AGU journal highlights -- Feb. 26, 2008

February 26, 2008

1. Sodium glow reveals Mercury's comet-like tail

Mercury's gravity is too weak to hold a permanent atmosphere. Thus, when atoms are evaporated from the surface by solar photons or other energetic processes, some of these atoms are accelerated to escape velocity by solar radiation pressure. The neutral components of this escaped gas form a comet-like tail that points away from the Sun. Baumgardner et al. study this tail by observing the bright yellow-orange light emitted by the sodium atoms in the tail. The authors find that the tail, previously detected out to 15 times the radius of Mercury, actually extends more than 100 times that distance, or 2.5 million kilometers (1.6 million miles) away from the planet. Further analysis shows that the time of flight for the sodium atoms in the tail, from the time they are sputtered from the surface to when they reach the tail's maximum observed extent, is approximately 15 hours. The authors note that although sodium atoms compose only a small fraction of the atoms sputtered from the surface, the bright emission from the sodium can serve as a tracer for other constituents invisible to ground-based observers.

Title: Imaging the sources and full extent of the sodium tail of the planet Mercury

Authors: Jeffrey Baumgardner, Jody Wilson, and Michael Mendillo: Center for Space Physics, Boston University, Boston, Massachusetts, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032337, 2008;

2. Satellite tracks summer Arctic ice motion

For more than 15 years, daily sea ice drifts in winter (October through May) have been monitored with satellite passive microwave sensors, including the 89-GHz channel of the Earth Observing System's Advanced Microwave Scanning Radiometer (AMSR-E) on the Aqua satellite. Noting that reliable estimates of summer ice drift are often obscured by weather, atmospheric moisture, and surface melt effects, Kwok seeks to determine how best to fill in the 3- to 4-month annual gaps in the satellite data. He finds that summer sea ice motion can be derived reliably from AMSR-E's 18-GHz channel, which has improved spatial resolution and lower sensitivity to atmospheric moisture than other channels previously used. Comparisons with buoy drifts reveal that the data retrieved from this channel are fairly accurate. Using this channel, Kwok examines 5 years of summer ice area exchange between the Pacific and Atlantic sectors of the Arctic Ocean and finds that sea level pressure patterns during the summer of 2006 and 2007 favored the export of sea ice into the Atlantic sector.

Title: Summer sea ice motion from the 18 GHz channel of AMSR-E and the exchange of sea ice between the Pacific and Atlantic sectors

Author: Ron Kwok: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032692, 2008;

3. Subtle wind variations may spur abrupt climate change

Simulating the climate during the Last Glacial Maximum (LGM), which occurred roughly 21,000 years ago, is a major challenge for climate modeling. In particular, the Atlantic meridional overturning circulation (AMOC), which regulates climate by distributing heat to the world's oceans and involves deepwater formation in the North Atlantic, is poorly constrained in model scenarios. To characterize the AMOC during the LGM, models must accurately simulate surface winds, which facilitate horizontal and vertical mixing in the ocean. Noting that wind fields during the LGM are not well understood, Montoya and Levermann model how changes in wind strength would affect AMOC strength. By assuming that LGM wind stresses are proportional to those experienced today, the authors discover that below certain thresholds of wind strength, North Atlantic deepwater formation takes place south of Greenland and the AMOC is relatively weak. Above this threshold, deepwater formation occurs farther north, leading to a vigorous AMOC. This suggests that subtle wind variations can significantly influence climate, perhaps even spurring abrupt climate changes.

Title: Surface wind-stress threshold for glacial Atlantic overturning

Authors: Marisa Montoya: Departamento de Astrofísica y Ciencias de la Atmósfera, Universidad Complutense de Madrid, Madrid, Spain;

Anders Levermann: Earth System Analysis, Potsdam Institute for Climate Impact Research, Potsdam, Germany; also at Institute of Physics, Potsdam University, Potsdam, Germany.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032560, 2008;

4. Carbon dioxide tied to air pollution mortality

Rising carbon dioxide (CO2) levels from burning fossil fuels have been linked to sea level changes, snowmelt, disease, heat stress, severe weather, and ocean acidification. Yet because it does not affect respiration directly, CO2 is not considered a classic air pollutant. Noting that increasing levels of CO2 cause temperature and water vapor content to rise, Jacobson uses photochemistry to determine that these factors independently feed back to increase ground-level ozone concentrations. This can harm lung function and irritate the respiratory system. Using a high-resolution model that correlates pollution levels to human health, the author finds that each one degree Celsius rise in temperature may increase U.S. annual air pollution deaths by about 1000. About 40 percent of these deaths may result from elevated ground-level ozone concentrations. The rest are likely from particles, which would increase due to CO2-enhanced stability, humidity, and biogenic feedbacks. The author notes that many of these deaths would occur in urban populations subject to smog, as are residents of some areas of California. Extrapolating U.S. deaths to global population yields about 22,000 excess deaths expected worldwide each year.

Title: On the causal link between carbon dioxide and air pollution mortality

Authors: Mark Z. Jacobson: Department of Civil and Environmental Engineering, Stanford University, Stanford, California, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL031101, 2008;

5. Natural ocean thermostat may protect some coral reefs

Several negative feedback mechanisms have been proposed to explain the stability of sea surface temperature in the western Pacific warm Pool (WPWP). These "ocean thermostat" mechanisms are hypothesized to cap sea surface temperatures in the WPWP at around 30-31 degrees Celsius. Noting that even slight changes in oceanic temperature can threaten coral survival by forcing corals to expel the colorful microscopic algae that provide them with nutrition (a phenomenon called coral bleaching), Kleypas et al. analyze data on the tropical ocean spanning 1950-2006, as well as data from simulations, and compare these with a database of coral bleaching reports. They find that between 1980 and 2005, only four episodes of bleaching were reported for reefs in the WPWP, much lower than in any other reef region. Further, sea surface temperatures from the WPWP in recent years have warmed less than elsewhere in tropical oceans, supporting the idea that thermostat mechanisms act to depress warming beyond certain thresholds.

(See press release:

Title: Potential role of the ocean thermostat in determining regional differences in coral reef bleaching events

Authors: Joan A. Kleypas and Gokhan Danabasoglu: National Center for Atmospheric Research, Boulder, Colorado, U.S.A.;

Janice M. Lough: Australian Institute of Marine Science, Townsville, Queensland, Australia.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL032257, 2008;

6. The dynamics of North American monsoons

The North American Monsoon Experiment is an international research program aimed at learning more about summertime precipitation over North America to improve precipitation prediction in models. In particular, several scientists interested in understanding more about summer precipitation over southwestern North America conducted a field study during the summer of 2004. For this study, scientists used GPS receivers, surface barometers, and surface thermometers to calculate the daily precipitable water vapor (PWV) content over northwestern Mexico. Through analyzing these data, Kursinski et al. find that the onset of the monsoon season can be seen by a large increase in PWV over several days, beginning in early July. Data from the Sierra Madre Occidental foothills reveal a dynamical transition in mid-August from smaller local convection patterns to larger, more regional scales. During the small-scale phase, a positive feedback helps precipitation-supplied moisture to initiate more moist convection. The authors note that precipitation is usually preceded by a rapid PWV rise and a sharp surface temperature decrease, implying that models must include moist convective downdrafts in the NAM area.

Title: Water vapor and surface observations in northwestern Mexico during the 2004 NAME Enhanced Observing Period

Authors: E. R. Kursinski: Department of Atmospheric Sciences, University of Arizona, Tucson, Arizona, U.S.A.;

R. A. Bennett: Department of Geosciences, University of Arizona, Tucson, Arizona, U.S.A.;

D. Gochis: Research Applications Laboratory, National Center for Atmospheric Research, Boulder, Colorado, U.S.A.;

S. I. Gutman and K. L. Holub: Earth System Research Laboratory, U.S. National Oceanic and Atmospheric Administration, Boulder, Colorado, U.S.A.;

R. Mastaler: Department of Atmospheric Sciences, University of Arizona, Tucson, Arizona, U.S.A.; now at Arizona Radio Observatory, Tucson, Arizona, U.S.A.;

C. Minjarez Sosa: Department of Atmospheric Sciences, University of Arizona, Tucson, Arizona, U.S.A.; also at Department de Fisica, Universidad de Sonora, Hermosillo, Sonora, Mexico;

I. Minjarez Sosa: Departmento de Geologia, Universidad de Sonora, Hermosillo, Sonora, Mexico;

T. van Hove: University Corporation for Atmospheric Research, Boulder, Colorado, U.S.A.

Source: Geophysical Research Letters (GRL) paper 10.1029/2007GL031404, 2008;
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