Monday, December 14, 2009

Deep Water

To start with, this headline and writer has got it exactly backwards.  The oceans are the great engine that actually dictates all the gross aspects of what we call science.  It absorbs most of the incoming solar energy.  What it fails to absorb is then absorbed a little by the atmosphere ore mostly bounced back into space or used to warm up rocks and encourage vegetation.  The latter two provides us with a very thin surface layer that is sometimes quite warm and in which the land based biosphere resides.

The ocean surface layer which captures effectively all this energy is about 200 meters thick.  After that the break into cold water is abrupt and approaches freezing temperatures as one gets deeper.

Our ignorance about the three dimensional geometry of this environment is presently abysmal.  All I have seen in the literature so far is excuses. I am sure that the first 200 meters is well mapped because of submarine activity.  They really need to know.   But once you get deeper, the work has barely begun.

I suspect it is critically important in understanding the drivers behind Arctic sea ice reduction.

It is going to take a deep diving device able to able a measure flow rate and direction continuously down to the deep sea bottom.  This must be done on a grid basis in a progressively tighter grid as discontinuities are picked up.  Obviously this will go on for years, but the equipment capability exists to do this.  Some part of it may even have begun.  If you know of any such effort, please let me know.

The reason this is important is clear.  A velocity change has already been observed in a known flow.  This means that somehow the geometry changed since the volume cannot change.

A change in velocity will change the energetics of the surface layer.  During this period it has been observed that the warmer surface waters have increased their contribution to the Arctic and the Antarctic.  The change of velocity may have changed little that is substantive but sufficient to inject additional annual heat into the surface layer.  We do not need very much at all and it will be difficult to measure.

The result has been the progressive destruction of all multiyear ice over the past three decades.  Merely assuming the heat contribution has not varied much is sufficient to explain present evidence.

December 10, 2009

In Deep Water: Will Essential Ocean Currents Be Altered by Climate Change? [Slide Show]

Scientists are struggling to get a grasp on the huge volumes of water flowing through the world's oceans

By Nancy Bazilchuk   
CURRENT QUESTIONS A key aspect of the Atlantic Ocean's large-scale circulation involves the sinking of cold, dense water--called deep-water formation--in Arctic and Antarctic waters. Scientists who are trying predict how climate change will play out in different parts of the globe want to find out how this vital circulation is altered in a warmer, wetter world--and how this alteration in turn will affect climate.

Every second, a vast quantity of cold, dense seawater equal to six times the combined flow of every land river on Earth streams over an ocean-floor ridge that stretches between Greenland and Scotland. This deep southbound current, flowing from the Norwegian, Iceland and Greenland seas into the North Atlantic, is the lower limb of the Gulf Stream and its northerly extension, a great conveyor belt of ocean heat and salt that transports warm tropical water north from the equator. Most climate change models predict global warming will slow these flows, in part by altering a key component of the Atlantic's circulation, called deep-water formation. If that happens, northern Europe will cool—or warm less severely—as the rest of the globe swelters.

Understanding the role that deep-water formation plays in driving this grand circulation pattern, more formally called the Atlantic Meridional Overturning Circulation (AMOC), will help scientists predict how global warming will affect climate—both in and beyond the Northern Hemisphere. Shifts in the Atlantic's circulation patterns will alter African and Indian monsoon rainfall as well as hurricane patterns in the South Atlantic, resulting in "a profound impact on the global climate system," according to a team of international scientists asked by the U.S. government to evaluate the potential for abrupt climate change.

Oceanographers have been using moored acoustic Doppler current profilers and temperature sensors for the last decade to measure deep water as it pours over the Greenland–Scotland Ridge on its way south. They're both trying to establish the natural yearly and decadal variations in its creation as well as look for evidence of changes from human-generated temperature increases. "Our assumption is that when we are studying the exchange between the North Atlantic and the Nordic Seas, that this deep conduction and cooling of ocean water is important to the AMOC," says Svein Østerhus, an oceanographer at the Bjerknes Center for Climate Research in Bergen, Norway, who has been investigating the flows.

Deep-water formation is just what it sounds like: As the Atlantic's surface waters travel north they become cooler and denser, so that by the time they reach the Arctic they are cold enough to sink to the ocean bottom. The sinking water pulls warm surface waters like the Gulf Stream north, which in turn leaves a void that pulls deep, colder water south. If global warming inhibits the formation of deep water, the flows across the Greenland–Scotland Ridge should slow.

But it's not that simple. Between the 1950s and the 1990s, the deep water in the Nordic Seas was both warmer and increasingly less salty. As a result, "we had quite remarkable changes in deep-water formation," Østerhus says. Nevertheless, the flow of deep water headed south over the Greenland–Scotland divide has remained stubbornly stable for the past 50 years, Østerhus and his colleagues reported a paper in Nature late last year. The reasons for these counterintuitive findings are not clear, he says. It may be that deep water is pooling behind the Greenland–Scotland Ridge, providing a reservoir from which older deep water can flow when production is slowed. Østerhus's colleague, Detlef Quadfasel, an oceanographer at the University of Hamburg, thinks that part of the explanation is that "this is a nonlinear system—it can simply jump from one state to another."

Now Østerhus will travel in January to the other place on the planet where deep water forms—the Antarctic. The Weddell Sea off Antarctica is home to the coldest, densest deep water on Earth. Østerhus's mission will be to retrieve data from a string of high-tech monitors put in place there last February, with the hope of understanding how Antarctic deep-water formation affects the churning of the Atlantic's currents far to the north. There is very little information on deep-water formation in the Antarctic, and Østerhus will collect basic data on temperatures, velocities and salinity that will form the foundation for later comparisons.

"Deep-water formation in the Arctic and Antarctica are of equal importance, and they are linked," he says. "Deep water formed in the Arctic can be traced the whole way south to Antarctica and deep water formed in the Weddell Sea can be traced as far north as Ireland. A change in the Antarctic deep-water formation may have an impact on the circulation of the North Atlantic."

As Østerhus and his colleagues scramble to understand what's going on at the most distant edges of the Atlantic, roughly two dozen other projects are measuring the Atlantic’s flows elsewhere, including a string of instruments from the Bahamas to Morocco. And in late September an international team proposed that the entire network be transformed into a more formal international monitoring effort in the hopes of helping humankind prepare for what global warming will bring.

"We rely on climate forecasting models to understand climate change, but not all models are in agreement," says William Johns, a University of Miami oceanographer who helped write the abrupt climate change report for the U.S. government. "We have to refine these models to get some idea of what we are going to have to get adapted to—and exactly how much warming we will get in the North Atlantic depends on how much this circulation really does slow down."

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