MOC stands for Meridional Overturning Circulation, and although it refers to the same global pattern of ocean currents (‘conveyor belt’) as the thermohaline circulation, this story shows why actually MOC is the more accurate name, as it is not just the ocean’s temperature and salt content that fuel the engine.
We know from ice age research that the Meridional Overturning Circulation can shut down. This has the effect of ‘stopping the Gulf Stream’ and bringing a chill to Europe, the Arctic, Atlantic North America and other regions on the northern hemisphere.
Unsurprisingly oceanographers have initially focused their attention at the North Atlantic. Because of the current positioning of the Earth’s continent, here a south-north corridor leads to exceptionally large differences in temperature and salt content (halinity).
It was therefore theorised here not only MOC’s main engine lies, but also its Achilles heel.
Fresh melt water from Pleistocene ice caps and now from Greenland could disrupt the sinking process. MOC however sinks at multiple sites, which could increase stability of the currents in the North Atlantic. And other new research indicates perhaps even at the Younger Dryas it wasn’t necessarily the Gulf Stream that initially shut down.
And new research published in Nature Geoscience invites us to look even further to the south. As the bipolar seesaw theory explains, when MOC shuts down it’s not just a binary state change for the Arctic regions, but also for Antarctica.
So if changes in the Southern Ocean are radical as well, wouldn’t that be where change would originate?
The ocean currents in the Southern Ocean are a bit more complex than in the North Atlantic. If firstly we ignore that spinning around the South Pole behaviour, there is another oddity.
The Southern Ocean is both an important zone of water upwelling, as it is an important sink – a location for deep water formation, as the image on the top of this story [from the Nature Geoscience publication] illustrates [AMOC shown across Atlantic from Antarctica (left) to Arctic Ocean (right)] the deep water formation occurs along the ice margins close to the continent of Antarctica, whereas upwelling occurs in a zone somewhat further to the north.
This Southern Ocean upwelling is of great importance for the Earth’s climate system, as it controls ‘the rate at which ocean reservoirs of heat and carbon communicate with the surface,’ the two researchers of MIT and Florida State University write [see two other recent publications on AMOC and ‘missing heat’].
And that is not just driven by temperature and salinity, but by winds instead. The Antarctic circumpolar winds could be increasing under the current climate change, and they could also retreat to the south. It is yet unclear how this affects the Southern Ocean upwelling, but if that were to increase, it would also increase the general Meridional Overturning Circulation and thereby both ocean temperature and carbon absorption and Arctic melting [as the Gulf Stream would become stronger, not weaker – and the Arctic Ocean may also become saltier].
If however either the upwelling or the sinking in the Southern Ocean were to be disturbed by changing wind patterns, that could prove to be another MOC power switch – and add a totally different ‘shut down hypothesis’ to the scientific climate debate.
“It has become clear over the past few years that the importance of Southern Ocean upwelling for our understanding of climate rivals that of North Atlantic downwelling,” the researchers conclude.
© Rolf Schuttenhelm | www.bitsofscience.org