Montoya & Levermann: Small variations in surface winds could have been cause of abrupt climate change during the Last Glacial Maximum

Changes in winds could have been cause of abrupt glacial climate change

ScienceDaily (July 21, 2008) — Spanish and German researchers have carried out a collaborative study that shows how during the last glacial period, small variations in the surface winds could have induced significant changes in the oceanic currents of the North Atlantic, and could even have played a role in the abrupt climate change that occurred at the time.

Scientists from the Complutense University of Madrid (UCM) and the Potsdam Institute for Climate Impact Research in Germany have carried out a study which identifies small alterations in the superficial sea winds as the factors with a key role in the abrupt climatic change that occurred over the last glacial period whose origin is not yet fully understood.

This study, carried out by researchers Marisa Montoya and Anders Levermann, concluded that there is a precise point from which a small variation in the speed of sea winds corresponds to a dramatic change in the Atlantic circulation intensity. According to Marisa Montoya, “If the glacial climate had been in the vicinity of that point, small wind changes could have caused sudden and significant climatic changes during that period.”

The study was based on climatic simulations called Last Glacial Maximum (LGM) (the period of maximum extension of the perpetual ice sheets that took place over 21,000 years ago). These simulations have demonstrated the existence of a threshold after which a small change in wind speed causes disproportionately large changes in the sea current speed. The results indicate that these changes in wind speed could have had a particularly important role in the abrupt climatic change of the last ice age.

Climate simulation of the Last Glacial Maximum is one of the principal challenges for experts in this area. The comparison of results from these simulations with climatic reconstructions based on data gathered from natural elements, such as sea sediments or the oldest ice samples; permit the evaluation of the climatic models in conditions independent from the ones used for their design. The results confirm the relevance of the small variations and help further substantiate the hypothesis about the physical mechanisms responsible for the climatic changes observed in the reconstructions.

Both, the climatic simulations as well as the reconstructions, indicate that variations in the Atlantic Ocean circulation could have been the key mechanism responsible for the abrupt climatic change that took place over the ice age. This circulation plays a fundamental role in the regulation of climate on a global scale, since it transports large quantities of relatively warm water from low latitudes to northern regions, softening the climate of countries like Norway or Ireland in comparison with other regions in the same latitude, but with much harsher climates, like Alaska or New York. This study therefore suggests that the changes in oceanic circulation could have been caused by changes in the speed of surface winds.

M. Montoya & A. Levermann. 2008. Surface wind-stress threshold for glacial Atlantic overturning. Geophysical Research Letters, 35(3) L03608; DOI: 10.1029/2007GL032560

J. A. Johannessen et al., Direct ocean surface velocity measurements from space: Improved quantitative interpretation of Envisat ASAR observations

Geophys. Res. Lett., 35, L22608; doi:10.1029/2008GL035709

Direct ocean surface velocity measurements from space: Improved quantitative interpretation of ENVISAT ASAR observations

Direct ocean surface velocity measurements from space: Improved quantitative interpretation of Envisat ASAR observations

J. A. Johannessen (Nansen Environmental and Remote Sensing Center, Bergen, Norway), B. Chapron (Institute Francais de Recherche pour l'Exploitation de la Mer, Plouzané, France), F. Collard (BOOST Technologies, Brest, France), V. Kudryavtsev (Nansen International Environmental and Remote Sensing Center, St. Petersburg, Russia), A. Mouche (BOOST Technologies, Brest, France), D. Akimov (Nansen International Environmental and Remote Sensing Center, St. Petersburg, Russia), and K.-F. Dagestad (Nansen Environmental and Remote Sensing Center, Bergen, Norway)

Previous analysis of Advanced Synthetic Aperture Radar (ASAR) signals collected by ESA's Envisat has demonstrated a very valuable source of high-resolution information, namely, the line-of-sight velocity of the moving ocean surface. This velocity is estimated from a Doppler frequency shift, consistently extracted within the ASAR scenes. The Doppler shift results from the combined action of near surface wind on shorter waves, longer wave motion, wave breaking and surface current. Both kinematic and dynamic properties of the moving ocean surface roughness can therefore be derived from the ASAR observations. The observations are compared to simulations using a radar imaging model extended to include a Doppler shift module. The results are promising. Comparisons to coincident altimetry data suggest that regular account of this combined information would advance the use of SAR in quantitative studies of ocean currents.

(Received 15 August 2008; accepted 2 October 2008; published 29 November 2008.)

Citation: Johannessen, J. A., B. Chapron, F. Collard, V. Kudryavtsev, A. Mouche, D. Akimov, & K.-F. Dagestad (2008), Direct ocean surface velocity measurements from space: Improved quantitative interpretation of Envisat ASAR observations, Geophys. Res. Lett., 35, L22608; doi:10.1029/2008GL035709.

Link to abstract: http://www.agu.org/pubs/crossref/2008/2008GL035709.shtml

Improving measurements of ocean surface velocity from space

Advanced synthetic aperture radar (ASAR) signals collected by the European Space Agency's Envisat help scientists map surface currents in the world oceans. As demonstrated, ASAR is a valuable source of high-resolution information on the line-of-sight roughness and velocity of the moving ocean surface. The velocity is estimated from careful examination and analysis of residual Doppler frequency shifts extracted from the ASAR scenes at a spatial resolution of about 5-10 km (3-6 miles). Using this methodology, Johannessen et al. consistently derive both kinematic and dynamic properties of the ocean surface roughness. The authors compare the observations with advanced simulations as well as altimeter-based surface current measurements, finding good agreement to motivate further research to integrate the derived characteristics of ocean surface motion into broader models. This can help scientists learn more about mesoscale and submesoscale upper ocean dynamics, coupled physical-biogeochemical processes, and air-sea interactions in intense current regimes.

The Suntory Mermaid II, Moved Only by Waves, Sails to a Seafaring First

by John J. Geoghegan, New York Times, Science section, July 8, 2008

The Suntory Mermaid II successfully completed late Friday night a 4,350-mile trip from Honolulu, Hawaii, to the Kii Channel off the east coast of Japan, marking the longest known voyage by a wave-powered boat.

The journey was undertaken by a Japanese team to demonstrate that an environmentally sensitive propulsion system powered exclusively by waves can operate in real-world conditions.

The bow-mounted mechanism, which harnesses wave power to provide a dolphinlike tail kick from two independently mounted flippers, was designed and built by Dr. Yutaka Terao of the department of naval architecture and ocean engineering at the Tokai University School of Marine Science and Technology in Japan.

The design team originally estimated that the 31-foot-long, three-ton catamaran would average 3-4 knots and arrive off the east coast of Japan about 60 days after its departure on March 16. But, unusually good weather and calm seas resulted in the boat traveling an average of only 1.5 knots, and the Mermaid’s maiden voyage ended up taking 111 days. Nevertheless, Dr. Terao and his team were satisfied with the result.

“We were able to prove that our propulsion system delivers a 7,000-kilometer voyage,” Dr. Terao said in an e-mail interview from Japan. “And we can easily improve the speed. In fact, the improvements have already started.”

Kenichi Horie, the ecologically minded sailor who captained the Mermaid, has set two world records for piloting environmentally sensitive boats, the first in 1993 for the longest distance traveled in a human-powered pedal boat (4,660 nautical miles) and the second in 1996 for the fastest Pacific crossing in a solar-powered boat (148 days).

At a dockside celebration on Sunday at Shin Nishinomiya Yacht Harbor, Mr. Horie told the gathering: “The time has come for us to shift from fossil fuels. I hope this voyage will increase awareness and interest in natural energy.”

Mr. Horie, 69, appeared energetic if noticeably thinner after his three and a half months at sea.

“I had some food left, so I could have enjoyed the trip a bit longer,” he said with a smile. “But I think I’ll save it for the next voyage.”

Link to article: http://www.nytimes.com/2008/07/08/science/08wave.html