The Atlantic Meridional Overturning Circulation

The word meridional in AMOC indicates movement that is in a north-south direction. The AMOC is of immense importance to the global climate and is responsible for transferring heat from the tropics northward to the poles in the Atlantic. AMOC is the Atlantic component of the ocean conveyor belt. Evaporation of surface water at the tropics causes this water to be very salty, because only water is evaporated, leaving behind a high concentration of salt. At the same time, however, the density of the water is lowered through warming by the strong tropical sun. This is because water is maximally dense at 39 degrees F (4 degrees C); above or below this its density decreases. In the North Atlantic, water circulates clockwise because of the Coriolis effect. From the tropics, part of this conveyor belt takes water into the Caribbean basin and along the east of the West Indies.

These two flows unite to form the Gulf Stream, which passes along the east coast of North America. The Gulf Stream influences the climate of the east coast of North America. For example, it keeps the waters of southeast Florida considerably warmer than the rest of the North America in the winter, while in the summer it maintains warmer temperatures in Nantucket and Martha's Vineyard than are found slightly more inland, in Massachusetts Bay. Southeast of Newfoundland, Canada, the Gulf Stream turns easterly across the North Atlantic, where it breaks into numerous branches.

One branch of the warm, salty water moves northward along Great Britain and Scandinavia, shedding heat and warming that part of the world. As it cools, its density increases. It sinks and returns southward as deep water, mostly along the western side of the Atlantic. Eventually, it will recirculate to the tropics, where it will warm once again and rise to the surface. This process explains why it is termed an overturning circulation.

A warming climate is predicted to increase freshwater input in the northern reaches of the Atlantic via increased precipitation, continental run-off, and melting of Arctic Greenland ice. Pulses of freshwater and ice from the Arctic during the latter third of the 20th century appear to have diluted much of the upper waters in the high-latitude North Atlantic. Potentially, such freshening could weaken the circulation by decreasing the density of surface water and by pulling heat from the northern current. Uncovering a trend in the circulation has proven to be difficult because the strength of the circulation varies greatly by season and year and researchers are still learning the extent of this variation. There is presently a shortage of data to make any confident conclusions about changes in the overturning. Longer-term temperature and salinity data covering a greater range of latitudes are needed; more work along these lines is currently underway.

There is circumstantial evidence linking cooling and glacial events several thousand or tens of thousands of years ago to changes in the ocean heat conveyor. Some researchers believe that it is unlikely that a sufficient injection of freshwater is available to shut down the AMOC, or that a substantial slowing is centuries away from occurring; whereas others believe that this tipping point may be approaching. Of intense interest is the Greenland Ice Sheet, which has been losing mass in recent years. In some models, a rapid input of ice and water from Greenland could have a significant effect on the overturning circulation; other models show a melting Greenland Ice Sheet to be insufficient for such an occurrence.

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