Whether you call it the Meridional Overturning Current, or the Meridional Overturning Circulation, it’s the ocean current that makes a complete lap around the Atlantic Ocean from the Arctic to the Antarctic. Because the current's movement reaches to about 60 degrees of latitude, both above and below the Equator -- the Earth's "zero" meridian -- it's called a "Meridional" circulation. It once made the journey from the New World to Europe faster and it moves the heat stored in the ocean around the world. For this reason, it sometimes called the Atlantic Conveyor.
The Meridional Overturning Current helps moderate weather along the coastal areas of the Atlantic Ocean and contributes its share of energy to the worldwide ocean circulation. Some early oceanographers called it a thermohaline current, thinking it was the result of heat and salt levels alone, but it begins as a wind-driven surface current originating near the Florida coast, where it's called the Gulf Stream. There, it's a warm, wind-driven surface current several miles wide that moves warm tropical waters northwestward where the water's remaining heat energy helps moderate the weather on the western coast of the United Kingdom. Along the way, the current stores the wind’s kinetic energy as potential energy.
By the time it reaches the coast of Norway, it’s spent much of the heat that it carried north. As its temperature drops, its molecules move more slowly and it becomes heavier and denser. If you weigh a cubic foot of water collected on the edge of the North Sea, it would be heavier and saltier than a cubic foot of water collected on the coast of Florida: the water’s salinity increases as it picks up more salt from the colder Arctic Ocean.
As the current picks up the salt from the Arctic waters it beings to sink, hence the name "overturning current" or "overturning circulation." The wind energy that drove it northward is still carried in the current and, as it sinks, the potential energy that the wind transferred to the current becomes kinetic energy once more, driving the current forward as it sinks.
The current moves southward, toward warmer water, as if to warm itself. It’s said that nature abhors a vacuum; like all of nature, water seeks moderation in pressure and temperature. As it approaches the warmer waters near the Equator, it heats. As it heats, it sheds its burden of salt and rises to the surface.
Newton’s First Law of Motion says a body in motion stays in motion until acted upon by an outside force. This applies to movement in the water, as well. As the current rises, it continues in its southerly direction until it encounters the colder waters of the Southern Ocean that surrounds Antarctica.
Chilled once more, the current picks up salt, and sinks. Part of the current continues to travel around Antarctica, rising later in the Pacific Ocean’s upwellings, where cold waters surface, sometimes suddenly.
Part of the current splits off and moves along the eastern coast of Africa, rising as it moves back into the southern Atlantic. As it reaches the Straits of Florida, its thermal energy dissipated, it once more becomes a wind-driven current, and the cycle repeats itself, perhaps several decades or even centuries after it began.