The Great Atlantic-Pacific Asymmetry


Ocean Volumes (excluding adjacent seas). Note how much greater the Pacific is than either the Indian or Atlantic Oceans.
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).


Simple cartoon of the Oceanic Conveyor Belt and the resulting Atlantic-Pacific Asymmetry. Orange indicates warm shallow water, while blue indicates cold deeper water. (From: Jim Kennett & Jeff Johnson, University of California Santa Barbara.
The Oceanic Conveyer Belt
A good way to follow the deep water on its course and to observe how it ages is to map out the deep-water oxygen content. 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.
Atlantic-Pacific Asymmetry and Climate Change
How will this overall Atlantic-Pacific asymmetry be affected by global climate change? The answer hinges on what happens to the production of North Atlantic Deep Water in the seas around southern Greenland. If this deep-water source is diminished or shut off, the fundamental cause of the asymmetry is largely eliminated. Atlantic and Pacific will become more similar. The Atlantic deep waters will have more nutrients, and the Pacific will have less than now. The intermediate point, the Circumpolar Ocean, also will have less nutrients, presumably. The change would take place on the time scale of deep-ocean mixing - on the time scale of a thousand years. It would be much less noticeable, with respect to ocean productivity, than any changes brought by altering the wind fields, and decreasing the intensity of upwelling, as discussed when considering the competition between trade-winds and monsoon circulation.