The Antarctic Half of the Global Thermohaline Circulation Is Faltering

Shocking news: "the southern branch of the global thermohaline circulation has slowed dramatically." This is already begun to affect global climate, including Arctic ice melt, increased droughts in the US and storminess in the North Atlantic. Climate models hadn't predicted this behavior.

The cold dense brine that forms at the margin of freezing Antarctic sea ice normally sinks to the ocean bottom to drive the thermohaline circulation. Now it's is being deflected instead of sinking. That cold water stays near the top, changing the equator to pole heat gradient, which speeds wind currents.

The global oceans' coldest water, Antarctic bottom water forms in several key spots around Antarctica. The water is so cold and dense that it spreads out along the bottom all of the major ocean basins except the north Atlantic and Arctic. Multiple recent reports provide strong evidence that the formation of Antarctic bottom water has slowed dramatically in response to massive subsurface melting of ice shelves and glaciers. The meltwater is freshening a layer of water found between depths of 50 and 150 meters. This lightened layer is impeding the formation of Antarctic bottom water, causing the Antarctic half of the global thermohaline circulation to falter.

This post is based on a synthesis of very recent research reports. The key report, that found the layer of fresh water between 50 and 150 meters deep, was just published.

That lens of (relatively) fresh water that is forming around Antarctica is challenging, and changing, almost everything in global circulation patterns.  It freezes sooner (and at a higher temperature).  That shields the water from the wind, and reduces wind-driven mixing.  It reduces, perhaps to the point of stopping altogether, the present global ocean circulation patterns.  That in turn will change global atmospheric weather.

Nobody knows exactly what comes next.  We've never seen it happen ...
... intensifying winds are speeding up below surface currents bringing more above freezing water in contact with deep ice around Antarctica. Twenty of the ice shelves and many of the glaciers that feed them are melting from below.

The melting from below is creating a layer of relatively fresh water 50 to 150 meters below the surface around Antarctica. This layer of light fresh water is floating above a  salty layer below. When ice forms at the surface in the Antarctic winter it creates cold dense salty water that tends to sink to the bottom, forming bottom water. However, this layer of light melt water is tending to block the water in the top 50 meters from sinking. The area of Antarctic sea ice has expanded because the layer of cold water has stayed on top and expanded outwards instead of sinking.

Brine ... isn't sinking because it is ponding above a freshening layer located at depths between 50 and 150 meters.

Analysis of potential temperatures, which are temperatures adjusted for the effects of increasing pressure with depth, shows the surface water in the top hundred meters is cooling over a vast area from 40°S to 80°S while the water in that vast area below 150 meters is warming.
The decline in Antarctic bottom water formation, combined with the southward expansion of warm subtropical water in the south Pacific and south Indian oceans has led to the rapid heating of intermediate and deep ocean water in the southern hemisphere.

As the earth has warmed in response to the effects of increasing levels of greenhouse gases the southern subtropical belt in the oceans and atmosphere has expanded, tightening the rings of winds and ocean currents around Antarctica. Enormous volumes of warm subtropical water have been added to the southern ocean at depths greater than 300 meters (greater than approximately 1000 feet).

Another recent detailed study of the water properties of the southern ocean has independently determined that the southern branch of the global thermohaline circulation has slowed dramatically, contributing to a large uptake of heat by the deep southern ocean.

The slowdown of the southern branch of the thermohaline circulation and the cooling of the surface waters close to Antarctica are enhancing the thermal gradient from the tropics to the pole, speeding up the winds in the southern hemisphere. These increases in wind speeds are likely increasing the flow of water from the Pacific to the Atlantic ocean, enhancing the northward flow of water, salt and heat from the south to the north Atlantic.

Thus, increased melting of Arctic sea ice may be related to declines in Antarctic bottom water formation. Likewise, the cool Pacific, warm Atlantic pattern causing increased U.S. droughts and storminess in the north Atlantic may be tied to these changes in ocean circulation patterns. Paleoclimate studies have consistently shown oscillations between Antarctic and north Atlantic bottom water formation and between relative coolness around Antarctica and north Atlantic warmth.

The Arctic melt down that is far exceeding model predictions is connected to the slow down in Antarctic bottom water formation. Climate modelers will be challenged to model the connections and the details. The cooling waters around Antarctica, while apparently good news, are not. The rapid melting of the Arctic will be enhanced.

The red layer at the top (the surface) is the salty cold water that should be sinking. The blue layer is fresher warmer water coming from bottom of melting Antarctic Ice Shelves, which deflects the salty cold water.

Given the importance of the Global Conveyor Belt (thermohaline circulation) I recommend reading the article yourself with a critical eye. This is a major climate tipping point.

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Replies to This Discussion

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Thermohaline Circulation isn't only slowing in the Southern hemisphere, folks. It's already slowing in the Atlantic too. We now realize the risk of complete shutdown of the Atlantic Meridional Overturning circulation this century is greater than 10%, which would a huge Climate Destabilization tipping point.

Gulf Stream system: Atlantic Ocean overturning, responsible for mil...

"Now we have detected strong evidence that the global conveyor has indeed been weakening in the past hundred years, particularly since 1970,"...

The recent changes found by the team are unprecedented since the year 900 AD, strongly suggesting they are caused by human-made global warming.

"So the human-caused mass loss of the Greenland ice sheet appears to be slowing down the Atlantic overturning -- and this effect might increase if temperatures are allowed to rise further,"...

"Common climate models are underestimating the change we're facing,... "That is another example where observations suggest that climate model predictions are in some respects still overly conservative when it comes to the pace at which certain aspects of climate change are proceeding."

"If the slowdown of the Atlantic overturning continues, the impacts might be substantial," says Rahmstorf. "Disturbing the circulation will likely have a negative effect on the ocean ecosystem, and thereby fisheries and the associated livelihoods of many people in coastal areas. A slowdown also adds to the regional sea-level rise affecting cities like New York and Boston. Finally, temperature changes in that region can also influence weather systems on both sides of the Atlantic, in North America as well as Europe."

If the circulation weakens too much it can even break down completely -- the Atlantic overturning has for long been considered a possible tipping element in the Earth System. This would mean a relatively rapid and hard-to-reverse change. The latest report by the Intergovernmental Panel on Climate Change (IPCC) estimates there to be an up to one-in-ten chance that this could happen as early as within this century. However, expert surveys indicate that many researchers assess the risk to be higher. [emphasis mine]



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