Meridional Overturning Circulation

The oceanic circulation moves water in all three directions, including upward motion at the equator. Once at the surface, the sun heats the cold water from below. The water then flows poleward; it loses its heat to the atmosphere, sinks, and returns to the equator below the ocean surface. This up–down and north–south motion is referred to as the meridional overturning component of the circulation, which also has a zonal, east–west component.

The ocean has not one but several meridional overturning circulations. The best-known circulation is associated with the slow, deep thermohaline circulation that maintains remarkably uniform temperatures and salinities below the surface layers. It involves the sinking of cold, saline water in the northern Atlantic. Afterward, the water flows southward, ultimately returns to the surface, and then proceeds back to the northern Atlantic.

Wind-Driven Gyres

At least as important but less well known are the meridional overturning components of the swift, shallow, wind-driven gyres that are confined to the surface layers and include intense jets such as the Gulf Stream in the Atlantic and the Kuroshio Current in the Pacific. These jets off the eastern coasts of continents move warm water poleward.

That water returns equatorward, very slowly, in weak currents across the rest of the ocean basin. The gyres are not strictly horizontal, but also have a meridional overturning component. The surface waters sink in certain subtropical subduction zones, flow equatorward at shallow depths, and rise back to the surface at the equator and other oceanic upwelling zones where the surface waters are exceptionally cold. In these regions, in the vicinity of the Galapagos Islands, and off the coasts of California and Peru, for example, the cold water is heated by the sun and in due course is carried poleward by warm jets such as the Gulf Stream; it then loses heat to the atmosphere before returning to the regions of sinking. In the Pacific, this shallow meridional overturning circulation of the wind-driven gyres, which does not involve the thermohaline circulation, is entirely responsible for the poleward transportation of heat. In the Atlantic, the wind-driven circulation must contribute significantly to the maintenance of the temperate climate of western Europe.

The connections between the deep and shallow meridional overturning components are poorly understood, but it is known that the winddriven and thermohaline circulations merge in the westward Antarctic Circumpolar Current of the Southern Ocean. The meridional circulations and their transportation of heat can be shut off should the surface waters become too buoyant in the regions where sinking occurs. Should this happen to the thermohaline circulation, the climate of western Europe would likely be affected. It can also happen to the wind-driven circulations, with climatic consequences as dramatic as those associated with El Niño. The cold tropical surface water that the sun heats up, instead of cooling off in higher latitudes, accumulate in low latitudes so that sea surface temperatures rise. El Niño demonstrates how such a change can affect the global climate. Whereas the temperature changes associated with El Niño are a consequence of an adiabatic, horizontal redistribution of warm surface waters and can occur rapidly in a matter of months, those that result from a shutdown of the meridional overturning component of the wind-driven circulation are diabatic

and take on the order of decades.