AGU Journal Highlights - 17 February 2004


The following highlights summarize research papers in Geophysical Research Letters (GL), Journal of Geophysical Research--Solid Earth (JB), and Journal of Geophysical Research--Atmospheres (JD). The papers related to these Highlights are printed in the next paper issue of the journal following their electronic publication.

1. Deadly European heat wave may be repeated

Increasing greenhouse gases may make extreme temperature events like the summer 2003 heat wave that struck western Europe a commonplace occurrence in the near future. Martin Beniston shows that temperature and climate simulations that included the effects from enhanced atmospheric greenhouse gas concentrations over Europe nearly matched the observed conditions last summer. Beniston analyzed the statistical features of past and current extreme temperature events observed in Switzerland and suggests that alterations in the atmospheric balance may be a harbinger of future climate trends. He notes that existing models predict regional and possibly global climate shifts, such as an average 4 degree Celsius [7 degrees Fahrenheit] temperature rise in the late 21st century summer as a response to increased greenhouse gases.

Title: The 2003 heat wave in Europe: A shape of things to come? An analysis based on Swiss climatological data and model simulations

Martin Beniston, University of Fribourg, Fribourg, Switzerland.

Source: Geophysical Research Letters (GL) paper:
10.1029/2003GL018857, 2004

2. Earth's formations help transfer energy from earthquakes

Earth's upper mantle and lower crust regions play a significant role in "reloading" a fault zone after an earthquake, according to research that used the 1906 San Francisco earthquake to model the transfer of accumulated stresses along the San Andreas fault line. S. J. Kenner suggests that improving seismic hazard estimates in such complex fault systems requires a better understanding of the structure and deformation of rock formations deep underground. Kenner shows that post-seismic activity can provide up to 80 percent of the stress released during large earthquakes, depending on the geometry and patterns of stress in shear zones near a fault. Her observations are consistent with the activity prior to the powerful 1906 quake, providing estimates indicating that much of the energy transferred to distant tectonic fields comes from underground motion.

Title: Rheological controls on fault loading rates in northern California following the 1906 San Francisco earthquake

Shelley J. Kenner, University of Kentucky, Lexington, Kentucky.

Source: Geophysical Research Letters (GL) paper:
10.1029/2003GL018903, 2004

3. Amazon discharge drives tropical Atlantic carbon sink

Data obtained from a German research vessel in the tropical Atlantic Ocean revealed the strong impact of the Amazon River discharge on the region's carbon balance. Arne Kortzinger found that the Amazon's average discharge of approximately 200,000 cubic meters [50 million gallons] of river water per second gives rise to a marked carbon dioxide sink in the open ocean that can be traced thousands of kilometers [miles] away from the coast. The finding is in contrast to typical surface waters in the tropical Atlantic, which act as sources of atmospheric carbon dioxide. Kortzinger's research provides a biological and hydrochemical explanation for the change from highly carbon dioxide- supersaturated Amazon River water to markedly carbon dioxide-undersaturated plume water in the open ocean.

Title: A significant CO2 sink in the tropical Atlantic Ocean associated with the Amazon River plume

Arne Kortzinger, Kiel University, Kiel, Germany.

Source: Geophysical Research Letters (GL) paper:
10.1029/2003GL018841, 2003

4. First tropical warm rain estimates could improve global climate models

Warm rain (rainfall derived from non-ice water droplets in mid- and low-level clouds) accounts for most of the precipitation in warm climates like the tropics, according to a new paper that breaks down the type of rainfall in the tropical zones. Lau and Wu present microwave images and radar data from the Tropical Rainfall Measuring Mission and found for the first time that approximately 72 percent of the total rain area and 31 percent of the total rain amount in the tropics comes from warm rain. Previously, there were no reliable estimates for the amount of warm rain. The authors also measured the relationship between liquid water in a cloud and the rain rate, which is used in climate models to represent convection cycles and their role in global warming. Warm rain is important in regulating the tropical water cycle, including potentially changing the amount of high clouds that produce cold rain and upper-level cloudiness in tropical and subtropical areas.

Title: Warm rain processes over tropical oceans and climate implications

William Lau, NASA/Goddard Space Flight Center, Greenbelt, Maryland;
H. T. Wu, Science and Systems Applications Inc., Lanham, Maryland.

Source: Geophysical Research Letters (GL) paper:
10.1029/2003GL018567, 2003

5. Martian basin may lend insight to planet's climate history

A Martian drainage basin with a different type of erosion than normally seen on the planet may indicate the way liquid once flowed on Mars. Moore et al. analyzed the shape and size of presumably liquid-formed channels recently observed in a crater by the Mars Orbiter. Laser altimetry data show deep, river-like features on the surface that, if produced by periodic precipitation similarly as on Earth, would likely take thousands of years to carve. The authors, however, cannot rule out the possibility that the channels resulted from a few shorter episodes of heavy precipitation, perhaps caused by a major impact. They note that their report is consistent with previous studies that proposed that the early Martian climate was dry with brief episodes of warm, wet conditions following meteor impacts.

Title: Martian layered fluvial deposits: Implications for Noachian climate scenarios

Jeffrey M. Moore, NASA Ames Research Center, Moffett Field, California;
Alan D. Howard, University of Virginia, Charlottesville, Virginia;
William E. Dietrich, University of California-Berkeley, California;
Paul M. Schenk, Lunar and Planetary Institute, Houston, Texas.

Source: Geophysical Research Letters (GL) paper:
10.1029/2003GL019002, 2003

6. Arctic tide model improves ocean assessments

A new high-resolution Arctic tidal model that improves simulations of tide height variability and ocean currents can be used for predicting the Arctic Ocean circulation and sea ice conditions. Padman and Erofeeva use software that merges ocean physics with tide height data taken from coastal tide gauges and satellite radar to create an accurate map of Arctic tidal conditions. The authors then confirmed their data by comparing their results to a previous model, using actual tidal patterns recorded by more than 300 coastal tide gauges as a measure. Previous tidal predictions for the Arctic were hindered by the lack of high-latitude satellite data and poor resolution of the polar region's complex topography. The new model uses a 5 kilometer [3 mile] grid, rather than the 14 kilometer [9 mile] grid used by the previous best model, which provides a much-improved assessment of tides in the narrow channels of the Canadian Arctic Archipelago and the adjacent Baffin Bay and Labrador Sea.

Title: A barotropic inverse tidal model for the Arctic Ocean

Laurence Padman, Earth and Space Research, Corvallis, Oregon;
S. Erofeeva, Oregon State University, Corvallis, Oregon.

Source: Geophysical Research Letters (GL) paper:
10.1029/2003GL019003, 2004

7. Tropical sea temperatures did not cause glacial melting

Tropical sea surface temperatures likely did not trigger the melting of the polar ice sheets at the end of the last ice age. Rodgers et al. report that temperature and precipitation responses to variations in tropical sea surface temperatures tend to balance out with the net ice accumulation. Their comparison using three different model scenarios suggests that periods with warmer sea surface temperatures during the last glacial maximum also affected the atmospheric moisture supply, enhancing cloud cover that increased precipitation and decreased solar radiation over the ice sheets. Previous studies had proposed that warmer tropical ocean temperatures would have a significant affect on reducing the ice sheet mass. While the researchers found a wide variety in the sensitivity of the ice sheets to different sea surface temperature scenarios, they conclude that other factors were more likely to induce the deglaciation.

Title: Sensitivity of Northern Hemispheric continental ice sheets to topical SST during deglaciation

Keith B. Rodgers, Sylvie Charbit, Masa Kageyama, Gwenaelle
Philippon, Gilles Ramstein, Laboratories of Climate and Environmental Science, Gif-sur-Yvette, France;
Catherine Ritz, Laboratories of Environmental Geophysics and Glaciology, Saint-Martin d'Heres, France;
Gerrit Lohmann, Bremen University, Bremen, Germany;
Stephan J Lorenz, Max Planck Institute for Meteorology, Hamburg, Germany;
Myriam Khodri, Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York.

Source: Geophysical Research Letters (GL) paper:
10.1029/2003GL018375, 2004

8. First estimate of CFCs in global ocean

A global inventory provides the first estimate for the amount of chlorofluorocarbons [CFCs] deposited in the world's oceans. Willey et al. report that approximately 75,000 metric tonnes [83,000 tons] of CFC-11 has been deposited in the oceans, accounting for nearly one percent of the total CFC-11 emitted into the atmosphere through 1994. The authors used measurements from the World Ocean Circulation Experiment and found that the vast majority of the CFC-11 is in the upper 1000 meters [3,000 feet] of the oceans, where it is biologically and chemically inert. They note that the manmade compounds, introduced in the 1930s, are slightly soluble in seawater and have a higher solubility at cold water temperatures. Their research also suggests that the estimated CFC distribution can be used to infer the dominant form of circulation that transfers such compounds into the ocean interior, providing information that can be used to refine global climate change simulations.

Title: Global oceanic chlorofluorocarbon inventory

Debra A. Willey, Rana A Fine, University of Miami, Florida;
Rolf E. Sonnerup, Mark J. Warner, University of Washington, Seattle, Washington;
John L. Bullister, NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington;
William M. Smethie Jr., Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York.

Source: Geophysical Research Letters (GL) paper:
10.1029/2003GL018816, 2004

9. How volcanic eruptions cause tsunamis

Researchers have found the key element that influences tsunami formation after a hot flow of rock and gas from volcanic eruptions reaches water. Watts and Waythomas suggest that the dense lower portion of pyroclastic flows has the greatest impact on large waves caused by the coastal eruptions. The authors analyzed several possible mechanisms that occur when the particle-rich flows encounter water, ranging from steam generation to pressure changes on the water surface, and found that the flow of basal material into and under the water generates the most significant waves. They also report that the initial portion of the flow tends to be consumed in the water's splash zone and creates steam that contributes less than one percent of the thermal energy needed to make water waves. The researchers conclude that the volume and density of the basal flow has a close correlation with the wave's amplitude and wavelength, which can be used to model the water movement in lakes, bays and oceans.

Title: Theoretical analysis of tsunami generation by pyroclastic flows

Phil Watts, Applied Fluids Engineering Inc., Long Beach, California;
C. F. Waythomas, Alaska Volcano Observatory, U.S. Geological Survey, Anchorage, Alaska.

Source: Journal of Geophysical Research-Solid Earth (JB) paper:
10.1029/2002JB002265, 2003

10. Better data needed for accurate U.S. ozone standards

[See also AGU press release 04-01:]

The U.S. Environmental Protection Agency may be overestimating natural ozone concentrations, potentially making air quality standards weaker. Fiore et al. found significant variability in their simulation of background ozone concentrations throughout the United States that are used to set policy for the regulatory agency. The authors used a three-dimensional model to simulate ozone concentrations across North America and found that natural, or background, ozone levels are lower than current EPA assumptions and vary widely by season and region. The agency currently sets its standards by comparing the background levels to measurements of ozone concentrations that are likely elevated by manmade pollution. The researchers point out that the variability of background levels means that the EPA should set its background ozone concentrations on a more local level and suggest testing the ozone quantities across a larger range of terrains and seasons.

Title: Variability in surface ozone background over the United States: Implications for air quality policy

Arlene Fiore, D. Jacob, H. Liu, R. M. Yantosca, T. D. Fairlie, Q. Li, Harvard University, Cambridge, Massachusetts.

Source: Journal of Geophysical Research-Atmospheres (JD) paper:
10.1029/2003JD003855, 2003

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