I like to move it, move it. Yah like to move it, move it. The oceans move it, move it. We like to….move it!
Ocean circulation plays a major role in shaping Earth’s climate. The great ocean conveyor belt moves 100 times the amount of water in the Amazon river through the world’s oceans; transporting heat, moisture, and nutrients around the globe. This massive water movement can be thought to begin in the North Atlantic off the coast of Greenland. Cold air from the Arctic rushes down and chills the ocean surface, causing the water to become more dense and sink (details can be found in my previous post “Well, Hot Salt!”: Ocean Power). Ice formation is another contributor to creating dense water. As this cold, dense water sinks to the bottom of the Atlantic Ocean, it pulls neighboring water behind it, like a conga line into the depths of the ocean. It doesn’t stop there! Flowing along the bottom of the ocean, this deep water completes an inter-polar journey from the Artic to the Antarctic. It is then caught up by the Antarctic Circumpolar Current and swung around the pole and into different oceans: the Pacific, Indian, or back to the Atlantic. Exhausted from the pole dance (yes, I meant to), deep water sluggishly moves north and mixes with warmer water. As it warms, it rises. Helped by surface winds, the conga line continues, pulling water from the depths in areas of upwelling. Deep water rises to the surface and is whisked off to fuel warm surface currents elsewhere. One well known example of these surface currents is the Gulf Stream in the Atlantic Ocean. Transporting warm water from the equator, the gulf stream flows past the Eastern U.S. coast and, at North Carolina, swings towards Europe and then up to Greenland to start the conga all over again!
I know that’s a lot of information so let’s break it down:
Gulf Stream/North Atlantic Current and North Atlantic Drift Current (NADC) – the “beginning” of the cycle
The Gulf Stream-North Atlantic Current system is an area of shallow, warm water currents that transport heat from the equator, up the Eastern U.S. coast, and across the Atlantic. Southern areas of the Gulf Stream are relatively fast-moving but as these waters reach northern latitudes they slow down and begin to drift…. becoming the North Atlantic Drift Current (NADC). This current travels further north and meets polar waters, cooling and initiating meridional overturning circulation (commonly referred to as “thermohaline circulation” and the “ocean conveyor belt”) through the formation of North Atlantic Deep Water.
North Atlantic Deep Water (NADW) – where things get… deep
Warm water from the Gulf Stream and North Atlantic Drift Current cools and sinks deep into the North Atlantic (hence the name North Atlantic Deep Water… very creative). NADW flows back along the western part of the Atlantic basin (EAST coast of N. America), basically tracing a path underneath the Gulf Stream. Warm waters of the Gulf Stream trap deep water at the bottom, but it has to go somewhere! So the NADW moves south towards the equator. Eventually, NADW reaches the Southern Hemisphere and encounters the Antarctic Circumpolar Current (ACC).
Antarctic Circumpolar Current (ACC) – ’round and ’round we go
Water from the deep Atlantic (NADW) joins and mixes with the Antarctic Circumpolar Current (circumpolar because it circles the pole). The ACC is massive, reaching from waters at the surface all the way to the ocean floor (up to 4000 meters deep), 2000 kilometers wide, and transporting 100-150 sverdrups (the volume of water from all of the rivers flowing into all of the oceans is 1.2 sverdrups)! Water swirls around the South Pole and parts of the ACC break off into the Pacific Ocean, Indian Ocean, and back into the Atlantic Ocean.
Pacific Ocean and Indian Ocean – bringing it back
First the Pacific: water escapes from the ACC and flows along the eastern edge of Australia (still near the bottom of course) and veers towards the West Coast of the U.S. Along the way, it mingles a bit with warmer water but remains near the bottom of the Pacific basin. Fortunately, our water will once again see the light. Upwelling along the Pacific coast of the U.S. allows our deep water to rise to the surface, carrying much-needed nutrients like nitrogen and iron with it. The water then warms and is transported back to the south, traveling between Australia and Indonesia and into the Indian Ocean.
Now, the same kind of upwelling happens in the Indian Ocean. Water from the ACC travels north, along the eastern coast of Africa and towards India where it rises to the surface and moves south. Water from the Pacific and the Indian Ocean merge at South Africa and round the corner, back into the Atlantic.
Atlantic Ocean – making ends meet
The now-warm surface current of the Pacific/Indian Oceans flows up the west coast of Africa and crosses the Atlantic to join the Gulf Stream once again, completing the circle of the ocean conveyor belt. Of course this is a simplified version of the ocean currents but gives you a sense of how heat, nutrients, and carbon dioxide are transported throughout the world.
Recap: Gulf Stream flows north and cools → sinks to form North Atlantic Deep Water → water flows south and joins Antarctic Circumpolar Current → breaks off and moves into the Pacific and Indian Oceans → upwelling brings deep water to the surface → water flows around Africa and back into the Atlantic, joining the Gulf Stream. Tah-Dah!
For more information on meridional circulation take a look at “Introduction: The Ocean’s Meridional Overturning Circulation” by Andreas Schmittner, John C.H. Chiang, and Sidney R. Hemming.
For more detailed information on the ACC check out “The Antarctic Circumpolar Ocean Current: A review of its influence on global ocean currents and climate within Antarctica and Europe” by James S. B. Mason from the Department of Antarctic Studies and Research, University of Canterbury.