Stefan Rahmstorf: What's going on in the North Atlantic?

by Stefan Rahmstorf, RealClimate, March 23, 2015

The North Atlantic between Newfoundland and Ireland is practically the only region of the world that has defied global warming and even cooled. Last winter there even was the coldest on record – while globally it was the hottest on record. Our recent study (Rahmstorf et al., 2015) attributes this to a weakening of the Gulf Stream System, which is apparently unique in the last thousand years.

The whole world is warming. The whole world? No! A region in the subpolar Atlantic has cooled over the past century – unique in the world for an area with reasonable data coverage (Fig. 1). So what’s so special about this region between Newfoundland and Ireland?

Figure 1. Linear temperature trend from 1900 to 2013. The cooling in the subpolar North Atlantic is remarkable and well documented by numerous measurements – unlike the cold spot in central Africa, which on closer inspection apparently is an artifact of incomplete and inhomogeneous weather station data.

It happens to be just that area for which climate models predict a cooling when the Gulf Stream System weakens (experts speak of the Atlantic meridional overturning circulation or AMOC, as part of the global thermohaline circulation). That this might happen as a result of global warming is discussed in the scientific community since the 1980s – since Wally Broecker’s classical Nature article “Unpleasant surprises in the greenhouse?” Meanwhile evidence is mounting that the long-feared circulation decline is already well underway.

The Atlantic circulation (AMOC) as part of the global overturning circulation of the oceans in an animation from NASA.

Difficult to measure

Climate models have long predicted such a slowdown – both the current 5th and the previous 4th IPCC report call a slowdown in this century “very likely,” which means at least 90% probability. When emissions continue unabated (RCP8.5 scenario), the IPCC expects 12% to 54% decline by 2100 (see also the current probabilistic projections of Schleussner et al., 2014). But the actual past evolution of the flow is difficult to reconstruct owing to the scarcity of direct measurements. Therefore, in our study we use data on sea surface temperatures in order to infer the strength of the flow: we use the temperature difference between the region most strongly influenced by the AMOC and the rest of the Northern Hemisphere.

Figure 2. Schematic of the Atlantic circulation. Surface currents in red, deep currents in blue, sea-ice cover in winter in white. (Source: Rahmstorf, Nature 1997)

Now we are not the first to have inferred from temperature data that the flow must have weakened.  Evidence for this was already presented by Dima and Lohmann (2010) or Drijfhout et al. (2012), among others (for further references see the introduction of our paper).

What is new is that we have used proxy reconstructions of large-scale surface temperature (Mann et al., 2009) previously published by one of us (study co-author and RealClimate co-founder Mike Mann) that extend back to 900 AD (see “What we can learn from studying the last millennium (or so)”) to estimate the circulation (AMOC) intensity over the entire last 1,100 years (Fig. 3). This shows that despite the substantial uncertainties in the proxy reconstruction, the weakness of the flow after 1975 is unique in more than a thousand years, with at least 99% probability. This strongly suggests that the weak overturning is not due to natural variability but rather a result of global warming.

Figure 3. Time series of the temperature difference between the subpolar North Atlantic and the entire Northern Hemisphere, which can be interpreted as an indicator of the strength of the Atlantic circulation.

Also in 2014 we again find a remarkable cold bubble over the northern Atlantic – as a look at the NASA website shows. 2014 was globally the warmest year on record, 1 °C warmer than the average for 1880-1920. But the subpolar Atlantic was 1-2 °C colder than that baseline. 

And even more recently, NOAA last week released the stunning temperature analysis for the past winter shown in Figure 4. That winter was globally the warmest since records began in 1880. But in the subpolar North Atlantic, it was the coldest on record! That suggests the decline of the circulation has progressed even further now than we documented in the paper.

Figure 4. Temperature anomaly map for the past December-January-February, from NOAA.

The role of Greenland

Another new aspect is the importance of the increasing mass loss of the Greenland ice sheet, which causes extra freshwater to enter the North Atlantic that dilutes the sea water. We have joined forces with Greenland expert Jason Box who has reconstructed the mass balance of the Greenland ice sheet since 1840 (Figure 6 of our paper, see also his blog). The ice loss amounts to a freshwater volume which should have made an important contribution to the observed decrease in salinity in the northern Atlantic – probably including the “great salinity anomaly” of the 1970s, famous amongst oceanographers.

Meltwater on the Greenland ice sheet. Photo: Ian Joughin.

What are the impacts of a slowdown?

The consequences of a large reduction in ocean overturning would look nothing like the Hollywood film The Day After Tomorrow. But they would not be harmless either – e.g., for sea level (Levermann et al., 2005) particularly along the US east coast (Yin et al., 2009), marine ecosystems, fisheries and possibly even storminess in Europe (Woollings et al., 2012). We have studied these consequences some years ago in an interdisciplinary project with colleagues from Bremerhaven, Hamburg and Norway – the results are summarized in Kuhlbrodt et al. (2009).

If our analysis is correct, then this indicates that climate models underestimate the weakening of the Atlantic circulation in response to global warming – probably because the flow in these models is too stable (see Hofmann and Rahmstorf, 2009). Although these models predict a significant weakening for the future, they do not suggest this as early as the observations show it (see Figure 2 of our paper). That the real flow may be more unstable than previously thought would be bad news for the future.

If the circulation weakens too much it can even completely break down – the AMOC has a well-known “tipping point” (Lenton et al., 2008). The latest IPCC report (just like the previous one) estimates a probability of up to 10% that this could happen as early as this century. However, this assessment is based on models that may underestimate the risk, as mentioned above. Expert surveys indicate that many researchers assess the risk higher than the (generally conservative) IPCC, as is the case for sea level. In a detailed survey (Kriegler et al., 2009), the 16 experts interviewed saw already at moderate global warming (2-4 °C) a probability of a ‘tipping’ (major reorganisation) of the flow between 5 and 40 percent. With strong global warming (4-8 °C) this probability was even estimated as between 20 and 65 percent.

Figure 5. Sea surface temperature anomaly on 20 March 2015. Note that this is relative to a baseline 1979-2000, which is already a cold period in the subpolar Atlantic. Source: Climate Reanalyzer.

Links

Thermohaline Ocean Circulation (Encyclopedia of Quaternary Sciences)

The Washington Post on our paper

And The Independent

Vox includes comments by some colleagues: Tom Delworth and Gerald Meehl

Peter Sinclair has posted a short video on our paper with interview clips with Mike Mann, Jason Box and myself

Climate Central weighs in with more comments from other scientists on our paper

And here a brand new paper on the impacts of an AMOC shutdown (hat tip to our reader Yvan Dutil)

By coincidence, this came out the same day as our paper: Ten years of ocean monitoring uncovers secrets of changing UK winters

References

Rahmstorf, S., Box, J., Feulner, G., Mann, M., Robinson, A., Rutherford, S., Schaffernicht, E. (2015). Exceptional twentieth-century slowdown in Atlantic Ocean Overturning Circulation, Nature Climate Change (online)

W.S. Broecker (1987). Unpleasant surprises in the greenhouse? Nature, vol. 328, pp. 123-126. http://dx.doi.org/10.1038/328123a0

C. Schleussner, A. Levermann, and M. Meinshausen (2015). Probabilistic projections of the Atlantic overturning, Climatic Change, vol. 127, pp. 579-586. http://dx.doi.org/10.1007/s10584-014-1265-2

M. Dima, and G. Lohmann (2010). Evidence for two distinct modes of large-scale ocean circulation changes over the last century, Journal of Climate, vol. 23, pp. 5-16. http://dx.doi.org/10.1175/2009jcli2867.1

S. Drijfhout, G.J. van Oldenborgh, and A. Cimatoribus (2012). Is a decline of AMOC causing the warming hole above the North Atlantic in observed and modeled warming patterns? Journal of Climate, vol. 25, pp. 8373-8379. http://dx.doi.org/10.1175/jcli-d-12-00490.1

M.E. Mann, Z. Zhang, S. Rutherford, R.S. Bradley, M.K. Hughes, D. Shindell, C. Ammann, G. Faluvegi, F. Ni (2009). Global signatures and dynamical origins of the Little Ice Age and Medieval Climate Anomaly, Science, 326, 1256-1260. http://dx.doi.org/10.1126/science.1177303

A. Levermann, A. Griesel, M. Hofmann, M. Montoya, and S. Rahmstorf (2005). Dynamic sea level changes following changes in the thermohaline circulation, Climate Dynamics, vol. 24, pp. 347-354, 2005. http://dx.doi.org/10.1007/s00382-004-0505-y

J. Yin, M.E. Schlesinger, and R.J. Stouffer (2009). Model projections of rapid sea-level rise on the northeast coast of the United States, Nature Geoscience, vol. 2, pp. 262-266. http://dx.doi.org/10.1038/ngeo462

T. Woollings, J.M. Gregory, J.G. Pinto, M. Reyers, and D.J. Brayshaw (2012). Response of the North Atlantic storm track to climate change shaped by ocean–atmosphere coupling, Nature Geoscience, vol. 5, pp. 313-317. http://dx.doi.org/10.1038/NGEO1438

T. Kuhlbrodt, S. Rahmstorf, K. Zickfeld, F.B. Vikebø, S. Sundby, M. Hofmann, P.M. Link, A. Bondeau, W. Cramer, and C. Jaeger (2009). An integrated assessment of changes in the thermohaline circulation, Climatic Change, vol. 96, pp. 489-537. http://dx.doi.org/10.1007/s10584-009-9561-y

M. Hofmann, and S. Rahmstorf (2009). On the stability of the Atlantic meridional overturning circulation, Proceedings of the National Academy of Sciences, vol. 106, pp. 20584-20589. http://dx.doi.org/10.1073/pnas.0909146106

T.M. Lenton, H. Held, E. Kriegler, J.W. Hall, W. Lucht, S. Rahmstorf, and H.J. Schellnhuber (2008). Tipping elements in the Earth’s climate system, Proceedings of the National Academy of Sciences, vol. 105, pp. 1786-1793. http://dx.doi.org/10.1073/pnas.0705414105

E. Kriegler, J.W. Hall, H. Held, R. Dawson, and H.J. Schellnhuber (2009). Imprecise probability assessment of tipping points in the climate system, Proceedings of the National Academy of Sciences, vol. 106, pp. 5041-5046. http://dx.doi.org/10.1073/pnas.0809117106

http://www.realclimate.org/index.php/archives/2015/03/whats-going-on-in-the-north-atlantic/

Climate Roulette: Elmar Kriegler et al.

Climate roulette

by James Dacey, physicsworld.com, March 16, 2009

Mankind is playing a Russian roulette with the climate, according to a study published today in the Proceedings of the National Academy of Sciences.

Elmar Kriegler, of the Potsdam Institute for Climate Impact Research, and his colleagues sought to find out what leading scientists really think will happen to the climate.

So Kriegler surveyed 43 scientists to gauge the impact of rising temperatures on five major components of the global climate system.

They calculate a 1-in-6 chance that a “tipping event” will occur if the temperature increases by 2-4 C in the next 200 years.

The 5 systems concerned are:

• Major changes in the North Atlantic Ocean circulation
• The Greenland and West Antarctic ice sheets
• The Amazon rainforest and El Nino

They define a tipping point as “the event of initiating the transition, or making its future initiation inevitable.” 

Essentially they are saying that beyond these points the climate will reach a kind of elastic limit -- beyond which, we will feel the wrath of the climate, and there’ll be nothing we can do about it.

Realizing that previous surveys have been met with a fair degree of apathy, they used “imprecise probabilities” -- a part of Bayesian statistics.

This new mathematics has been controversial, but advocates say it can weigh up a given hypothesis in a more rounded way than classical statistics.

Developed in the 1980s and 1990s, Bayesian statistics seem to have gained most traction in the field of operations research and economic decision making.

“The currently discussed long-term targets of 50% reduction globally by 2050 (and 80% reduction for the industrial countries), with a continuing reduction after 2050, is an important step in this direction, but does not guarantee the reaching of the 2 degree target,” Kriegler told physicsworld.com.

This may sound like a very gloomy forecast, but Kriegler was a bit more pragmatic about taking coordinated international action:

“Nevertheless, these [targeted reductions] are a useful benchmark to focus the minds of politicians and society. Reaching this goal requires at least the following -- in the order of importance:

1. A massive decarbonization of the energy system, starting in the electricity sector
2. A strong increase in energy efficiency
3. A stop to tropical deforestation, and an increase of the forest area in the tropics in the long run
4. A massive reduction of CH4 and N2O emissions from the agricultural sector

http://www.iop.org/mt4/mt-tb.cgi/2860

James Hansen: accelerating Greenland ice loss will cause North Atlantic cooling, worse storms

Greenland melt threatens to shift northern climate

The Sydney Morning Herald, May 18, 2013
 
Greenland ice melting at an expanding pace may begin cooling the North Atlantic and increasing the severity of storms by 2075, said James Hansen, the former NASA scientist who raised concerns about global warming in the 1980s.

• Greensland glacier breaks off

“If we stay on this path where the rate of mass loss from Greenland doubles every 10 years, we would get to a situation by about 2075 or 2080 where the mass loss is so fast that it causes the whole North Atlantic to be colder,” Hansen said in London.

The findings are from computer models using current rates of ice melt and will be detailed in a paper that Hansen plans to finish writing in the summer, he said in an interview.

Inflows of cold, fresh water from Greenland would slow deep currents that carry cold water south, cooling the North Atlantic as tropical waters get warmer, Hansen said. That would increase a “temperature gradient” that’s conducive to stronger storms.

“It could happen sooner” than 2075, Hansen said. “If you look at how fast the mass loss is increasing, it looks like the doubling time is between five and 10 years,” he said.

The findings add to research showing the system of Atlantic currents that channels the warm Gulf Stream to the northeast and moves colder, deeper waters south may slow this century because of climate change. Possible effects include cooler temperatures for Europe -- or a moderation of rising temperatures -- and higher sea levels in New York, past studies have found.

The United Nations Intergovernmental Panel on Climate Change will this year publish the first section of a four-part report of its first comprehensive review of the climate since 2007. A leaked draft in December said it’s “very likely” the Atlantic Meridional Overturning Circulation -- the system of currents in the ocean -- will weaken this century. It is “very unlikely” it will collapse completely, the report showed.

http://m.smh.com.au/environment/climate-change/greenland-melt-threatens-to-shift-northern-climate-20130518-2jsvo.html

Ocean Conveyor's 'Pump' Switches Back On

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.)
 

 

Enlarge Image In the North Atlantic Ocean, the contrast between frigid, dry winter air and warm water draws heat from the ocean into the atmosphere and leaves ocean water colder and denser. The denser waters sink and feed into the lower limb of a global system of currents often described as the Ocean Conveyor. The process, called deep convection, has far-reaching affects on climate. (Illustration by Jack Cook, 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

M Vizcaíno et al Clim.Dyn. Long-term ice sheet–climate interactions under anthropogenic greenhouse forcing simulated with a complex Earth System Model

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

Several multi-century and multi-millennia simulations have been performed with a complex Earth System Model (ESM) for different anthropogenic climate change scenarios in order to study the long-term evolution of sea level and the impact of ice sheet changes on the climate system. The core of the ESM is a coupled coarse-resolution Atmosphere–Ocean General Circulation Model (AOGCM). Ocean biogeochemistry, land vegetation and ice sheets are included as components of the ESM. The Greenland Ice Sheet (GrIS) decays in all simulations, while the Antarctic ice sheet contributes negatively to sea level rise, due to enhanced storage of water caused by larger snowfall rates. Freshwater flux increases from Greenland are one order of magnitude smaller than total freshwater flux increases into the North Atlantic basin (the sum of the contribution from changes in precipitation, evaporation, run-off and Greenland meltwater) and do not play an important role in changes in the strength of the North Atlantic Meridional Overturning Circulation (NAMOC). The regional climate change associated with weakening/collapse of the NAMOC drastically reduces the decay rate of the GrIS. The dynamical changes due to GrIS topography modification driven by mass balance changes act first as a negative feedback for the decay of the ice sheet, but accelerate the decay at a later stage. The increase of surface temperature due to reduced topographic heights causes a strong acceleration of the decay of the ice sheet in the long term. Other feedbacks between ice sheet and atmosphere are not important for the mass balance of the GrIS until it is reduced to 3/4 of the original size. From then, the reduction in the albedo of Greenland strongly accelerates the decay of the ice sheet.

From 2004: Satellites record weakening North Atlantic Current

19.04.2004

Dominant Pattern of Variability of the Sea-surface Height in the 1990s

This image shows the dominant pattern of variability of the sea-surface height in the 1990s. The slope of the sea-surface height is in balance with ocean currents, much in the way weather maps of pressure relate to winds. The large blue region in the northern Atlantic represents a slowing of the counter-clockwise,
cyclonically circulating subpolar gyre. For this image, the researchers used TOPEX/Poseidon data, which has been combined with ERS-1/2 data into the NASA Pathfinder data set. The Pathfinder data set also includes the Seasat and Geosat data which are referenced to TOPEX/Poseidon data. The next few years will reveal how sea surface height is going to evolve as the altimetric time series continues with JASON-1 observations. Credit: Sirpa Hakkinen, NASA GSFC



Terra MODIS Sea Surface Temperatures for North Atlantic Ocean

This image of North Atlantic Ocean sea surface temperatures represents an eight-day composite from Sept 6 - Sept 13, 2001 from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on Terra. The red and orange colors represent warmer water, while the blue shades represent cold water in the higher latitudes. The Gulf Stream is evident along the U.S. eastern seaboard. The color bar is in units of degrees Celsius. Credit: Ronald Vogel, SAIC for NASA GSFC

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

S. Murakami et al., J. Climate, 19, 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

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

Casey Saenger et al., Nature Geosci., 2009, Surface-temperature trends and variability in the low-latitude North Atlantic since 1552

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

Abstract

Sea surface temperature variability in the North Atlantic Ocean recorded since about 1850 has been ascribed to a natural multidecadal oscillation superimposed on a background warming trend^{1, 2, 3, 4, 5, 6}. It has been suggested that the multidecadal variability may be a persistent feature^{6, 7, 8}, raising the possibility that the associated climate impacts may be predictable⁹. However, our understanding of the multidecadal ocean variability before the instrumental record is based on interpretations of high-latitude terrestrial proxy records^{7, 8}. Here we present an absolutely dated and annually resolved record of sea-surface temperature from the Bahamas, based on a 440-year time series of coral growth rates. The reconstruction indicates that temperatures were as warm as today from about 1552 to 1570, then cooled by about 1 ^°C from 1650 to 1730 before warming until the present. Our estimates of background variability suggest that much of the warming since 1900 was driven by anthropogenic forcing. Interdecadal variability with a period of 15–25 years is superimposed on most of the record, but multidecadal variability becomes significant only after 1730. We conclude that the multidecadal variability in sea-surface temperatures in the low-latitude western Atlantic Ocean may not be persistent, potentially making accurate decadal climate forecasts more difficult to achieve.

1. Massachusetts Institute of Technology and Woods Hole Oceanographic Institution Joint Program in Oceanography, Woods Hole, MA 02543, U.S.A.
2. Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, U.S.A.
3. US Geological Survey (retired) 13765 2600 Rd., Cedaredge, CO 81413, U.S.A.
4. University of California San Diego, Scripps Institution of Oceanography, La Jolla, CA 92093, U.S.A.

Correspondence to: Casey Saenger¹ e-mail: csaenger@mit.edu

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

M.G. McPhee, A. Proshutinsky, J. H. Morison, M. Steele, M. B. Alkire, GRS, 36, Rapid change in freshwater content of the Arctic Ocean

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

Rapid change in freshwater content of the Arctic Ocean

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.)

Abstract

The dramatic reduction in minimum Arctic sea ice extent in recent years has been accompanied by surprising changes in the thermohaline structure of the Arctic Ocean, with potentially important impact on convection in the North Atlantic and the meridional overturning circulation of the world ocean. Extensive aerial hydrographic surveys carried out in March–April, 2008, indicate major shifts in the amount and distribution of fresh-water content (FWC) when compared with winter climatological values, including substantial freshening on the Pacific side of the Lomonosov Ridge. Measurements in the Canada and Makarov Basins suggest that total FWC there has increased by as much as 8,500 cubic kilometers in the area surveyed, effecting significant changes in the sea-surface dynamic topography, with an increase of about 75% in steric level difference from the Canada to Eurasian Basins, and a major shift in both surface geostrophic currents and freshwater transport in the Beaufort Gyre.

(Received 28 January 2009; accepted 21 April 2009; published 21 May 2009.)

McPhee, M. G., A. Proshutinsky, J. H. Morison, M. Steele, & M. B. Alkire (2009), Rapid change in freshwater content of the Arctic Ocean, Geophysical Research Letters, 36, L10602; doi:10.1029/2009GL037525.

Link to abstract: http://www.agu.org/pubs/crossref/2009/2009GL037525.shtml

A.S. Bower, M.S. Lozier, S.F. Gary, C.W. Böning, Nature 459, Interior pathways of the North Atlantic meridional overturning circulation

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)

Abstract

To understand how our global climate will change in response to natural and anthropogenic forcing, it is essential to determine how quickly and by what pathways climate change signals are transported throughout the global ocean, a vast reservoir for heat and carbon dioxide. Labrador Sea Water (LSW), formed by open ocean convection in the subpolar North Atlantic, is a particularly sensitive indicator of climate change on interannual to decadal timescales^{1, 2, 3}. Hydrographic observations made anywhere along the western boundary of the North Atlantic reveal a core of LSW at intermediate depths advected southward within the Deep Western Boundary Current (DWBC)^{4, 5, 6, 7, 8, 9}. These observations have led to the widely held view that the DWBC is the dominant pathway for the export of LSW from its formation site in the northern North Atlantic towards the Equator^{10, 11}. Here we show that most of the recently ventilated LSW entering the subtropics follows interior, not DWBC, pathways. The interior pathways are revealed by trajectories of subsurface RAFOS floats released during the period 2003–2005 that recorded once-daily temperature, pressure and acoustically determined position for two years, and by model-simulated 'e-floats' released in the subpolar DWBC. The evidence points to a few specific locations around the Grand Banks where LSW is most often injected into the interior. These results have implications for deep ocean ventilation and suggest that the interior subtropical gyre should not be ignored when considering the Atlantic meridional overturning circulation.

Correspondence to: Amy S. Bower¹M. Susan Lozier²

Correspondence and requests for materials should be addressed to A.S.B. (e-mail: abower@whoi.edu) and M.S.L. (e-mail: mslozier@duke.edu).

Link to abstract: http://www.nature.com/nature/journal/v459/n7244/full/nature07979.html