Somewhere in the North Atlantic, cold salty water is sinking into the abyss. Perhaps this action occurs in the Labrador Sea, or maybe the descent takes place in the waters between Greenland and Scotland. But the water is disappearing from the surface, sinking to the seafloor and feeding a deep ocean current known as the “global conveyor belt.” This movement of water from shallow to deep is the driver of the Atlantic Meridional Overturning Circulation (AMOC).
The Gulf Stream owes its existence to the AMOC. The term “overturning circulation” refers to the transfer of water from the ocean surface to the ocean bottom and back again. This process is also called thermohaline circulation.
Some of the water sinking to the bottom of the North Atlantic starts a journey that will last 1600 years before the water surfaces again in the North-Central Pacific Ocean. This journey takes water from the Arctic to the Antarctic, and then around the globe in the Southern Ocean. When the water reaches the Pacific Ocean, it spins off into a current that travels northward, traversing the length of that ocean. It is a hell of a journey.
The journey
The volume of water moving along the conveyor belt is immense, measuring more than 100 times the flow of the Amazon River. But life on the bottom of the ocean moves at a leisurely pace with the current flowing just a few centimeters per second. When the North Atlantic waters reach the seafloor, they flow southward. So, the first leg of the journey travels the length of the Atlantic Ocean from north to south.
Thermohaline circulation relies on gravity for its energy. Hence the cold, dense North Atlantic waters flow downslope seeking the lowest points on the ocean floor. The current hugs the deep ocean between the Mid Atlantic Ridge and the coast of South America until it reaches the Southern Ocean off the coast of Antarctica.
When the current enters the Southern Ocean, geography takes over as a controlling force. The narrow bit of ocean between the tip of South America and Antarctica is Drake Passage. The ocean floor rises up beneath this passage, forming a barrier to the density-driven conveyor belt. Thus, forcing the current to travel east towards the Indian and Pacific Oceans.
Some of the current’s water then splits off and travels northward up the east coast of Africa and into the Indian Ocean. However, the rest of the water continues on past Australia before it takes a northward turn into the Pacific Ocean.
Return to the surface
When the deep currents make their way northward in the Indian and Pacific Oceans, the water they carry warms. Warm water is less dense than cold water, so these deep waters rise and return to the ocean surface. This process of upwelling is an essential part of marine ecosystems.
During the long journey along the seafloor, the waters of the global conveyor belt pick up carbon dioxide and nutrients. When they return to the surface, these nutrient-rich waters feed the plankton at the base of the food chain. If the plankton thrive, then the fish and mammals farther up the chain also thrive.
But the journey is only halfway over upon the return to the surface. From the North Pacific, the water travels in surface currents on a journey westward. The surface currents carry the water between Indonesia and Australia and then across the Indian Ocean and around the southern tip of Africa. Once back in the Atlantic Ocean, the currents flow northwest across the ocean and into the Caribbean. The water warms in the heat of the tropical sun and then travels northeast via the Gulf Stream. After a journey of close to two thousand years, the warm salty water returns home to the North Atlantic. Here it gives up its heat to warm Europe, and when it has cooled enough, the water sinks to the bottom of the Atlantic Ocean, beginning the journey again.
ArcheanWeb:
Thermohaline circulation (Source: ArcheanWeb) – https://archeanweb.com/2020/04/03/thermohaline-circulation/ Also:
Gulf Stream and the Little Ice Age (Source: ArcheanWeb) – https://archeanweb.com/2019/12/30/gulf-stream-and-the-little-ice-age/ Also:
The Antarctic Circumpolar Current: An Ouroboros (Source: ArcheanWeb) – https://archeanweb.com/2020/01/29/the-antarctic-circumpolar-current-an-ouroboros/ Also:
Sources
What is the Atlantic Meridional Overturning Circulation? (Source: Met Office) – https://www.metoffice.gov.uk/weather/learn-about/weather/oceans/amoc Also:
The Global Conveyor Belt (Source: NOAA) – https://oceanservice.noaa.gov/education/kits/currents/06conveyor2.html Also:
Feature Image (Blue arrows are deep currents and red arrows are surface currents): Thermohaline circulation (By Brisbane) (Modified) – https://commons.wikimedia.org/wiki/File:Thermohaline_circulation.png – This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license – https://creativecommons.org/licenses/by-sa/3.0/deed.en
