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AGU journal highlights -- June 15, 2011

The following highlights summarize research papers that have been recently published in Water Resources Research (WRR), Geophysical Research Letters (GRL), Journal of Geophysical Research-Atmospheres (JGR-D), Paleoceanography (PA), and the Journal of Geophysical Research-Oceans (JGR-C).

In this release:

  1. How yearly cholera outbreaks propagate in the Bengal Delta
  2. Surge in North Atlantic hurricanes due to better detectors, not climate change
  3. Potential for Atlantic current collapse hinted by complex global circulation model
  4. Formation of Indonesian Archipelago destroyed Australian rainforests
  5. Air loss may play role in lowering of an Antarctic ice shelf
  6. First satellite measurements of elusive sulfur compound in a volcanic plume
  7. Different patterns but same El Nio, study suggests
  8. What triggers Canary Current's seasonal drift?
  9. Proposed NASA mission could detect climate effects, simulation indicates

Anyone may read the scientific abstract for any already-published paper by clicking on the link provided at the end of each Highlight. You can also read the abstract by going to and inserting into the search engine the full doi (digital object identifier), e.g. 10.1029/2010WR009914. The doi is found at the end of each Highlight below.

Journalists and public information officers (PIOs) at educational or scientific institutions who are registered with AGU also may download papers cited in this release by clicking on the links below. Instructions for members of the news media, PIOs, and the public for downloading or ordering the full text of any research paper summarized below are available at

1. How yearly cholera outbreaks propagate in the Bengal Delta

Cholera, a deadly waterborne disease, remains a major threat in many areas of the world, including the Bengal Delta region. In this region, cholera outbreaks have two annual peaks; the first occurs during the dry season in the spring, and the second occurs in the fall following the wet season. However, the large-scale hydroclimatic processes underlying the propagation of the disease have not been well understood.

Akanda et al. show that cholera outbreaks in the Bengal Delta region propagate from the coast to inland and from spring to fall following two distinct transmission cycles. The first outbreak begins in the spring near the coast when northward movement of plankton-rich seawater and increasing salinity promote the growth of cholera-causing bacteria in rivers, which are used for irritation, sanitation, and consumption. The second outbreak begins in the fall, after summer floods and monsoons affects sanitation conditions that aid in bacterial transmission by contaminating waters over much of Bangladesh.

The authors find that while spring cholera outbreaks appear to affect further outbreaks in the subsequent fall season, fall outbreaks do not affect cholera outbreaks in the following spring. This analysis could help in using dry season water management as a tool for reducing cholera burden throughout the year and developing climate-based warning of cholera outbreaks, and inform prevention and intervention strategies in affected regions.

Source: Water Resources Research, doi: 10.1029/2010WR009914, 2011

Title: Hydroclimatic influences on seasonal and spatial cholera transmission cycles: Implications for public health intervention in the Bengal Delta

Authors: Ali Shafqat Akanda and Antarpreet S. Jutla: Water and Environmental Research, Education, and Actionable Solutions Network, Department of Civil and Environmental Engineering, Tufts University, Medford, Massachusetts, USA;

And others (for a complete list of authors, check

2. Surge in North Atlantic hurricanes due to better detectors, not climate change

A spate of research has indicated there may be a link between climate change and the prevalence of North Atlantic tropical cyclones. Upon closer inspection, however, researchers have noted that the prominent upswing in tropical cyclone detections beginning in the mid twentieth century is attributable predominantly to the detection of "shorties," tropical cyclones with durations of less than 2 days. That the apparent surge in cyclone activity could be attributable to changes in the quality and quantity of detections has gained ground as a potential alternative explanation.

Using a database of hurricane observations stretching back to 1878, Villarini et al. try to tease out any detectable climate signal from the records. The authors note that between 1878 and 1943 there were 0.58 shorty detections per year, and between 1944 and 2008 there were 2.58 shorty detections per year. This increase in shorties, which the authors propose may be related to the end of World War II and the dawn of air-based reconnaissance and weather tracking, was not mirrored by an increase in tropical cyclone activity for storms longer than 2 days.

The authors compare the rate of shorty detections against a variety of climate parameters, including North Atlantic sea surface temperature, mean tropical sea surface temperature, the North Atlantic Oscillation, and the Southern Oscillation Index. The authors find that North Atlantic sea surface temperatures were related to tropical cyclones of longer than 2 days' duration but were not related to the rate of short detections. Additionally, for every decade after 1950s the occurrence of shorties seems to be related to a different climate parameter. Both of these findings are highly suggestive of data quality problems for the shorties record. The researchers note that their finding does not rule out the possibility of a climate-driven increase in shorties over the twentieth century. Rather, any existing trend will be imperceptible, as it is masked by data quality issues.

Source: Journal of Geophysical Research-Atmospheres, doi:10.1029/2010JD015493, 2011

Title: Is the recorded increase in short-duration North Atlantic tropical storms spurious?


Gabriele Villarini: Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey, USA; and Willis Research Network, London, UK;

James A. Smith: Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey, USA;

Gabriel A. Vecchi and Thomas R. Knutson: Geophysical Fluid Dynamics Laboratory, National Oceanic and Atmospheric Administration, Princeton, New Jersey, USA.

3. Potential for Atlantic current collapse hinted by complex global circulation model

The Atlantic meridional overturning circulation (AMOC) is a gigantic heat, salt, and nutrient mixer that spans the length of the Atlantic Ocean. Drawing warming surface waters up from the south through the Gulf Stream and along the North Atlantic Current, the system has a large amount of control over the climate of western Europe. Once in the North Atlantic the water cools, becoming more dense and sinking to between 3,000 and 5,000 meters (between 1.9 and 3.1 miles) in depth before commencing a return journey south. Both paleoclimate evidence and simplified ocean circulation models suggest that the AMOC may have two stable states (either its current behavior or an "off" mode), and this has left some researchers worried that the system may experience a sudden and drastic cessation.

While threshold behavior for the AMOC has been readily observed in simple climate models, Hawkins et al. reproduce the dynamic in a much more complex atmosphere-ocean coupled general circulation model (AOGCM) for the first time. Re-creating more than 56,000 years of ocean activity in the Fast Met Office/Universities Simulator (FAMOUS) AOGCM, the authors find that by progressively adding freshwater into the North Atlantic they are able to trigger the transition from a healthy functioning AMOC to a nonexistent one, which does not recover when the freshwater addition is subsequently decreased. Further, on the basis of the FAMOUS simulations and recent observations, the authors suggest that measurements of the direction of the net flux of freshwater at the southern edge of the Atlantic could serve as indicators that the AMOC actually has two stable states and thus has the potential to exhibit a threshold transition.

Source: Geophysical Research Letters, doi: 10.1029/2011GL047208, 2011

Title: Bistability of the Atlantic overturning circulation in a global climate model and links to ocean freshwater transport

Authors: E. Hawkins, R. S. Smith, L. C. Allison and T. J. Woollings: NCAS-Climate, University of Reading, Reading, UK;

J. M. Gregory: NCAS-Climate, University of Reading, Reading, UK; and Met Office, Exeter, UK;

H. Pohlmann: Met Office, Exeter, UK;

B. de Cuevas: National Oceanography Centre, Southampton, UK.

4. Formation of Indonesian Archipelago destroyed Australian rainforests

Just over 3 million years ago, a climatic upheaval forever changed Australia's western coast. Over the span of 200,000 years the southward flowing waters of the Leeuwin Current cooled by 2 to 3 degrees Celsius (3.6 to 5.4 degrees Fahrenheit), decreasing coastal precipitation and converting northwestern Australia's rainforests to semiarid grasslands. By measuring the isotope ratios and chemical composition of preserved shells left by ancient single-celled organisms, Karas et al. reconstruct the hydrologic signature of the geologic upheaval that triggered the sudden cooling.

During the Pliocene (approximately 4 million to 3 million years ago), tectonic activity led to the formation of a number of small islands in the Indonesian Archipelago. This protruding land restricted the Indonesian Throughflow (ITF), a current that carries warm, fresh, tropical Pacific water through the island chain and feeds the Leeuwin Current. On the basis of their shell-based reconstruction, the authors identify a strong divergence between the temperature of the Leeuwin Current and the nearby tropical Indian Ocean since about 3.3 million years ago. The finding of a relative cooling of the coastal Leeuwin Current was indicative of a weakened ITF rather than other regional changes. Colder waters flowing along the Australian shore would have reduced the amount of precipitation making its way inland, allowing tectonic activity in Indonesia to change the climate of the land down under.

Source: Paleoceanography, doi: 10.1029/2010PA001949, 2011

Title: Pliocene Indonesian Throughflow and Leeuwin Current dynamics: Implications for Indian Ocean polar heat flux

Authors: Cyrus Karas: Leibniz Institute of Marine Sciences (IFM-GEOMAR), University of Kiel, Kiel, Germany; and Goethe-University Frankfurt, Frankfurt am Main, Germany;

Dirk Nrnberg: Leibniz Institute of Marine Sciences (IFM-GEOMAR), University of Kiel, Kiel, Germany;

Ralf Tiedemann: Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany;

Dieter Garbe-Schnberg: Institute of Geosciences, University of Kiel, Kiel, Germany.

5. Air loss may play role in lowering of an Antarctic ice shelf

The surface elevation of the Larsen Ice Shelf in the Antarctic Peninsula has lowered by about 0.1 meters per year (3.9 inches per year) since 1992. This could be caused either by an increase in the density of firn (compacted snow) at the surface of the ice shelf or by an increase in melting of ice at the base of the ice shelf. Attributing the observed lowering to one of these causes would help researchers to know whether atmospheric or ocean warming was impacting the ice shelf. To do this, it is necessary to know the density or air content of the firn. However, firn air content has been measurable only with labor-intensive ground-based techniques.

To overcome those challenges, Holland et al. developed a new method that uses radar sounding measurements to estimate air content. Using their method, the authors observe spatial variations in the air content of the shelf. Overall, they find that the air content suggests that loss of air (increased firn density) could account for the observed lowering of the surface of the ice shelf, though the result does not rule out a contribution from melting at the base of the ice shelf.

Source: Geophysical Research Letters, doi:10.1029/2011GL047245, 2011

Title: The air content of Larsen Ice Shelf

Authors: Paul R. Holland, Hugh F. J. Corr, Hamish D. Pritchard, David G. Vaughan, Robert J. Arthern, and Adrian Jenkins: British Antarctic Survey, Cambridge, UK;

Marco Tedesco: Earth and Atmospheric Sciences, City College of New York, New York, New York, USA.

6. First satellite measurements of elusive sulfur compound in a volcanic plume

For the first time, satellites have measured hydrogen sulfide in a volcanic plume. Volcanoes release hydrogen sulfide in large quantitiesyearly global estimates range from 1 to 37 teragrams (1.1 to 40.7 million US tons). Along with sulfur dioxide, which is estimated to be emitted from volcanoes at about 15 to 21 teragrams per year (16.5 to 23.1 million US tons per year), hydrogen sulfide is a major portion of the volcanic sulfur released into the atmosphere. The ratio of hydrogen sulfide to sulfur dioxide emissions is useful for studying source conditions, sulfur chemistry, and magma-water interactions.

However, unlike sulfur dioxide, hydrogen sulfide has been challenging to measure in the atmosphere. Clarisse et al. use infrared satellite observations to characterize hydrogen sulfide from the 7-8 August 2008 eruption of the Kasatochi volcano in the Aleutian Islands. The eruption consisted of five explosive events; the observations indicated that the hydrogen sulfide plume was likely associated with the earlier events. The study shows that volcanoes are significant sources of hydrogen sulfide in the atmosphere and demonstrates for the first time that satellites can observe hydrogen sulfide plumes.

Source: Geophysical Research Letters, doi:10.1029/2011GL047402, 2011

Title: Infrared satellite observations of hydrogen sulfide in the volcanic plume of the August 2008 Kasatochi eruption

Authors: Lieven Clarisse, Pierre-Franois Coheur, Simon Chefdeville, Jean-Lionel Lacour, and Daniel Hurtmans: Spectroscopie de l'Atmosphre, Service de Chimie Quantique et Photophysique, Universit Libre de Bruxelles, Brussels, Belgium;

Cathy Clerbaux: Spectroscopie de l'Atmosphre, Service de Chimie Quantique et Photophysique, Universit Libre de Bruxelles, Brussels, Belgium; and LATMOS, IPSL, CNRS, INSU, Universit Pierre et Marie Curie, Paris, France.

7. Different patterns but same El Nio, study suggests

The traditional view of El Nio is that it starts with a warm sea surface anomaly off the west coast of South America. This aberrant warming drifts west from the coast, triggering peak sea surface temperatures in the central Pacific during the southern summer. However, as the observational record grows and the full range of variability comes into view, some researchers have suggested that a recent slate of El Nio events that focus on the central Pacific, skipping the eastern warming, is indicative of the system being composed of at least two unique climate dynamics. The central Pacific-focused El Nio Modoki has been increasing in prevalence over recent decades, and researchers expect this trend to continue with global warming.

New research by Takahashi et al., however, indicates that El Nio Modoki and the classical type are, in fact, one but that the perceived divergent behavior highlights the dynamic range of the system. Using El Nio observations stretching back to 1870, the authors perform a statistical analysis, breaking down the data set to isolate hidden spatial and temporal patterns. The Modoki and traditional El Nio records do not show the clustering that would be expected for separate climate phenomena. What do stand out, however, are the extreme warm events that occurred in the eastern Pacific in 1982-1983 and 1997-1998. On the basis of their findings, the authors propose two indices to describe the development of El Nio events: one for the moderate warming of the central Pacific and another for the extreme heat sometimes seen in the east. Therefore, the propagation of a given El Nio would be a combination of these two variables.

Source: Geophysical Research Letters, doi:10.1029/2011GL047364, 2011

Title: ENSO regimes: Reinterpreting the canonical and Modoki El Nio

Authors: K. Takahashi: Instituto Geofsico del Per , Lima, Per;

A. Montecinos: Departamento de Geofsica, Universidad de Concepcin, Concepcin, Chile;

K. Goubanova and B. Dewitte: Instituto Geofsico del Per , Lima, Per and Laboratoire d'Etudes en Gophysique et Ocanographie Spatiale, CNES/CNRS/IRD/UPS, Toulouse, France.

8. What triggers Canary Current's seasonal drift?

Along the northwestern coast of Africa lies an important fishery, stimulated by an upwelling of cold, nutrient-rich, deep-ocean water. Driven by a complex convergence of ocean currents, the waters between the coast, the Portuguese island of Madeira, and the Canary Islands are known to vary dramatically throughout the year, seeing coastal current reversals near the shore and the location of the large-scale Canary Current drifting seasonally, moving offshore in the winter before returning toward the coast in the summer. To sort out the trigger for this seasonal drift, Mason et al. produced a high-resolution model of the Canary Current that captures details of its interaction with the coastal region where the deep water upwelling occurs.

The authors find a pair of circular seasonal anomalies that they suggest control the strength and location of the Canary Current. The first, formed in late autumn, is a persistent, clockwise-spinning region of elevated sea surface height and increased flow rates. Its counterpart, a counterclockwise-rotating sea surface depression, is formed in the spring. Both anomalies spawn near the African coast and meander westward at around 2.6 kilometers per day (1.6 miles per day), pushing their way out of the region over the course of a year. The researchers report that the position of the Canary Current consistently lines up with the southward flowing edges of these anomalies and that this in turn explains the drift of the Canary Current. By selectively modifying parameters within their model, the authors determined that the anomalies are driven primarily by small-scale seasonal variations in wind stress along the coast.

Source: Journal of Geophysical Research-Oceans, doi:10.1029/2010JC006665, 2011

Title: Seasonal variability of the Canary Current: A numerical study

Authors: Evan Mason: Departament d'Oceanografa Fsica, Institut de Cincies del Mar, CMIMA-CSIC, Barcelona, Spain; and Departamento de Fsica, Universidad de Las Palmas de Gran Canaria, Las Palmas de Gran Canaria, Spain;

Pablo Sangr: Departamento de Fsica, Universidad de Las Palmas de Gran Canaria, Las Palmas de Gran Canaria, Spain;

Francois Colas, Jeroen Molemaker, Alexander F. Shchepetkin, and James C. McWilliams: Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, USA;

Charles Troupin: GeoHydrodynamics and Environment Research, Universit de Lige, Lige, Belgium.

9. Proposed NASA mission could detect climate effects, simulation indicates

Rising concentrations of carbon dioxide and other greenhouse gases in the atmosphere change how much solar radiation is reflected back into space. A proposed NASA mission, Climate Absolute Radiance and Refractivity Observatory (CLARREO), would make benchmark measurements of shortwave reflectance and longwave radiance spectra and of atmospheric refractivity to help researchers understand the changing solar reflectance and its effects on climate.

To help mission planners and to determine the utility of solar reflectance data from CLARREO for understanding climate forcings and feedbacks, Feldman et al. introduce an observational system simulation experiment that calculates the signals of future climate forcings and feedbacks in top-of-atmosphere reflectance spectra. The experiment allows mission planners to simulate CLARREO instrument measurements. The authors find that expected forcings from increases in anthropogenic emissions would be detectable and the proposed mission should help scientists understand climate forcings and feedbacks.

Source: Journal of Geophysical Research-Atmospheres, doi:10.1029/2010JD015350, 2011

Title: CLARREO shortwave observing system simulation experiments of the twenty-first century: Simulator design and implementation

Authors: Daniel R. Feldman: Department of Earth and Planetary Science, University of California, Berkeley, California, USA;

Chris A. Algieri: Climate Sciences Department, Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA;

Jonathan R. Ong: Department of Mathematics, University of California, Berkeley, California, USA;

William D. Collins: Department of Earth and Planetary Science, University of California, Berkeley, California, USA; and Climate Sciences Department, Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA.


Contact: Maria-Jos Vias
American Geophysical Union

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