AMOC Stability

In fact, the plausibility of substantialvariation of the heat flow into the Arcticmakes all atmospheric causation models risible. It took me a long time to reach that particular conclusion.

What cinched it was the optimumheat flow available to the sea bed during the Bronze Age optimum.  We have never come near it since.

The take home is that heat transferis not stable at all and is prone to shifting abruptly, particularly downward.

It is also plausibly trackingalong a one thousand year cycle that is at least somewhat convincing.  It is getting warmer for three more centuriesanyway.

That heat flow is what keeps usfrom having a fresh Arctic icecap.  It isnot surprising that a modest variation is likely to kick our ass.

Somehow it varies.  We are otherwise unclear as the cycle isstill too long for us to clarify much.

Needless to say , I am somewhatdismissive of scientists mistaking the weather for climate causation.

How stable is the ocean circulation?

Nov 11, 2009

The Atlantic Meridional Overturning Circulation (AMOC) is one of the mostimportant large-scale ocean current systems controlling the Earth's climate.But just how stable will it remain as climate changes and how well do modelsrepresent its stability? That's what a team of scientists from Germany'sPotsdam Institute for Climate Impact Research has been investigating.

"Since the 1980s, scientists have been discussing and studying thepossibility of instabilities in this major ocean circulation," MatthiasHofmann told environmentalresearchweb. "Such instabilities were verylikely the cause of at least twenty abrupt climate shifts during the last IceAge, recorded for example in the Greenland icecores."

The AMOC transports around 1015 W of heat from the tropics towardhigh northern latitudes, bringing Europe arelatively warm and mild climate. So any slowdown or switch-off of the currentscould have a dramatic effect. Today, the circulation brings water from the tipof South Africa right upinto the Arctic Ocean. It then sinks indeep-water formation regions in the Nordic and Labrador Seasand returns south at depths of 2000–3000 m.

The more vigorous hydrological cycle predicted under climate change islikely to increase rainfall over the northern Atlantic and boost the amount offreshwater entering the ocean via rivers. Melting of the GreenlandIce Sheet will also add freshwater to the mix. These factors will reducesalinity and, together with higher sea surface temperatures, this will reduceseawater density and inhibit deep-water formation.

"The North Atlantic as the majorregion of deep water formation is a highly vulnerable area in times of climatechange," said Hofmann. "It has to be regarded as a crucial tippingelement in the Earth's climate system – or even its 'Achilles Heel', as thefamous USocean scientist Wally Broecker once put it."

According to Hofmann, over the last few decades numerous model studieshave revealed that global warming and North Atlanticfreshening could provoke a weakening or even a shutdown of the circulation andhence trigger an abrupt and dangerous climate change. The last IPCC reportconcluded there is an up to 10% risk of "a large and abrupttransition" of the Atlantic Oceancirculation occurring this century.

"To understand the stability of the AMOC, scientists usually studyhow it responds in models to different amounts of freshwater influx, as wouldoccur when Greenland's ice continues tomelt," said Hofmann. "We used an improved model which eliminatesspurious mixing that previous studies had suggested might affect the stabilityof ocean currents in models."

Hofmann and colleague Stefan Rahmstorf, also at the Potsdam Institutefor Climate Impact Research, Germany, found that the basic AMOC instability mechanismis a very robust feature that also occurs in this low-mixing model, where theAMOC is predominantly wind-driven. "However, the question of how close toa dangerous threshold we are remains unresolved," he said.

The pair also reviewed recent findings by Dutch colleagues, who foundstrong evidence that most current climate models could have a systematic biastowards the AMOC being far too stable, compared to the real ocean. "So wemay still be underestimating the risk, based on the current climatemodels," said Hofmann. "These models are good at computing simplethings like global mean temperature, but they are still unreliable when itcomes to highly non-linear threshold responses like an instability in oceancurrents."

Now the pair plan to continue their research by trying to derive asimple measure of the stability of the AMOC, employing a concept recentlyproposed by their Dutch colleagues (de Vries and Weber, GRL, 2005; Dijkstra,Tellus A, 2007). "The goal is to find better ways to check the stabilityof ocean currents in models with data from the real ocean, to test and improvethe reliability of our models," said Hofmann.

Hofmann and Rahmstorf reported their work in PNAS.

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