Ocean Conveyor's 'Pump' Switches Back On

How will climate warming affect ocean circulation? The answer isn't so simple.

by Lonny Lippsett, Woods Hole Oceanographic Institution, January 9, 2009


The Greenland tip jet is a sporadic, low-level atmospheric jet stream characterized by fierce winds on the lee side of Cape Farewell on the southern tip of Greenland. As storms pass through from the southwest, high-level winds descend the glacial slopes on the eastern side of Greenland, accelerating as they drop down over the ocean. In the process, they draw cold air into a relatively small area over the southern Irminger Sea. This phenomenon appears to play a critical role in chilling North Atlantic waters so that they sink to great depths and drive part of the global ocean circulation and climate system. Using NOAA's QuikSCAT satellite, MIT/WHOI Joint Program graduate student Kjetil Våge compiled this image of a tip jet on Dec. 5, 2002. Color indicates wind speeds in meters per second; arrows indicate wind direction. (Courtesy of Kjetil Våge, Woods Hole Oceanographic Institution.)
 

 


One of the  “pumps” that helps drive the ocean’s global circulation suddenly switched on again last winter for the first time this decade. The finding surprised scientists who had been wondering if global warming was inhibiting the pump and did not foresee any indications that it would turn back on.

The “pump” in question is in the western North Atlantic Ocean, where pools of cold, dense water form in winter and sink beneath less-dense warmer waters. The sinking water feeds into the lower limb of a global system of currents often described as the Great Ocean Conveyor (View animation (Quicktime)). To replace the down-flowing water, warm surface waters from the tropics are pulled northward along the Conveyor’s upper limb.

The phenomenon has far-reaching impacts on climate. It transports tropical heat to the North Atlantic region, keeping winters there much warmer than they would be otherwise. And it draws down the man-made buildup of carbon dioxide from air to surface waters and eventually into the depths, where the greenhouse gas is stored for centuries and offset global warming.

The pump is driven by the contrast between warm water and frigid, dry winter air, which draws heat from the ocean into the atmosphere and leaves ocean water colder and denser. Over the last 15 years, the sinking of cold water in the North Atlantic has been either absent or too shallow to feed into the deep Conveyor. Scientists have speculated that a cause could be generally warming air temperatures, which also melts polar ice and adds less-dense fresh water to the ocean. That overall trend didn’t change in 2007, and in fact, Arctic Ocean sea ice disappeared to a record minimum in the summer of 2007.

Yet the sinking of cold water in the North Atlantic resumed vigorously, a research team led by Kjetil Våge and Robert Pickart of Woods Hole Oceanographic Institution reported in the December 23, 2008, issue of Nature Geoscience. “The obvious question is, why?” wrote Våge, Pickart, and colleagues.

A fleet of floats
Their investigations turned up a myriad of interrelated, nuanced factors that make it difficult to predict future changes in ocean circulation and climate, concluded the research team, which also included Virginie Thierry (Laboratoire de Physique des Océans), Gilles Reverdin (Laboratoire d'Océanographie Dynamique et de Climatologie), Craig M. Lee (University of Washington), Brian Petrie (Bedford Institute of Oceanography), Tom A. Agnew and Amy Wong (Meteorological Service of Canada), and Mads H. Ribergaard (Danish Meteorological Institute).

The researchers examined new data collected by robotic floats that have been drifting for several years in the Labrador and Irminger Seas around southern Greenland. These Argo floats—part of a fleet of 3,000 dispatched since 2000 through the world’s oceans—descend to depths of 1.25 miles (2,000 meters), collect temperature and salinity data as they periodically rise toward the surface, and then transmit the data via satellite before descending again (View animation (Quicktime)). Unlike ships that usually (and wisely) avoid rough North Atlantic seas in winter, the Argo floats provide a way to detect the sinking of cold waters in the season that it occurs.

The Argo float data showed that in the winter of 2007-2008, cold water sank significantly beyond 0.62 miles (1,000 meters) deep in northern seas for the first time in eight years and for only the second time since the mid-1990s. Beyond that depth, waters can be swept into lower limb of the Conveyor and carried around the world.

Sinking was undoubtedly enhanced last winter by air temperatures over the North Atlantic that were 9-11 °F (5-6 °C) colder than in the previous seven years. That often occurs when a seesawing pattern of high- and low-pressure air masses, called the North Atlantic Oscillation, is in its “positive” position, bringing frigid westerly winds from Canada streaking across the North Atlantic. But, curiously, that was also the case in 2006-2007, in which sinking did not occur.
 
The lack of substantial sinking throughout the decade meant that there was no “preconditioning”—that is, colder waters could not build up from previous winters to a point where they are easily pushed over a density threshold and sink the following year, the research team said. That made the sudden reappearance of sinking in 2007-2008 all the more surprising.

Tip jets and exported ice
Digging deeper, the researchers found that local wind patterns, which occurred in 2007-2008 (but not the preceding winter), may have played a role. In particular, storms tended to track farther to the south, pulling cold air off the ice edge of eastern Canada into the Labrador Sea. The same storms also continued past Cape Farewell at the southern tip of Greenland, creating a phenomenon known as Greenland tip jets: High winds from the west deflect around the glacial slopes of Greenland, accelerating as they draw cold, ocean-chilling air into a relatively small area over the southern Irminger Sea.

A final clue emerged. Analyzing satellite and in-situ ocean data, the researchers said a large amount of pack ice and fresh water was exported into the northwest Labrador Sea in the summer of 2007. This froze the following winter, significantly extending the ice edge farther offshore. As a consequence, cold air from the North American continent traveled farther over ice, instead of warmer ocean waters, remaining cold until it hit warmer open water in the middle of Labrador Sea. The resulting temperature contrast helped trigger the sinking process.

The scientists noted “that the increased liquid and frozen freshwater flux into the Labrador Sea was probably tied to the large export of sea ice from the Arctic Ocean that contributed to the record minimum in sea-ice extent observed in the summer of 2007. Ironically, this disappearance of Arctic sea ice, which has been linked to global warming, may have helped trigger the return of deep wintertime [water sinking] to the North Atlantic.”

A National Science Foundation grant supported this research. Originally published January 9, 2009; last updated September 3, 2009

Link:  http://www.whoi.edu/page.do?pid=12455&tid=282&cid=54347

Climate Dynamics, Vol. 31, No. 6 (1 November 2008), pp. 665-690; DOI: 10.1007/s00382-008-0369-7

Long-term ice sheet–climate interactions under anthropogenic greenhouse forcing simulated with a complex Earth System Model

Miren Vizcaíno^{1, 2} , Uwe Mikolajewicz¹, Matthias Gröger^{1, 3}, Ernst Maier-Reimer¹, Guy Schurgers^{1, 4} and Arne M. E. Winguth

(1)	Max-Planck-Institut für Meteorologie, Bundestrasse 53, 20146 Hamburg, Germany

(2)	Department of Geography, University of California, Berkeley, CA, U.S.A.

(3)	Present address: IFM-GEOMAR, Kiel, Germany

(4)	Present address: Department of Physical Geography and Ecosystems Analysis, Lund University, Lund, Sweden

(5)	Department of Atmospheric and Oceanic Sciences, Center for Climatic Research, Madison, WI, U.S.A.

(Received 16 April 2007, accepted 9 January 2008, published online 30 January 2008.)

Abstract

From 2004: Satellites record weakening North Atlantic Current

19.04.2004

A North Atlantic Ocean circulation system weakened considerably in the late 1990s, compared to the 1970s and 1980s, according to a NASA study.

Sirpa Hakkinen, lead author and researcher at NASA’s Goddard Space Flight Center, Greenbelt, Md. and co-author Peter Rhines, an oceanographer at the University of Washington, Seattle, believe slowing of this ocean current is an indication of dramatic changes in the North Atlantic Ocean climate. The study’s results about the system that moves water in a counterclockwise pattern from Ireland to Labrador were published on the Internet by the journal Science on the Science Express Web site at:

http://www.sciencexpress.org or http://www.aaas.org

The current, known as the sub polar gyre, has weakened in the past in connection with certain phases of a large-scale atmospheric pressure system known as the North Atlantic Oscillation (NAO). But the NAO has switched phases twice in the 1990s, while the subpolar gyre current has continued to weaken. Whether the trend is part of a natural cycle or the result of other factors related to global warming is unknown.

"It is a signal of large climate variability in the high latitudes," Hakkinen said. "If this trend continues, it could indicate reorganization of the ocean climate system, perhaps with changes in the whole climate system, but we need another good five to 10 years to say something like that is happening." Rhines said, "The subpolar zone of the Earth is a key site for studying the climate. It’s like Grand Central Station there, as many of the major ocean water masses pass through from the Arctic and from warmer latitudes. They are modified in this basin. Computer models have shown the slowing and speeding up of the subpolar gyre can influence the entire ocean circulation system."

Satellite data makes it possible to view the gyre over the entire North Atlantic basin. Measurements from deep in the ocean, using buoys, ships and new autonomous "robot" Seagliders, are important for validating and extending the satellite data. Sea-surface height satellite data came from NASA’s Seasat (July, August 1978), U.S. Navy’s Geosat (1985 to 1988), and the European Space Agency’s European Remote Sensing Satellite1/2 and NASA’s TOPEX/Poseidon (1992 to present).

Hakkinen and Rhines were able to reference earlier data to TOPEX/Poseidon data, and translate the satellite sea-surface height data to velocities of the subpolar gyre. The subpolar gyre can take 20 years to complete its route. Warm water runs northward through the Gulf Stream, past Ireland, before it turns westward near Iceland and the tip of Greenland.

The current loses heat to the atmosphere as it moves north. Westerly winds pick up that lost heat, creating warmer, milder European winters. After frigid Labrador Sea winters, the water in the current becomes cold, salty and dense, plunges beneath the surface, and heads slowly southward back to the equator. The cycle is sensitive to the paths of winter storms and to the buoyant fresh water from glacial melting and precipitation, all of which are experiencing great change.

While previous studies have proposed winds resulting from the NAO have influenced the subpolar gyre’s currents, this study found heat exchanges from the ocean to the atmosphere may be playing a bigger role in the weakening current. Using Topex/Poseidon sea-surface height data, the researchers inferred Labrador Sea water in the core of the gyre warmed during the 1990s. This warming reduces the contrast with water from warmer southern latitudes, which is part of the driving force for ocean circulation.

The joint NASA-CNES (French Space Agency) Topex/Poseidon oceanography satellite provides high-precision data on the height of the world’s ocean surfaces, a key measure of ocean circulation and heat storage in the ocean.

NASA’s Earth Science Enterprise is dedicated to understanding the Earth as an integrated system and applying Earth System Science to improve prediction of climate, weather and natural hazards using the unique vantage point of space. NASA, the National Oceanic and Atmospheric Administration, and the National Science Foundation funded the study.

Krishna Ramanujan | Source: GSFC
Further information: www.gsfc.nasa.gov/topstory/2004/0415gyre.html

Link: http://www.innovations-report.com/html/reports/earth_sciences/report-28159.html

Journal of Climate, 19, 5008-5033, doi:10.1175/2008JCLI2104.1

Global-scale energy and freshwater balance in glacial climate: A comparison of three PMIP2 LGM simulations

S. Murakami, R. Ohgaito (Frontier Research Center for Global Change, JAMSTEC, Yokohama, Japan) A. Abe-Ouchi (Center for Climate System Research, University of Tokyo, Kashiwa, and Frontier Research Center for Global Change, JAMSTEC, Yokohama, Japan), M. Crucifix (Institut d’Astronomie et de Géophysique Georges Lemaître, Université catholique de Louvain, Louvain-La-Neuve, Belgium) and B. L. Otto-Bliesner (National Center for Atmospheric Research, Boulder, CO, U.S.A.)

Figure 1. Geographical maps of (top) vertically integrated northward DSE flux transported by transient eddies, (middle) rms 250-hPa height anomaly from the annual mean climatology, and (bottom) synoptic-scale waves using the MIROC for the (left) CTL and (right) LGM simulations. Contour intervals are 0.05 × 108 W m^-1, 30 m, and 30 m, respectively. High resolution figure

Abstract

Three coupled atmosphere–ocean general circulation model (AOGCM) simulations of the Last Glacial Maximum (LGM: about 21 000 yr before present), conducted under the protocol of the second phase of the Paleoclimate Modelling Intercomparison Project (PMIP2), have been analyzed from a viewpoint of large-scale energy and freshwater balance. Atmospheric latent heat (LH) transport decreases at most latitudes due to reduced water vapor content in the lower troposphere, and dry static energy (DSE) transport in northern midlatitudes increases and changes the intensity contrast between the Pacific and Atlantic regions due to enhanced stationary waves over the North American ice sheets. In low latitudes, even with an intensified Hadley circulation in the Northern Hemisphere (NH), reduced DSE transport by the mean zonal circulation as well as a reduced equatorward LH transport is observed. The oceanic heat transport at NH midlatitudes increases owing to intensified subpolar gyres, and the Atlantic heat transport at low latitudes increases in all models whether or not meridional overturning circulation (MOC) intensifies. As a result, total poleward energy transport at the LGM increases in NH mid- and low latitudes in all models. Oceanic freshwater transport decreases, compensating for the response of the atmospheric water vapor transport. These responses in the atmosphere and ocean make the northern North Atlantic Ocean cold and relatively fresh, and the Southern Ocean relatively warm and saline. This is a common and robust feature in all models. The resultant ocean densities and ocean MOC response, however, show model dependency.

Murakami, S., R. Ohgaito, A. Abe-Ouchi, M. Crucifix & B.L. Otto-Bliesner. 2008: Global-scale energy and freshwater balance in glacial climate: A comparison of three PMIP2 LGM simulations. Journal of Climate, 19, 5008-5033, doi:10.1175/2008JCLI2104.1.

Link to abstract: http://ams.allenpress.com/perlserv/?request=get-document&doi=10.1175%2F2008JCLI2104.1&ct=1

Nature Geoscience, published online 21 June 2009; doi:10.1038/ngeo552

Surface-temperature trends and variability in the low-latitude North Atlantic since 1552

Casey Saenger¹, Anne L. Cohen², Delia W. Oppo², Robert B. Halley³ and Jessica E. Carilli⁴

Link to abstract: http://www.nature.com/ngeo/journal/vaop/ncurrent/abs/ngeo552.html

Geophysical Research Letters, 36, L10602; doi:10.1029/2009GL037525.

Rapid change in freshwater content of the Arctic Ocean

M. G. McPhee (McPhee Research Co., Naches, WA, U.S.A.), A. Proshutinsky (Woods Hole Oceanographic Institute, Woods Hole, MA, U.S.A.), J. H. Morison (Polar Science Center, Applied Physics Laboratory, University of Washington, Seattle, WA, U.S.A.), M. Steele (Polar Science Center, Applied Physics Laboratory, University of Washington, Seattle, WA, U.S.A.), and M. B. Alkire (College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR, U.S.A.)

Nature 459, 243–247 (14 May 2009) | doi: 10.1038/nature07979; received 24 April 2008; accepted 5 March 2009.

Interior pathways of the North Atlantic meridional overturning circulation

Amy S. Bower (Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, MA 02540, U.S.A.), M. Susan Lozier, Stefan F. Gary (Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, MA 02540, U.S.A.), and Claus W. Böning (IFM-GEOMAR Leibniz-Institut für Meereswissenschaften, Kiel, 24105, Germany)