Changing Hadley and Walker circulations due to a warming Arctic

Comments from Neven's excellent Arctic Sea Ice News blog:

Werther,

re: NH cells This is, in my opinion, the largest effect of climate change. The weakening of the polar jet is indicating a gradual merge of the ferrell and polar cells. It is this global restructuring of global weather patterns that will produce the most immediate impacts on society and humanity's food production capabilities. This will occur due to increases in droughts in current food producing regions and floods in current population centers.

What we are also seeing in this regime of climate change is a gradual strengthening of the hadley cell as tropical evaporation increases. This is increasing desertification. The northward expansion of the 30'N desert belt will also significantly impact grain and livestock production.

Studies of these effects and predictions of their impacts are in the peer-reviewed record produced over the last 20 years. The fact that we have done nothing to act on the impacts that have been observed indicates that only a "breakthrough" event will produce real motivation for change. The longer that we wait, the more painful climate change will be and the more difficult it will be to fix it. In fact, we are approaching a point where it is becoming very likely that modernity will not survive the next 65 years.

With the arctic ice pack being the "canary in the coal mine" and the most effective messenger of these developing threats, it is important that we more fully understand and and are able to explain what is happening in the arctic today.

That is why I monitor these blogs, and sincerely appreciate the work that is being done here.

http://onlinelibrary.wiley.com/doi/10.1029/2010JD015197/abstract

"poleward shift of the subtropical dry zones (up to 2° decade−1 in June-July-August (JJA) in the Northern Hemisphere "

Posted by: Jai Mitchell | July 10, 2013 at 17:19

Werther 

In fact, what has been going on since autumn last year, is a restructuring of the three NH cells. The gradient between them is fading. The behavior of the Polar Jet Stream is directly related to that.

The geopotential difference is lower, meaning that the height of the atmospheric pressure zones, especially over the Arctic and Boreal zones have gone up.

Thanks Werther, but I'm not sure I understand this logic.
Why would the height of the atmospheric pressure zones, especially over the Arctic, go up if the gradient between the NH cells fades ?

What I understand is that in 2012 the gradient between the NH cells was low, (and the polar vortex was weak, and that is why we had a warm Arctic.

So why would 2013 show the opposite ?

Posted by: Rob Dekker | July 11, 2013 at 08:45

Rob Dekker wrote: July 11, 2013 at 08:45

Why would the height of the atmospheric pressure zones, especially over the Arctic, go up if the gradient between the NH cells fades?

Hi Rob,

I think the increased height of the troposphere is a direct consequence of more water vapour in the atmosphere due to a warmer earth, rather than the gradient between circulation cells. Wikipedia says:

As a rule, the "cells" of Earth's atmosphere shift polewards in warmer climates (e.g. interglacials compared to glacials)

So it seems the Hadley and Ferrel cells themselves are crowding the Polar cell, raising the height of the atmosphere in the Arctic. Or more exactly pushing the Polar cell further to the North.

However, as the jet stream weakens, one would also expect the loss of this natural barrier between cells to raise the average height of the polar cells.

Have you seen the polar jet stream this week? It's running from 70N to 83N in the CAA, and 75N to 85N over the Laptev sea right now (00z 11 Jul 2013).

Highly unusual, and very far North.

Cheers,
Lodger

Posted by: Artful Dodger | July 11, 2013 at 10:22

http://neven1.typepad.com/blog/2013/07/so-how-slow-was-this-start.html

Recent trends of the tropical hydrological cycle inferred from Global Precipitation Climatology Project and International Satellite Cloud Climatology Project data

Journal of Geophysical Research: Atmospheres, Vol. 16, No. D9 (16 May 2011); doi:10.1029/2010JD015197

Recent trends of the tropical hydrological cycle inferred from Global Precipitation Climatology Project and International Satellite Cloud Climatology Project data

Y P. Zhou, Kuan-Man Xu, Y. C. Sud and A. K. Betts

Abstract

[1]Scores of modeling studies have shown that increasing greenhouse gases in the atmosphere impact the global hydrologic cycle; however, disagreements on regional scales are large, and thus the simulated trends of such impacts, even for regions as large as the tropics, remain uncertain. The present investigation attempts to examine such trends in the observations using satellite data products comprising Global Precipitation Climatology Project precipitation and International Satellite Cloud Climatology Project cloud and radiation. Specifically, evolving trends of the tropical hydrological cycle over the last 20–30 years were identified and analyzed. The results show (1) intensification of tropical precipitation in the rising regions of the Walker and Hadley circulations and weakening over the sinking regions of the associated overturning circulation; (2) poleward shift of the subtropical dry zones (up to 2° decade⁻¹ in June-July-August (JJA) in the Northern Hemisphere and 0.3–0.7° decade⁻¹ in June-July-August and September-October-November in the Southern Hemisphere) consistent with an overall broadening of the Hadley circulation; and (3) significant poleward migration (0.9–1.7° decade⁻¹) of cloud boundaries of Hadley cell and plausible narrowing of the high cloudiness in the Intertropical Convergence Zone region in some seasons. These results support findings of some of the previous studies that showed strengthening of the tropical hydrological cycle and expansion of the Hadley cell that are potentially related to the recent global warming trends.

http://onlinelibrary.wiley.com/doi/10.1029/2010JD015197/abstract

Jeff Masters: Devastating flooding continues in Colombia

Devastating flooding continues in Colombia


by Jeff Masters, wunderblog, May 16, 2011


Devastating flooding has hit South America in Colombia, where exceptionally heavy spring rains have killed at least 425 people so far this year, with 482 others missing. Damages are in the billions, and there are 3 million disaster victims. "Some parts of the country have been set back 15 to 20 years," said Plan’s Country Director in Colombia, Gabriela Bucher. "Over the past 10 months we have registered five or six times more rainfall than usual," said the director of Colombia's weather service, Ricardo Lozano. 


Up to 800 mm (about 32 inches) of rain has fallen along the Pacific coast of Colombia over the past two weeks (Figure 3). The severe spring flooding follows on the heels of the heaviest fall rains in Colombia's History. Weather records go back 42 year in Colombia. Colombia's president Juan Manuel Santos said, "the tragedy the country is going through has no precedents in our history." The 2010 floods killed 571 people -- the second deadliest year for floods in Colombian history, next to 1987. The floods did over $1 billion in damage, and affected 2.8 million people. In many places, the flood waters from this great disaster never fully receded, and are now rising again due to this latest round of intense flooding. More rain is in the forecast -- the latest forecast from the GFS model calls for an additional 5-10 inches (200-400 mm) across much of western and northern Colombia in the coming week.




Figure 2. Satellite-observed rainfall over Colombia during the past two weeks shows a region of up to 800 mm (about 32 inches) has fallen near the Pacific coast. Image credit: Navy Research Lab, Monterey, Calif.

Colombia's rainy season usually has two peaks: one the fall in October, then then another in the spring in April-May. The heavy rains are due to the presence of the Intertropical Convergence Zone, the area encircling the earth near the Equator where winds originating in the northern and southern hemispheres come together. When these great wind belts come together (or "converge", thus the name "Convergence Zone"), the converging air is forced upwards, since it has nowhere else to go. The rising air fuels strong thunderstorm updrafts, creating a band of very heavy storms capable of causing heavy flooding rains. In La Niña years, when a large region of colder than average water is off the Pacific coast of Colombia, rainfall tends to increase over Colombia. La Niña was moderate to strong during the fall 2010 rains and floods in Colombia and was largely to blame for Colombia's deadly rainy season. However, in recent months, La Niña has waned. April sea surface temperatures off the Pacific coast of Colombia (0°-10° N, 85°-75° W) warmed to the 13th highest temperatures in the past 100 years, 0.68 °C above average. Thus, this month's flooding in Colombia may not be due to La Niña.

See also my December 2010 post, Heaviest rains in Colombia's history trigger deadly landslide; 145 dead or missing




Figure 3. Dramatic video of flooding in Colombia over the weekend. Flood waters swept away cars and buses in a busy street in the city of Barranquilla, and passengers climbed on the roofs of their vehicles in order to escape the flood waters. Video credit: BBC.


http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=1802#

Jeff Masters: Heaviest rains in Colombia's history trigger deadly landslide; 145 dead or missing

Heaviest rains in Colombia's history trigger deadly landslide; 145 dead or missing

by Jeff Masters, wunderblog, December 6, 2010

Colombia's heaviest rains in history triggered a landslide in the poor hillside community of Bello on Sunday, killing at least 20 people and leaving 125 missing. 

This year's unprecedented rainy season had already killed 176 people prior to Sunday, making it one of the deadliest flooding years in Colombia's history, according to the director of Colombia's national disaster management office, Luz Armanda Pulido. In 2009, 110 people died in flooding disasters, and 48 were killed in 2008, according to Colombian Red Cross director of national relief operations Carlos Ivan Marquez. This year's rains are the heaviest in the 42 years since Colombia's weather service was created and began taking data, agency director Ricardo Lozano said. The resulting flooding has destroyed or damaged the homes of 1.6 million people. Colombia's president Juan Manuel Santos said the number of homeless from the flooding could reach 2 million, and said "the tragedy the country is going through has no precedents in our history." 

Neighboring Venezuela has also been hard-hit by this year's severe rainy season -- at least 30 people are dead from floods and mudslides, and tens of thousands homeless. 

More rain is in the forecast -- the latest forecast from the GFS model (Figure 2) -- calls for an additional 4-6 inches (100-150 mm) across much of western and northern Colombia in the coming week.


Figure 1. Satellite-observed rainfall over Colombia during the past two weeks shows a region of 100-200 mm (4-8 inches) has fallen near Medellin, close to where Sunday's landslide in Bello occurred. Image credit: Navy Research Lab, Monterey.

Colombia's rainy season usually peaks in October, then gradually wanes in November and December. The heavy rains are due to the presence of the Intertropical Convergence Zone, the area encircling the earth near the Equator where winds originating in the northern and southern hemispheres come together. When these great wind belts come together (or "converge", thus the name "Convergence Zone"), the converging air is forced upwards, since it has nowhere else to go. The rising air fuels strong thunderstorm updrafts, creating a band of very heavy storms capable of causing heavy flooding rains. This year is a La Niña year, which means there is a large region of colder than average water off the Pacific coast of Colombia. Colder than average water off the Pacific coast enhances rainfall over Colombia, and this year's La Niña, which is at the borderline between the "moderate" and "strong" categories, is largely to blame for Colombia's deadly rainy season.


Figure 2. Rainfall forecast from today's run of the GFS model predicts that region to the north and west of Bogota, Colombia may see another 100-150 mm (4-6 inches) during the coming week (red colors.) Image credit: NOAA/CPC.

See also my November 22 post, Colombia rainy season floods kill 136.

Link:  http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=1706

Graham Cogley: Global snowline altitudes and climate change

The snowline and the climate

by Graham Cogley, environmentalresearchweb, September 6, 2010

If the climate were to change, you would expect the snowline altitude to change. It does, and we can show that it has in recent centuries, but we can also turn the proposition around. The snowline makes a very good tool with which to think about the climate. Here, again, is a graph of the global snowline.

A global approximation of the climatic snowline. South Pole on the left, North Pole on the right. Each little square is at an altitude which is the average of many “mid-altitudes,” each of which is the average of one glacier’s minimum and maximum altitude.

It isn’t just that the snowline makes sense of the glaciologist’s definition of “maritime” and “continental.” The temperature at the snowline varies in the graph from purple (very cold and continental, on the ice cap covering Illimani and on other peaks above 6 km in the Bolivian Andes) to dark red (very warm and maritime, in the northern mid-latitudes).

Why is the “line” fat in the northern mid-latitudes? Obviously there are a good many glaciers to sample there, but it is not obvious until we colour the little squares that the fatness is because regional climates vary in continentality. In southern Alaska, for example, the shoreline runs crudely east-west, continentality increases inland, and the snowline actually rises towards the pole.

Putting aside regional variations, why doesn’t the global snowline define a neat triangle, highest at the equator and lowest at the poles? The answer lies in the so-called general circulation of the atmosphere. The snowline dips in the tropics, between 30° S and 30° N, because that is the region through which the Inter-Tropical Convergence Zone travels as it follows the Sun. Here the airflow derived from subsidence over the desert belts of each hemisphere converges on the ITCZ. The subsidence implies warming of the air and therefore reduction of its relative humidity, which is why the desert belts are desert belts. Glaciologically, the subsidence means that you don’t need much heat to melt what little snow accumulates, so the snowline (strictly, the equilibrium line) is very cold and therefore very high. Between the desert belts, convergence at the ITCZ forces the air to rise and cool, provoking snowfall. The extra snow requires more heat, and a lower and therefore warmer equilibrium line, than in the desert belts.

I wonder if I can convince you that in the mid-latitudes of each hemisphere the snowline is concave up? It is a subtle but physically genuine depression of the equilibrium line, and as at the ITCZ it is due to convergence and thus to lifting and cooling of air. Again, the cooling provokes more snowfall and in turn a lowering of the equilibrium line. This time the converging airmasses are flowing poleward from the desert belts and equatorward from the poles.

Did you notice the asymmetry of the hemispheres? Anywhere poleward of the tropics, the snowline is hundreds of metres or more lower in the southern hemisphere than at the equivalent latitude in the northern hemisphere. It reaches sea level at about 60-65° S, but where we run out of land at 84° N it is still a few hundred metres above sea level.

The temperature at the surface of the Antarctic Ice Sheet is about 25 °C colder than at the surface of the Arctic Ocean. Something like 18 °C worth of the difference is simply because the ice sheet is about 3 km above sea level. The remainder, and the depression of the snowline throughout the southern extra-tropics relative to the north, are due to the chilling effect of the ice sheet on the general circulation.

Finally, a question that always makes my head spin. What would the altitude of the snowline be if there were no mountain range? There would be no orographic moisture trap, and no glaciers of course. If we knew the temperature of the snowline, we could go to the atmospheric temperature records and find where the snowline would be if there were land. But, first, by supposition there isn’t any land. Second, we know that if there were it would draw the snowline down to meet it, that being why glaciers start out maritime at the coast and become more continental the further inland you go. Third, that means that if there were land the temperature would be different from what it is in the free atmosphere, which returns us to where we started from but with the realization that we ought not to have started from there.

So I can’t produce an answer to the question. But I can see that the snowline teaches us a lot about the climate, including the proposition that the general circulation, the temperature and the topography are all mixed up in it together.

Link:  http://environmentalresearchweb.org/blog/2010/09/the-snowline-and-the-climate.html

NASA's JPL: Launch of Aquarius/SAC-D designed to provide monthly global maps of how salt concentration varies on the ocean surface – a key indicator of ocean circulation and its role in climate change, May 2010

Dear Readers,

This might sound to you like the most boring topic known to man, but in fact the measurement of salt concentrations in the ocean indicates its interactions with the atmosphere. They need good measurements to improve their ocean-atmosphere coupled models. For example, at times there is a difference between the salt concentrations in the western Atlantic (around Central America)and the eastern Pacific great enough to very significantly affect weather patterns and where lots of rain falls and where it doesn't. (How's that for layman's language!)




Alan Buis 818-354-0474
Jet Propulsion Laboratory, Pasadena, Calif.
Alan.buis@jpl.nasa.gov

Sandra Torrusio 011-54-11-4331-0074, ext. 288
Comisión Nacional de Actividades Espaciales, Buenos Aires, Argentina
storrusio@conae.gov.ar

News release: 2009-149                                                                      Oct. 1, 2009

International Science Teams Selected for Aquarius/SAC-D Mission

PASADENA, Calif. – NASA and Argentina's Comisión Nacional de Actividades Espaciales (CONAE), with support from the Argentine Ministry of Science, Technology and Innovative Production (MinCyT), have selected additional members of the international scientific investigating team for the Aquarius/Satélite de Aplicaciones Científicas (SAC)-D mission, scheduled to launch in 2010. The new team members include two from NASA's Jet Propulsion Laboratory, Pasadena, Calif.

The joint minimum three-year mission will carry a suite of instruments into space onboard the Argentine-built SAC-D spacecraft. NASA's sensor, Aquarius, is the primary instrument on the mission. Aquarius is designed to provide monthly global maps of how salt concentration varies on the ocean surface – a key indicator of ocean circulation and its role in climate change. Seven CONAE-sponsored instruments will provide environmental data for a wide range of applications, including natural hazards, land processes, epidemiological studies and air quality issues.

NASA and CONAE conducted a joint solicitation and selection of scientific investigations and innovative application demonstration projects using Aquarius/SAC-D observations. NASA selected 15 projects that it will fund over the next four years for a total of $8 million. CONAE/MinCyT selected 15 Argentine projects with participation of scientists from Chile and Brazil, which will be funded for a total of $1.3 million. An additional 10 proposals were selected from scientists in Italy and Japan.

The primary focus of the selected projects is to prepare the scientific community to use Aquarius/SAC-D observations to better understand the interactions between global ocean circulation, the water cycle and Earth's climate. Several projects will concentrate on socio-economic applications of the mission's observations in such areas as fishery management, disease and flood forecasting, and monitoring volcanic eruptions and fires.

The principal investigators for the NASA-funded projects are:

- William Asher, University of Washington, Seattle
- Frederick Bingham, University of North Carolina, Wilmington
- Shannon Brown, NASA's Jet Propulsion Laboratory, Pasadena, Calif.
- Antonio Busalacchi, University of Maryland, College Park
- Ichiro Fukumori, Jet Propulsion Laboratory, Pasadena, Calif.
- Arnold Gordon, Lamont-Doherty Earth Observatory, Columbia University, Palisades, N.Y.
- Thomas Jackson, U. S. Department of Agriculture, Beltsville, Md.
- W. Linwood Jones, University of Central Florida, Orlando
- Roger Lang, George Washington University, Washington, D.C.
- William Large, National Center for Atmospheric Research, Boulder, Colo.
- Nikolai Maximenko, University of Hawaii, Honolulu
- Rui Ponte, Atmospheric and Environmental Research, Inc., Cambridge, Mass.
- Stephen Riser, University of Washington, Seattle
- Douglas Vandemark, University of New Hampshire, Durham
- Frank Wentz, Remote Sensing Systems, Santa Rosa, Calif.

The principal investigators for the CONAE/MinCyT-funded projects are:

- Miguel Bertolami, National University of the Patagonia San Juan Bosco, Chubut, Argentina
- Marcelo Cassini, National University of Luján, Buenos Aires, Argentina
- Carlos Cotlier, National University of Rosario, Santa Fe, Argentina
- Dora Goniadzki, National Water Institute, Buenos Aires, Argentina
- Raúl Guerrero, National Institute for Fisheries Research and Development, Buenos Aires, Argentina
- Haydee Karszenbaum, University of Buenos Aires, Buenos Aires, Argentina
- Maite Narvarte, Marine Biology Institute Alte. Storni, Chubut, Argentina
- Alberto Piola, Naval Hydrographic Service, Buenos Aires, Argentina
- Gloria Pujol, National Meteorological Service, Buenos Aires, Argentina
- Mirta Raed, National University of Luján, Buenos Aires, Argentina
- Raúl Rivas, National University of Centro, Buenos Aires, Argentina
- Cristina Rodriguez, Mariscope Chilena Department of Oceanography, Puerto Montt, Chile
- Hector Salgado, Naval Hydrographic Service, Buenos Aires, Argentina
- Paola Salio, National University of Buenos Aires, Buenos Aires, Argentina
- Cristina Serafini, National University of Luján, Buenos Aires, Argentina

NASA is providing the Aquarius instrument (which was built by JPL), along with launch services and Aquarius science data processing. JPL manages the Aquarius mission for NASA. NASA's Goddard Space Flight Center, Greenbelt, Md., is providing the Aquarius instrument radiometer. CONAE is providing the SAC-D spacecraft; additional instruments, including optical and thermal cameras, a microwave radiometer and other specific sensors (with participation from Italy, France, Canada and various Argentine institutions); and mission operations.

Launch of Aquarius/SAC-D onboard a Delta II rocket is scheduled for no earlier than May 2010 from Vandenberg Air Force Base in California.

For more information on the Aquarius/SAC-D mission, visit: http://aquarius.nasa.gov and http://www.conae.gov.ar/satelites/sac-d.html . JPL is managed for NASA by the California Institute of Technology in Pasadena.

Julian P. Sachs et al., Nature Geoscience, June 2009: Southward movement of the Pacific intertropical convergence zone AD 1400–1850

Nature Geoscience, 2 (2009) 519-525, published online 28 June 2009; doi:10.1038/ngeo554

Southward movement of the Pacific intertropical convergence zone AD 1400–1850

Julian P. Sachs^1,*, Dirk Sachse^1,4, Rienk H. Smittenberg^1,4, Zhaohui Zhang^1,4, David S. Battisti² and Stjepko Golubic³

Abstract

Tropical rainfall patterns control the subsistence lifestyle of more than one billion people. Seasonal changes in these rainfall patterns are associated with changes in the position of the intertropical convergence zone, which is characterized by deep convection causing heavy rainfall near 10^° N in boreal summer and 3^° N in boreal winter. Dynamic controls on the position of the intertropical convergence zone are debated, but palaeoclimatic evidence from continental Asia, Africa and the Americas suggests that it has shifted substantially during the past millennium, reaching its southernmost position some time during the Little Ice Age (AD 1400–1850). However, without records from the meteorological core of the intertropical convergence zone in the Pacific Ocean, quantitative constraints on its position are lacking. Here we report microbiological, molecular and hydrogen isotopic evidence from lake sediments in the Northern Line Islands, Galápagos and Palau indicating that the Pacific intertropical convergence zone was south of its modern position for most of the past millennium, by as much as 500 km during the Little Ice Age. A colder Northern Hemisphere at that time, possibly resulting from lower solar irradiance, may have driven the intertropical convergence zone south. We conclude that small changes in Earth's radiation budget may profoundly affect tropical rainfall.

1. School of Oceanography, University of Washington, Seattle, WA 98195, USA
2. Department of Atmospheric Sciences, University of Washington, Seattle, WA 98195, USA
3. Biological Science Center, Boston University, Boston, MA 02215, USA
4. Present addresses: DFG-Leibniz Center for Surface Process and Climate Studies, Institut für Geowissenschaften, Universität Potsdam, 14476 Potsdam, Germany (D.S.); Geological Institute, ETH Zürich, 8092 Zürich, Switzerland (R.H.S.); Department of Earth Sciences, Nanjing University, Nanjing, 210093, China (Z.Z.)

*Correspondence, e-mail: jsachs@u.washington.edu

Link to abstract: http://www.nature.com/ngeo/journal/v2/n7/abs/ngeo554.html

Intertropical Convergence Zone, the Earth's most prominent rainfall feature creeping northward

Earth's most prominent rainfall feature creeping northward

ScienceDaily, July 1, 2009 — The rain band near the equator that determines the supply of freshwater to nearly a billion people throughout the tropics and subtropics has been creeping north for more than 300 years, probably because of a warmer world, according to research published in the July issue of Nature Geoscience.

If the band continues to migrate at just less than a mile (1.4 km) a year, which is the average for all the years it has been moving north, then some Pacific islands near the equator – even those that currently enjoy abundant rainfall – may be drier within decades and starved of freshwater by midcentury or sooner. The prospect of additional warming because of greenhouse gases means that situation could happen even sooner.

The findings suggest "that increasing greenhouse gases could potentially shift the primary band of precipitation in the tropics with profound implications for the societies and economies that depend on it," the article says.

"We're talking about the most prominent rainfall feature on the planet, one that many people depend on as the source of their freshwater because there is no groundwater to speak of where they live," says Julian Sachs, associate professor of oceanography at the University of Washington and lead author of the paper. "In addition many other people who live in the tropics but farther afield from the Pacific could be affected because this band of rain shapes atmospheric circulation patterns throughout the world."

The band of rainfall happens at what is called the intertropical convergence zone. There, just north of the equator, trade winds from the northern and southern hemispheres collide at the same time heat pours into the atmosphere from the tropical sun. Rain clouds 30,000 ft. thick in places proceed to dump as much as 13 ft. (4 m) of rain a year in some places. The band stretching across the Pacific is generally between 3 and 10 degrees north of the equator depending on the time of year. It has recently been hypothesized that the intertropical convergence zone does not reside in the southern hemisphere for reasons having to do with the distribution of land masses and locations of major mountain ranges in the world, particularly the Andes mountains, that have not changed for millions of years.

The new article presents surprising evidence that the intertropical convergence zone hugged the equator some 350 years ago during Earth's "Little Ice Age," which lasted from 1400 to 1850.

The authors analyzed the record of rainfall in lake and lagoon sediments from four Pacific islands at or near the equator.

One of the islands they studied, Washington Island, is about 5 degrees north of the equator. Today it is at the southern edge of the intertropical convergence zone and receives nearly 10 ft. (2.9 m) of rain a year. But cores reveal a very different Washington Island in the past: It was arid, especially during the little ice age.

Among other things, the scientists looked for evidence in sediment cores of salt-tolerant microbes. On Washington Island they found that evidence in 400- to 1,000-year-old sediment underlying what is now a freshwater lake. Such organisms could only have thrived if rainfall was much reduced from today's high levels on the island. Additional evidence for changes in rainfall were provided by ratios of hydrogen isotopes of material in the sediments that can only be explained by large changes in precipitation.

Sediment cores from Palau, which lies about 7 degrees north of the equator and in the heart of the modern convergence zone, also revealed arid conditions during the Little Ice Age.

In contrast, the researchers present evidence that the Galapagos Islands, today an arid place on the equator in the Eastern Pacific, had a wet climate during the little ice age.

They write, "The observations of dry climates on Washington Island and Palau and a wet climate in the Galapagos between about 1420-1560/1640 provide strong evidence for an intertropical convergence zone located perennially south of Washington Island (5° N) during that time and perhaps until the end of the eighteenth century."

If the zone at that time experienced seasonal variations of 7 degrees latitude, as it does today, then during some seasons it would have extended southward to at least the equator, Sachs says. This has been inferred previously from studies of the intertropical convergence zone on or near the continents, but the new data from the Pacific Ocean region is clearer because the feature is so easy to identify there.

The remarkable southward shift in the location of the intertropical convergence zone during the little ice age cannot be explained by changes in the distribution of continents and mountain ranges because they were in the same places in the little ice age as they are now. Instead, the co-authors point out that the Earth received less solar radiation during the little ice age, about 0.1% less than today, and speculate that may have caused the zone to hover closer to the equator until solar radiation picked back up.

"If the intertropical convergence zone was 550 km, or 5 degrees, south of its present position as recently as 1630, it must have migrated north at an average rate of 1.4 km – just less than a mile – a year," Sachs says. "Were that rate to continue, the intertropical convergence zone will be 126 km – or more than 75 miles – north of its current position by the latter part of this century."

Link: http://www.sciencedaily.com/releases/2009/07/090701135535.htm

G. Leduc et al., Nature, 445: Moisture transport across Central America as a positive feedback on abrupt climatic changes

Nature 445, 908-911 (22 February 2007); doi:10.1038/nature05578; received 18 October 2006; accepted 4 January 2007.

Moisture transport across Central America as a positive feedback on abrupt climatic changes

Guillaume Leduc, Laurence Vidal, Kazuyo Tachikawa, Frauke Rostek, Corinne Sonzogni, Luc Beaufort and Edouard Bard (CEREGE, UMR6635, CNRS Université Paul Cézanne Aix-Marseille III, Collège de France, Europôle de l'Arbois, BP 80, 13545 Aix-en-Provence Cedex 04, France)

Abstract

Moisture transport from the Atlantic to the Pacific ocean across Central America leads to relatively high salinities in the North Atlantic Ocean¹ and contributes to the formation of North Atlantic Deep Water². This deep water formation varied strongly between Dansgaard/Oeschger interstadials and Heinrich events—millennial-scale abrupt warm and cold events, respectively, during the last glacial period³. Increases in the moisture transport across Central America have been proposed to coincide with northerly shifts of the Intertropical Convergence Zone and with Dansgaard/Oeschger interstadials, with opposite changes for Heinrich events⁴. Here we reconstruct sea surface salinities in the eastern equatorial Pacific Ocean over the past 90,000 years by comparing palaeotemperature estimates from alkenones and Mg/Ca ratios with foraminiferal oxygen isotope ratios that vary with both temperature and salinity. We detect millennial-scale fluctuations of sea surface salinities in the eastern equatorial Pacific Ocean of up to two to four practical salinity units. High salinities are associated with the southward migration of the tropical Atlantic Intertropical Convergence Zone, coinciding with Heinrich events and with Greenland stadials⁵. The amplitudes of these salinity variations are significantly larger on the Pacific side of the Panama isthmus, as inferred from a comparison of our data with a palaeoclimate record from the Caribbean basin⁶. We conclude that millennial-scale fluctuations of moisture transport constitute an important feedback mechanism for abrupt climate changes, modulating the North Atlantic freshwater budget and hence North Atlantic Deep Water formation.

Correspondence and requests for materials should be addressed to G.L. (e-mail: leduc@cerege.fr) and E.B. (e-mail: bard@cerege.fr).

Link to abstract: http://www.nature.com/nature/journal/v445/n7130/abs/nature05578.html