8.14: Ocean Circulation and Climate

Circulation of water in the oceans can be likened to a giant conveyor belt. Water is cooled near the poles; transported through the deep oceans, where it mixes with other water; returned to the surface far from where it sank; and warmed and transported back to a location where it cools and enters the cycle again.

The ocean circulation system carries heat from one part of the planet to another and transfers dissolved chemicals between surface and deep-water layers. Consequently, changes in circulation can cause and be caused by changes in climate, and they can cause changes in the distribution of dissolved chemicals that directly affect the biological communities of the oceans. Ocean circulation also carries some of the excess carbon dioxide from fossil fuel burning into the deep oceans (CC9). Therefore, deep-ocean circulation research is of great interest, particularly for global climate change studies.

Meridional Overturning Circulation: The Conveyor Belt

The Meridional Overturning Circulation (MOC), which is often referred to as the “conveyor belt circulation,” is a closed-loop circulation of water that travels through all of the world's oceans. The general features of the MOC are depicted in Figure 8-28. The complete circuit takes an average of about 1000 years, and the amount of water transported is enormous—about 20 times the combined flow of all the world’s rivers. The MOC is usually considered to start in the North Atlantic near Greenland and Iceland, where surface water is cooled by the cold air mass that flows from the Canadian Arctic. The cold, high-density water sinks to form North Atlantic Deep Water (NADW), which then flows south through the deep Atlantic Ocean, around Africa, where it flows over and mixes with the deep water formed near Antarctica. The deep water is transported around Antarctica, then north into the Indian and Pacific Oceans. The water mixes progressively toward the surface. The details of where and how this mixing takes place are not yet well known, but it is thought to take place slowly throughout most of the oceans and more quickly through turbulent mixing in some areas where currents and internal waves interact with seafloor topography. Eventually, the deep water mixes with surface waters and enters the mixed-layer current system. Complex exchanges and movements of surface water eventually return surface water to the Atlantic Ocean, replacing the water that originally formed NADW. This system is instrumental in transporting heat from low latitudes to high latitudes of the North Atlantic Ocean.

Figure 8-28. A simplified depiction of the Meridional Overturning Circulation (MOC), or “conveyor belt,” that starts with the formation of North Atlantic Deep Water in several locations near Greenland.

Surface water in the North Atlantic Ocean is warmer than NADW. As the Gulf Stream flows into the northeastern Atlantic Ocean near Europe, prevailing westerly winds blow over it, transferring heat and moisture into the atmosphere. The westerly wind air mass releases its heat and moisture over Europe, causing Europe’s climate to be extremely mild and wet in comparison with the climates of other land areas at the same latitude (Chap. 7). Thus, the MOC transfers heat from the Indian Ocean and Pacific Ocean to the North Atlantic region near Europe. At the same time, the MOC transports dissolved nutrients from the Atlantic Ocean to surface waters of the Indian and Pacific Oceans.

The MOC Climate Switch

Ocean sediment records have shown that, during the last ice age, which peaked about 18,000 years ago, the MOC generally operated more slowly and weakly than it does today, and varied in strength. The variances may have occurred abruptly at times. Some of those abrupt changes may have almost entirely turned the MOC off or back on again during the past tens of thousands of years. It is believed that such abrupt changes have, at least on some occasions, coincided with very abrupt (on the scale of decades) changes in climate. Evidence suggests that variations in the MOC circulation, at least during the cold, ice-age period, may have been chaotic (CC11) and that ocean circulation may have switched periodically between a very weak MOC and a strong MOC, which is seen today. 

About 15,000 years ago, the last ice age ended, although it may be that the current warm period is just a warm interglacial interval similar to other interglacial periods that have interspersed the ice age in the past. Nevertheless, about 15,000 years ago, the Earth’s average temperature increased by about 6°C in as little as 100 years. This abrupt change was accompanied by a 20% increase in atmospheric carbon dioxide concentration and a substantial, but not yet well quantified, increase in the intensity of the MOC.

During the past 13,500 years, the Earth’s climate has been warmer than at any other time during the past million or more years. However, this warm period was interrupted by a short cold period, the Younger Dryas period or “Little Ice Age”, that occurred between approximately 12,900 years ago and 11,500 years ago. At the beginning of the Younger Dryas period, western Europe’s climate cooled dramatically by about 5°C within a matter of decades or less and then returned just as abruptly to its former warm condition at the end of this period. Apparently, the Younger Dryas cold climate occurred when the MOC was severely slowed or stopped. The reason for the temporary slowing or stoppage of the MOC is not known, but might be related to the flow of meltwater from glaciers.

During the early postglacial period, about 13,000 years ago, glaciers extended far to the south, and most meltwater from the North American ice sheet probably drained down the Mississippi Valley to the Gulf of Mexico. By 13,000 years ago, as the glaciers continued to melt, a lake called Lake Agassiz covered much of Manitoba, western Ontario, northern Minnesota, eastern North Dakota, and Saskatchewan. At its greatest extent, Lake Agassiz may have covered as much as 440,000 km2, larger than any currently existing lake in the world. It is believed that the cold, fresh meltwater that was collected in this lake emptied rapidly either through the McKenzie River to the Arctic Ocean or through the Saint Lawrence River to the North Atlantic, or perhaps both. Because it was fresh or very low salinity water, it would have floated on the ocean water to form a stable, low-density surface layer over a large area of the Arctic Ocean and North Atlantic Ocean. The low-salinity surface layer would have acted as a virtual cap, severely restricting the formation of NADW and slowing the MOC. Within several hundred years after glacial meltwater first flooded out of Lake Agassiz in large volumes the freshwater flow to the North Atlantic probably diminished because, by that time, most glaciers had finished melting. At that point, NADW again formed, the MOC resumed, and Europe’s climate warmed rapidly.

We are not sure whether a number of additional abrupt climate changes that have occurred during the past 10,000 to 12,000 years coincided with significant changes in the rate of water transport within the MOC, but it appears that at least some such abrupt climate changes did coincide with such changes. We cannot yet determine for certain whether changes in the MOC caused these additional abrupt climate changes, or vice versa.

Recent studies of the MOC suggest that the rate of formation of North Atlantic Deep Water and thus the rate of flow in the MOC exhibits strong short-term variability that makes it very difficult to assess whether there are any long-term trends in its strength. However, some estimates suggest that the MOC may have slowed down, perhaps as much as 30% during the past several decades. This slowdown, if it can be confirmed, may be related to climate change. If so, and if this indicates that the MOC will continue to slow down as the planet warms due to anthropogenic climate change, one possible result could be the abrupt (years to decades) onset of a cold climate period in Europe, similar to the Younger Dryas period, and to other abrupt climate changes elsewhere. However, modeling studies (CC10) suggest that climate change is likely to slow the MOC but not to cause it to slow so much that catastrophic changes, including drastic cooling of Europe, will occur within the next century or two.