The Deep Atlantic Conveyor

Cartoon of the North Atlantic Heat Conveyor. Orange flow is surface water, blue flow is deep water. (Modified from: NASA)
The Nordic Heat Pump (again)
Consider again the strawberries in Norway. It is the warm currents entering the Norwegian Sea that provide the extra heat (as much as 30 percent greater than the “background” heat) to keep Norway warm enough to grow strawberries, as well as apples, pears and plums. Some of this water flows back into the North Atlantic as a cold current along the Greenland coast, laden with icebergs and pack ice, after some peregrinations. Much of it cools and sinks, causing deep convection in the Norwegian Sea. The cold deep water made in this fashion flows over sills between Greenland, Iceland and Denmark and feeds the deep waters of the North Atlantic.

The import of warm surface water and the export of cold deep water sets up a heat pump which keeps northern Europe pleasantly free of a Greenland-type ice cap. This is known as the "Atlantic Heat Conveyor" or also the "Nordic Heat Pump." In fact, the climate of all of northwestern Europe greatly depends on this heat piracy associated with the Nordic Heat Pump, which helps move heat from the Caribbean into the northern North Atlantic, and even from the tropical regions of the South Atlantic.

Simple cartoon of the Oceanic Conveyor Belt and the resulting Atlantic-Pacific Asymmetry in nutrient-rich waters. Orange indicates warm shallow water, while blue indicates cold deeper water. (From: Jim Kennett & Jeff Johnson, University of California Santa Barbara. CSULB Department of Geology )
The Oceanic Conveyer Belt
What if we now want to follow the deep water after it sinks and observe its course through a variety of ocean basins? The best way to study this is to map out the oxygen content of the deep-water. Where the deep water is young (that is, where it came from the surface quite recently), its oxygen content is high. Where it is old (that is, where it has not seen the surface for a long time), much of its oxygen has been spent in bacterial decay of organic matter, and in respiration. (The organic matter, of course, is produced in the sunlit surface waters and gets into the deep water by sinking, in particles.) As we should expect, from the distribution of deep-water sources, the Atlantic deep waters are young, and those of the North Pacific are old.

Although an oversimplified model, the pattern we observe is as follows: The North Atlantic is filled with young surface waters, which it largely receives because of the sinking of waters around Greenland. The North Pacific is filled with older deep waters which arrive from the south after a long voyage from Antarctica — and even there these waters were not entirely "young" when they sank, because they were not at the surface long enough to thoroughly reset their chemistry by exchange with the atmosphere.

In a gross (but memorable) oversimplification, we can say that the Atlantic gives its deep waters to the Pacific, and receives surface waters in exchange. This has important consequences for the nutrient distribution in these oceans. The Atlantic loses nutrient-rich deep water, while the Pacific gains them and sends nutrient-poor waters away via the Indian Ocean. Scripps oceanographer Joseph Reid puts it well: "They give us their best, we give them our worst." Thus, deep circulation removes nutrients from the Atlantic and piles them into the Pacific.

The oxygen content of longitudinal sections (from south to north) of the (A) Pacific and (B) Atlantic Oceans (Note that this graph exaggerates vertical proportions, especially the seafloor topography, in black). The scale for both is shown in (A) and is in units of micromoles of oxygen per kilogram of seawater. It can clearly be seen how the Pacific has older, nutrient-rich, oxygen-poor deep water while the Atlantic contains younger, nutrient-poor, oxygen-rich deep water.

How “Old” is the Water in our Oceans?
It takes, on the whole, one thousand years to renew the deep waters of the world’s ocean. This estimate is based on radiocarbon measurements from the CO2 dissolved within the ocean. Radiocarbon (14C) is made within the atmosphere by the action of cosmic radiation on nitrogen atoms (14N). Radiocarbon enters the ocean through dissolution in the surface water, and is then transported to the deep water by sinking water. In the deep ocean, the radiocarbon decays and a little more than one percent (1.2%) is lost every 100 years (half-life of about 6000 years).

In the deep North Pacific, about 10% of the original radiocarbon is missing. Using this information, we can calculate how "old" the water is, which is the same as determining how fast the deep-water sources produce sink-water. It turns out that the volume of the deep ocean is 1000 million cubic kilometers (without thermocline and surface water), and it takes a thousand years to renew the water in it. Hence the production is one million cubic km per year. This is the same as 30 million cubic meters per second (= 30 sverdrup).

Turning Down (or off) the Nordic Heat Pump
Computer models suggest that continued global warming might interfere with this heat pump. The pump (which depends on the production of deep water) might shut down if and when the surface waters becomes too warm to sink. How will the climate respond if the Atlantic Conveyor becomes less efficient in pumping heat into the Nordic Sea? The heat, as mentioned, feeds the Icelandic Low, a major center for generating storms. Without the heat pump, the storm belts of Iceland will move southward. Paris might then be in a situation to have to either the climate of Copenhagen (from weakening the Iceland Low) and that of Madrid (from general warming). It might get both, at different times, as would Frankfurt and Munich. Norway and Iceland, in this scenario, might actually get colder rather than warmer — or at least the contrast between summer and winter would increase.

Ocean Volumes (excluding adjacent seas). Note how much greater the Pacific is than either the Indian or Atlantic Oceans.
As of right now, we cannot say for sure whether or not the production of North Atlantic Deep Water will in fact be affected by global warming, and if so, how the climate would react. All we can say is that North America will be less affected than Europe by such changes, because the climate of North America depends largely on what happens in the Pacific. The question there is how the Aleutian Low will react, and the system of oscillations that make El Niño events. At this point, such questions are still a matter of research, and we do not know the answer.