Atlantic Meridional Overturning Circulation

Ocean currents, fueled by winds, circulate across the surface as well as into the depths of the world’s oceans. These currents carry heat and are critical to climate systems around the world. One of these currents is the Atlantic Meridional Overturning Circulation (AMOC), which according to the Encyclopedia of Earth is defined as a major current in the Atlantic Ocean that circulates warm, salty water northward in the upper Atlantic and colder water southward in the deep Atlantic (2012). Though often considered interchangeable, the AMOC is not to be confused with Thermohaline Circulation, which describes the general processes by which ocean waters rise and sink, driven by changes in heat and salinity as water cools and heats while moving from the equator to the poles (Survey, 2012). In the case of the AMOC, Thermohaline Circulation plays a role in the transition from warm, salty waters moving up the coast of North America towards the Nordic Seas. Once the warm waters reach the colder climates of the Arctic, the water cools and becomes denser which causes it to sink, creating the North Atlantic Drift. Ice formation in the North Atlantic also contributes to the density of water, as salt falls out of water when it freezes, contributing to the density of the cooling, sinking waters.

This circulation and heat exchange between the water in the Atlantic Meridional Overturning Circulation and the atmosphere helps to regulate the climate of the North Atlantic. The warmth of the waters carried by the Gulf Stream from the equator as it reaches the North Atlantic prevents excessive ice formation in the winter as well as warms the atmosphere, resulting in relatively warmer climate conditions in comparison to those of the North Pacific (Schmittner et al., 2007). In the grand scheme of ocean currents, the cold bottom waters from the North Atlantic end up flowing along the floor of the Atlantic Basin until it meets with the Antarctic cold currents in the Southern Hemisphere and gets reintegrated into the global conveyor belt.

There are signs that the Atlantic Meridional Overturning Circulation could be tied to ancient long term climate changes, such as the Younger Dryas, which was a brief abrupt period of climate change about 14,500 years ago in which the global temperatures dropped to near glacial conditions (Younger Dryas, 2008). It is generally accepted that slowing of the AMOC was the cause of this global cooling. However, the exact cause for the flow reduction is still in question.