Global Ocean Currents
The global ocean is made up of five major, connected ocean basins. The water in these basins is constantly flowing in streams called currents. Some currents are shallow, while others sink to the ocean floor and travel from pole to pole, taking hundreds of thousands of years before they rise to the surface again.
This system of global ocean currents mixes the ocean from top to bottom and all around the planet transferring heat, nutrients and minerals from one place to another. Shallow currents in the top 3000 feet (1000 meters) of the ocean help regulate the climate, for example by transporting warm water from the equator to the north and south, and bringing cool water to the equator, helping to keep the climate mild.
The largest and most influential system of ocean currents is often called the Global Conveyor Belt. It transports water from Antarctica to the Arctic via the equatorial regions distributing heat, nutrients and carbon across the global ocean.
How do ocean currents work?
There are two main types of ocean current: surface currents set in motion by wind, tides and the spin of the earth, and deep, slow-moving currents generated by differences in temperature and salinity (salt content). These density-driven currents are often called thermo-haline (thermo = relating to temperature, haline = relating to saltiness).
The great ocean pump
The Southern Ocean is like an engine room for ocean circulation. Around some parts of the Antarctic coastline strong winds and sea ice processes result in the formation of extremely cold, salty water. This water – Antarctic Bottom Water (ABW) – is some of the heaviest water on our planet, much heavier than the water around it. It sinks to the seafloor (around 1.8 miles [3000 meters] down) and relatively warm, less salty water moves in to replace it at the surface. Then the cycle begins again.
This great ocean pump, along with a similar pump found in the North Atlantic, powers the global conveyor belt. The extreme density of AABW makes these regions some of the most powerful ocean pumps on the planet.
Antarctic bottom water is declining
In 2012, Antarctic oceanographers reported that the volume of Antarctic Bottom Water (AABW) had reduced by around 60 percent since the record began 42 years before. The overall production of AABW was declining. Over the next five years Antarctic Bottom Water also became warmer, fresher and less dense.
Research suggests that ABW is becoming fresher because as the climate warms, glaciers around Antarctica are melting more quickly, flushing the coastline with fresh water from glacier ice. Other factors may also contribute including increased rainfall and reduced sea ice formation, both caused by the warmer climate.
This change in the volume of ABW could reduce the strength of the ocean pump, slowing the global conveyor belt and disrupting the global climate.
Changes to the global climate
WHAT ARE THE IMPACTS?
In August 2021, scientists reported that the Atlantic Meridional Overturning Circulation (AMOC), part of the global conveyor belt which includes the Gulf Stream in the North Atlantic, was showing signs of slowing. These changes were attributed to rising sea surface temperatures in the tropics and freshening of seawater caused by melting of the Greenland Ice Sheet.
Researchers are continuing to investigate the effects of fresher, less dense Antarctic Bottom Water on global currents. If lighter Antarctic Bottom Water weakens its pumping effect and slows the global conveyor belt down, this will have serious consequences for our global climate.
In a warmer climate global circulation could slow or even shut down. More frequent and severe storms, floods and extreme El Niño events could occur, oxygen levels in the ocean could fall, and marine ecosystems could collapse entirely.
The Southern Ocean flows around Antarctica.
Life on Ice
Reduced sea ice is impacting Antarctic life.
Carbon dioxide is making the ocean more acidic.
The Southern Ocean is warming up.
Life at Sea
Warmer oceans are affecting ecosystems.