AGU journal highlights - 11 November 2005

I. Highlights, including authors and their institutions

The following highlights summarize research papers in Geophysical Research Letters (GL). The papers related to these Highlights are printed in the next paper issue of the journal following their electronic publication.

You may read the scientific abstract for any of these papers by going to http://www.agu.org/pubs/search_options.shtml and inserting into the search engine the portion of the doi (digital object identifier) following 10.1029/ (e.g., 2005GL987654). The doi is found at the end of each Highlight, below. To obtain the full text of the research paper, see Part II.

1. Human activity has changed temperature extremes

Evidence suggests that manmade greenhouse gas emissions have warmed Earth over the last 50 years. While many studies have analyzed average temperatures and their dependence on external forces, few have analyzed extreme temperatures in the same manner. To study this, Christidis et al. applied a standard optimal detection approach to patterns of change in temperature extremes. The authors show that since 1950, the warmest nights and coldest days and nights of the year have heated by about 1 Kelvin [one degree Celsius, around two degrees Fahrenheit]. Comparing these observations with other climate model simulations showed that humans exert significant influence on patterns of change seen in extremely warm nights. Human influences also contributed to warming on cold nights, though the change is not as extreme. However, there was no detection of significant human influences on extremely warm days. The researchers add that although this model does not exactly match observations, both the model changes and the observations are significantly different than the model's estimate of variability generated internally within the climate system. Based on this study, the authors expect that extreme temperatures will intensify, possibly by 7 Kelvin [seven degrees Celsius, 10 degrees Fahrenheit] over the next hundred years. This could severely impact human health, they note.

Title: Detection of changes in temperature extremes during the second half of the 20th century

Authors:
Nikolaos Christidis, Peter A. Stott, Simon Brown, and John Caesar: Met Office, Hadley Centre for Climate Prediction and Research, Exeter, UK;
Gabriele C. Hegerl: Nicholas School of the Environment and Earth Sciences, Duke University, Durham, North Carolina, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL023885, 2005

2. Predicting catastrophic earthquakes

Quickly estimating the location and magnitude of an earthquake is essential to early warning systems. However, in large earthquakes, where the duration of rupture could range from a few seconds to tens of seconds, the estimation of earthquake magnitude is done before the rupture terminates. Since this estimation uses only a few seconds of wave data, the energy released by large earthquakes is often initially undervalued. Because determining the actual magnitude of an earthquake from initial P-wave [primary wave, the fastest wave and first to arrive] data is difficult, Iwata et al. propose a practical solution: when a small earthquake is detected in a region where a catastrophic one is expected, early warning systems can be modified to determine the probability that this earthquake will grow in magnitude. First, they offer a method to examine the probability of an earthquake growing into a catastrophic one. This method, based on the magnitude-frequency distribution of previous earthquakes in the region, was applied to the Nankai trough region off the coast of Japan. The researchers determined that if the observed earthquake magnitude reaches 6.5, the estimated probability that the final magnitude will reach 7.5 is between 25 and 41 percent.

Title: Probabilistic estimation of earthquake growth to a catastrophic one

Authors:
Takaki Iwata: The Institute of Statistical Mathemiatics, Tokyo, Japan;
Masajiro Imoto, Sigeki Horiuchi: National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Japan

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL023928, 2005

3. Hurricane intensification may be related to eyewall precipitation

Previous studies have shown that a relationship exists between hurricane wind intensification and tall precipitation cells along the eyewall. Kelley et al. Have better quantified this relationship by studying the frequency of tall precipitation. They used the Weather Surveillance Radar-1988 Doppler (WSR-88D) radars along the United States coast. The WSR-88Ds observed many hurricanes at four to six minute intervals when the hurricane eyewall was within a few hundred kilometers [yards] of landfall. To check the accuracy of the WSR-88D height measurements, the authors used very accurate height measurements from the Precipitation Radar on the Tropical Rainfall Measuring Mission (TRMM) satellite. The WSR-88D analysis showed that if the frequency of tall precipitation in the eyewall is at least 33 percent (one in three radar volume scans) then there was an 82 percent chance that hurricane winds will intensify. If this threshold was not met, the chance of wind intensification dropped to 17 percent. The authors suggest that this height-frequency threshold could aid forecasters during future hurricane seasons.

Title: Hurricane intensification detected by continuously monitoring tall precipitation in the eyewall

Authors:
Owen A. Kelley and John Stout: NASA Goddard Space Flight Center, Greenbelt, Maryland, USA, and Center for Earth Observing and Space Research, George Mason University, Fairfax, Virginia, USA;
Jeffrey B. Halverson: NASA Goddard Space Flight Center, Greenbelt, Maryland, USA, and Joint Center for Earth Systems Technology, University of Maryland Baltimore County, Baltimore, Maryland, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL023583, 2005

4. Natural ocean cycles will soon weaken thermohaline circulation

The Atlantic Multidecadal Oscillation (AMO) is a pattern of multi-decadal surface temperature variability centered on the North Atlantic Ocean, seen in analyses of global climate using measurements dating back to the 19th century. Shown to have been linked with Sahel drought, Brazilian rainfall rates, North American climate, and Atlantic hurricane frequency, studies of these 50-100 year fluctuations are hindered by short global climate records and insufficient subsurface ocean data. Thus, it is difficult to conclude whether the Atlantic Multidecadal Oscillation is genuinely oscillatory. To clarify this, Knight et al. examined a 1,400-year simulation that replicates the observed pattern and amplitude of the Atlantic Multidecadal Oscillation. Their results imply that the Atlantic Multidecadal Oscillation is a genuine quasi-periodic cycle, related to the variability in the oceanic thermohaline circulation. Using this relationship, they attempted to reconstruct past thermohaline circulation changes and observed an increase in thermohaline circulation strength over the last 25 years. Due to the Atlantic Multidecadal Oscillation's oscillatory nature, the authors inferred a decreasing thermohaline circulation strength over the next few decades, which would accelerate the anticipated human-caused weakening of the same phenomenon. They suggest that future climate change predictions need to account for natural changes expected from shifts in the Atlantic Multidecadal Oscillation.

Title: A signature of persistent natural thermohaline circulation cycles in observed climate

Authors:
Jeff R. Knight, Robert J. Allan, Chris K. Folland, and Michael Vellinga: Hadley Centre for Climate Prediction and Research, Met Office, Exeter, Devon U.K.;
Michael E. Mann: Department of Meteorology and Earth and Environmental Systems Institute (ESSI), Pennsylvania State University, Pennsylvania, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL024233, 2005

5. Natural and Manmade Variability in Ocean Heat Content

Simulations of Earths climate system suggest that the ocean's heat content has increased over the last 50 years due to higher levels of manmade greenhouse gases in the atmosphere. However, these models do not simulate the degree to which internal variability of the climate system causes oceanic heat content to fluctuate. To explore this issue, Levitus et al. analyzed yearly data collected on a one degree-square grid spanning the top 700 meters [2,000 feet] of the global ocean. This involved performing Empirical Orthogonal Function analysis on this data, a method that reduces large matrices into fundamental mathematical relationships that reflect variability seen in the original observations. Using this method, the authors determined that aside from the linear trend, heat variability in ocean is associated with both the El Nino phenomenon on interannual time scales and with the Pacific Decadal Oscillation on interdecadal scales. The authors suggest that models must account for the internal variability of the ocean's heat content in order to improve climate simulations.

Title: EOF analysis of the upper ocean heat content, 1956-2003

Authors:
Sydney Levitus, John Antonov, Tim Boyer, Hernan Garcia, Ricardo Locarnini: National Oceanographic Data Center, National Oceanic and Atmospheric Administration; Silver Springs, Maryland, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL023606, 2005

6. Aurora on Saturn

Based on previous studies and images taken from the Hubble Telescope during the Cassini spacecraft's approach trajectory to Saturn in early 2004, Saturn kilometric radiation (intense radio emissions escaping outward from the Saturn's auroral regions at frequencies above the local electron plasma frequency) has been shown to be associated with Saturn's auroral displays and dependent on solar wind conditions. Mitchell, et al. Have further quantified this correlation by analyzing data taken from the Magentospheric Imaging Instrument Ion and Neutral Camera aboard Cassini. These instruments recorded an abrupt increase in energetic neutral atom flux coming from the vicinity of Saturn's magnetotail, or the area of the magnetosphere on the night side of the planet not compressed by solar wind. These flashes of ion activity in the tail that match enhancements in Saturn's kilomentric radiation were imaged from outside Saturn's magnetosphere, allowing the authors to detect ion acceleration events while simultaneously measuring the magnetic field and plasma velocities in the magnetosheath [region close to the outer limit of the planet's magnetosphere]. Given the similarity between this observed phenomena and magnetic substorms on Earth, the authors conclude that Earth-like substorms and similar auroral processes occur within Saturn's magnetosphere.

Title: Energetic ion acceleration in Saturn's magnetotail: Substorms at Saturn?

Authors:
D. G. Mitchell, P. C. Brandt, E. C. Roelof, S. M. Krimigis, C. P. Paranicas, B. H. Mauk: Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland, USA;
J. Dandouras: Centre D'Etude Spatiale des Rayonnements, Toulouse, France;
N. Krupp: Max-Planck-Institut fuer Sonnensystemforschung, Lindau, Germany;
D. C. Hamilton: University of Maryland, College Park, Maryland, USA;
W. S. Kurth: Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa, USA;
P. Zarka: Observatory of Paris, Meudon, France;
M. K. Dougherty: Space and Atmospheric Physics, Imperial College, London, UK;
E. J. Bunce: Department of Physics & Astronomy, University of Leicester, Leicester, UK;
D. E. Shemansky: Department of Aerospace Engineering, University of Southern California Los Angeles, Los Angeles, California, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL022647, 2005

7. Central European temperatures rising faster than the rest of the Northern Hemisphere

In the Northern Hemisphere, temperatures over land have risen faster than in the ocean since 1980. However, surface temperature analysis of Central Europe show that temperatures there have risen three times faster than the Northern Hemisphere land average. Philipona, et.al. note that while temperatures and humidity across Europe changed uniformly for individual months, in accord with changing large-scale weather patterns, both temperature and humidity strongly increased in value from west to east for all months, an observation that cannot be due to air circulation. Previous measurements showed that solar shortwave radiation was decreasing; thus the rapid temperature increases were not due to solar warming. Instead, thermal longwave radiation from the atmosphere was strongly increasing under cloud-free skies and was highly correlated with increasing temperature. The authors show that 70 percent of the increase in the downward longwave radiation was due to increasing water vapor in the atmosphere, while 30 percent was due to increasing manmade greenhouse gases. These observations combine to suggest that the region is experiencing "positive water vapor feedback," in which carbon dioxide emissions warm the planet, causing more surface water to evaporate. This water vapor, also a greenhouse gas, accumulates in the atmosphere and further increases surface temperatures. [See also AGU press release 05-38: http://www.agu.org/sci_soc/prrl/prrl0538.html]

Title: Anthropogenic greenhouse forcing and strong water vapor feedback increase temperature in Europe

Authors:
Rolf Philipona: Physikalisch-Meteorologisches Observatorium Davos, and World Radiation Center, Davos Dorf, Switzerland;
Bruno Duerr: MeteoSwiss, Zurich, Switzerland;
Atsumu Ohmura and Christian Ruckstuhl: Institute for Atmospheric and Climate Science, Swiss Federal Institute of Technology (ETH), Zurich, Switzerland.

Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL023624, 2005

8. Tsunami warnings using ocean circulation models

The 26 December 2004 tsunami was the first of its magnitude to have occurred since the advent of both digital seismometry and satellite radar altimetry. Though the precise tsunami source and formation mechanisms are still not known, Song et al. note that numerical models that take advantage of the new data sets available can shed light on these issues. After the tsunami, digital seismic data was used to estimate the earthquake's fault parameters, including its slip function. The authors then used these parameters to generate a three-dimensional ocean-general-circulation-model, which is capable of isolating the tsunami waves from ocean circulation signals due to the wind, eddies, and other phenomena. They compared this model to the actual waves' propagation, recorded by satellites, and showed that the model matches the observed phenomenon consistently. This confirmed that the slip function is the most important condition controlling tsunami strength and that the geometry and the rupture velocity of the earthquake determine the spatial characteristics of the tsunami. The authors suggest that ocean-general-circulation-models coupled with fresh earthquake data can provide earlier warning to coastal communities at risk.

Title: The 26 December 2004 tsunami source estimated from satellite radar altimetry and seismic waves

Authors:
Y. Tony Song, L.L. Fu, and Victor Zlotnicki: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA;
Chen Ji and Vala Hjorleifsdottir: Seismological Laboratory, California Institute of Technology, Pasadena, California, USA;
C.K. Shum and Yuchan Yi, Space Geodesy and Remote Sensing, Ohio State University, Columbus, Ohio, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL023683, 2005

9. Monitoring Mount St. Helens

In September 2004, Mount St. Helens experienced a swarm of shallow earthquakes. In the following weeks, the crater floor deformed, steam and ash erupted, and lava extruded, accompanied by the formation and uplift of a new lava dome. During this period of unrest, Vaughan et al. took images of the volcano, using the NASA MODIS/ASTER (MASTER) airborne simulator paired with range-finding lidar, allowing the authors to match MASTER observations in the thermal infrared range with georeferenced elevation data. In addition, Forward-Looking Infrared cameras were deployed to collect ground- and helicopter-based thermal images. These data allowed the authors to view surface temperatures, calculate radiative cooling rates and corresponding radiative power, and observe structural deformation. Spectral analysis determined that the extruding lava was dacitic [a fine-grained rock], and MASTER and lidar data confirmed that the dome grew in areas that experienced elevated temperatures. The authors comment that airborne platforms not only provide higher spatial resolution than satellites, but that such platforms can be rapidly deployed to an erupting volcano to monitor hazards as they are created. In addition, they suggested that the combination of MASTER, thermal infrared range, and lidar data can lead to improved visualization and interpretation of an evolving volcanic system.

Title: Monitoring eruptive activity at Mount St. Helens with TIR image data

Authors:
R.G. Vaughan, S.J. Hook, V.J. Realmuto: Jet Propulsion Laboratory, Pasadena, California, USA
M.S. Ransey: Department of Planetary Science, University of Pittsburgh, Pittsburgh, Pennsylvania, USA;
D.J. Schneider: Alaska Volcano Observatory, U.S. Geological Survey, Anchorage, Alaska, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2005GL024112, 2005

II. Ordering information for science writers and general public

Journalists and public information officers of educational and scientific institutions (only) may receive one or more of the papers cited in the Highlights by sending a message to Jonathan Lifland [jlifland@agu.org], indicating which one(s). Include your name, the name of your publication, and your phone number. The papers will be e-mailed as pdf attachments.

Others may purchase a copy of the paper online for nine dollars:
1. Copy the portion of the digital object identifier (doi) of the paper following "10.1029/" (found under "Source" at the end of each Highlight).
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