Sunday, July 18, 2010

Atlantic/Pacific circulation drivers swapped 16 millennia ago

One way of looking at the global ocean current picture is that it's driven by gravity.

(Image: Left panel ‐ glacial current conveyor belt flow 21,000 years ago; Right panel ‐ reorganized conveyor belt flow 17,500‐15,000 years ago with deep‐water sinking in the North Pacific. Credit: IPRC.)

Cold water is heavier than warm water, so gravity lets it sink. Saltier water is heavier than less salty water, so gravity also forces it to sink.

Warm water flows northward in the Atlantic, gets chilled, and sinks. It sucks other water behind it, and pushes on the water in front of it, launching a system of currents that ultimately extends around the world.

Hawai'i scientist Axel Timmermann, of the University of Hawai'i's International Pacific Research Center (IPRC), is part of an international team of scientists who have studied what happened when this system stopped working 16 millennia ago. It was a period of dramatic climate change that scientists call the Mystery Interval.

They reported their findings in the July 9 issue of Science. A press release here.

A research team looked at sediment cores from 30 places around the Pacific, and studied the marine organisms that lived in the Pacific during the period in question.

They write that as the last ice age ended, between 17,500 and 15,000 years ago, melting glaciers dumped so much cold fresh water into the North Atlantic that it blocked the big current flow there.

But the world's ocean currents didn't stop. Instead, the North Pacific took over as the prime mover in current flow, said Timmermann's co-researcher and lead author in the Science paper, Yusuke Okazaki, of the Japan Agency for Marine-Earth Science and Technology:

“Around 17,000 years ago, the North Pacific surface waters grew saltier, and the resulting higher density there caused massive sinking. Newly formed icy deep water spilled out of the subarctic North Pacific at depths of 2000-3000 meters merging into a southward flowing deep western boundary current. A warm, strong poleward current, moreover, formed at the surface. It released much heat into the atmosphere and supplied water for the Pacific deep overturning circulation,” Okazaki said.

Timmermann said that the collapse of the North Atlantic current system might have launched a severe cooling episode, but that the new North Pacific current activity began moving water in such a way that equatorial heat was transferred northward, “and possibly prevented further cooling of the Northern Hemisphere.”

The new Pacific activity may also have roiled up deep-ocean waters rich in carbon, and increased the globe's carbon dioxide levels, causing still more warming, the authors say. The result may have been that instead of the dying Atlantic circulation causing more cooling, the new Pacific circulation may have had the opposite effect, said IPRC's Laurie Menviel, another author of the paper.

“This could have catalyzed further warming and accelerated the glacial meltdown,” Menviel said.

It's an intriguing theory. Timmermann said more tests are needed to confirm the conclusions about activity during the Mystery Interval.

“Our findings caution against the Atlantic-centric view of abrupt climate change that has prevailed amongst climate scientists for the last 20 years. They highlight the complicated adjustments happening in the global ocean during these periods of climate change, in which the North Pacific was definitely a player to be considered.”

Timmermann also said that it is unlikely a similar Atlantic-Pacific swap could occur today, mainly because the Bering Strait between North America and Russia-Siberia was once iced over and now is open. That means water movement would prevent a dramatic salinity change that launched the North Pacific circulation system more than 15,000 years ago.

If this stuff fascinates you, there's a three-part National Science Foundation video interview with Timmermann here.

The paper: Okazaki, Y., A. Timmermann, L. Menviel, N. Harada, A. Abe-Ouchi, M. O. Chikamoto, A. Mouchet, H. Asahi, 2010: Deep Water Formation in the North Pacific During the Last Glacial Termination, Science, July 9, 2010.

© Jan TenBruggencate 2010

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