually plays a more significant role in causing downwelling than previously thought. Moreover, mixing by small systems or eddies plays a large role in driving surface currents.