AGU journal highlights - 9 March 2005

03/09/05

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., 2004GL987654). The doi is found at the end of each Highlight, below. To obtain the full text of the research paper, see Part II.

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1. Improving hurricane prediction

During hurricane season, meteorologists often have one question in mind: Where is the hurricane going to touch land? Predicting the course of a hurricane is an extremely complex problem beyond the capabilities of existing computer models, because there are myriad atmospheric factors that can affect the course of a storm. General circulation models have the computational clout to account for these factors, but they lack the ability to produce detailed results on a regional scale. Atlas et al. report that a high-resolution general circulation model now being used at NASA's Goddard Space Flight Center and Ames Research Center doubles the resolution of most global weather models and dramatically advances scientists' ability to predict the course of hurricanes. They demonstrate the model's abilities by testing it on four storms: Gustav and Isadore in 2002 and Bonnie and Charley in 2004. The model performed well, but scientists still lack enough information about the initial phases of a tropical storm to get the modeling off to the best start. Better use of satellite data could improve this situation, according to the authors.

Title: Hurricane forecasting with the high-resolution NASA finite volume general circulation model

Authors:
R. Atlas, O. Reale, B.-W. Shen, J.-D. Chern, K.-S. Yeh, Laboratory for Atmospheres, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA;
S.-J. Lin, NOAA Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey, USA;
W. Putman, Earth and Space Data Computing Division, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA;
T. Lee, NASA Headquarters, Washington, D.C., USA;
M. Bosilovich, J. Radakovich, Global Modeling and Assimilation Office, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL021513, 2005

2. Special section on 2003 Halloween solar storms

The 16 February 2005 issue of Geophysical Research Letters features a collection of papers about the violent Sun-Earth connection during October-November 2003, when a series of powerful solar eruptions caused intense flare and solar wind activity. The special section is paired with similar analyses of the events in two other AGU journals: Journal of Geophysical Research-Space Physics and Space Weather. In the current issue, Gopalswamy et al. summarize the first results of the analyses of the event, which lasted over a three-week period from mid-October until early November 2003. The solar activity produced the largest geomagnetic storm of the solar cycle, reaching throughout the solar system and buffeting Earth's ionosphere, as well as spacecraft far beyond Earth's orbit, with energetic particles. The Geophysical Research Letters special section contains 11 articles that report on the storm's effects in locations ranging from Boston to the distant Voyager 2 spacecraft.

[Highlights 3 and 4, below, are based on papers in this special Section.]

Title: Introduction to the special section: Violent Sun-Earth connection events of October-November 2003

Authors:
N. Gopalswamy, L. Barbieri, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA;
G. Lu, NCAR High Altitude Observatory, Boulder, Colorado, USA;
S. P. Plunkett, Naval Research Laboratory, Washington, D. C., USA;
R. M. Skoug, Los Alamos National Laboratory, Los Alamos, New Mexico, USA..

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL022348, 2005

3. Solar emissions likely control Jupiter's X-rays

X-ray emissions from Jupiter's equatorial regions are likely controlled by the Sun and may represent an accurate proxy for solar activity, a finding that could allow researchers to monitor flares that are invisible to space weather satellites. Bhardwaj et al. provide observational evidence from the European Space Agency's XMM-Newton spacecraft, confirming their earlier speculation that Jovian low-latitude (known as disk) X-rays approximate solar X-ray radiation. The researchers found that the day-to-day variability in Jupiter's disk X-ray was nearly synchronized with solar emissions from the same period, including a large solar flare matched by a corresponding Jovian brightening. Earlier research by several of the same authors proposed that high-latitude X-rays on Jupiter are largely produced by the impact of fast-moving charged particles into the planet's upper atmosphere. Their new results, confirmed by other recent spacecraft data, offers the first direct evidence that the Sun controls the X-rays from Jupiter's disk.

Title: Solar control on Jupiter's equatorial X-ray emissions: 26-29 November 2003 XMM-Newton observation

Authors: Anil Bhardwaj, R. F. Elsner, NASA Marshall Space Flight Center, Huntsville, Alabama, USA;
G. Branduardi-Raymont, G. Ramsay, R. Soria, Mullard Space Science Laboratory, University College London, London, United Kingdom;
G. R. Gladstone, Southwest Research Institute, San Antonio, Texas, USA;
P. Rodriguez, XMM-Newton SOC, Madrid, Spain;
J. H. Waite, University of Michigan, Ann Arbor, Michigan, USA;
T. E. Cravens, University of Kansas, Lawrence, Kansas, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL021497, 2005

4. Zeroing in on natural ozone destruction

The Earth's ozone layer can be damaged not only by human use of ozone-depleting chemicals but by large solar storms. These "solar proton events" bombard the Earth with protons that deplete the ozone layer by triggering a cascade of chemical reactions. Protons produce large amounts of nitrogen oxides that diminish ozone levels for years. Water vapor breaks down to hydrogen oxides that, for a few hours or days, can destroy up to 70 percent of the ozone layer. The solar proton event on 28 October 2003 was the largest in over 40 years. Degenstein et al. describe ozone depletion between 50 and 80 kilometers [30 and 50 miles] in the atmosphere with a maximum observed depletion of 75 percent near 65 kilometers [40 miles]. They examined the impacts of this huge solar storm on the atmosphere in the Southern Hemisphere, using data from the OSIRIS instrument on the Odin satellite. The observations fit with previous observations and modeling studies, although the maximum zone of depletion was eight kilometers [five miles] lower than previously observed.

Title: Observations of mesospheric ozone depletion during the October 28, 2003 solar proton event by OSIRIS

Authors: D. A. Degenstein, N. D. Lloyd, A. E. Bourassa, R. L. Gattinger, E. J. Llewellyn, Institute of Space and Atmospheric Studies, Department of Physics and Engineering Physics, University of Saskatchewan, Saskatoon, Saskatchewan, Canada

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL021521, 2005

5. A broken climate "hockey stick"?

Most climatologists agree that the twentieth century was the hottest in the last 1,000 years. This consensus is based in part on the "hockey stick" record published by climatologist Michael Mann and colleagues in 1998. The hockey stick shows temperatures that decline slowly over most of the millennium before rising sharply in the mid-19th century and heading ever upwards to recent temperatures. To construct the hockey stick Mann and colleagues used numerous proxy temperature records based on tree rings, ice cores, corals, and other historical records. They used a statistical method called principal component analysis to compile these records and calibrate them to known temperatures where they could, and found the hockey stick. McIntyre and McKitrick report that the hockey stick has no statistical significance. They were able to reproduce the hockey stick by running meaningless data with no trend through the Mann et al. procedure. They also show that just a few measurements from a proxy temperature record based on a single species of tree dominates all the other records and controls the hockey stick's shape in the early portion of the temperature reconstruction.

Title: Hockey sticks, principal components, and spurious significance.

Authors:
Stephen McIntyre, Northwest Exploration Co., Ltd., Toronto, Ontario, Canada;
Ross McKitrick, Department of Economics, University of Guelph, Guelph, Ontario, Canada

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL021750, 2005

6. Getting a fix on the great southern current

The Antarctic Circumpolar Current is a key component of the global climate system. The largest of Earth's global currents, it transfers heat, salt, freshwater, and nutrients between the Pacific, Atlantic, and Indian oceans. Changes in the Antarctic Circumpolar Current have the potential to affect climate over large areas, so monitoring the current is an important priority for oceanographers and climatologists. But how closely must the current be monitored to reliably detect seasonal or yearly changes? Meredith and Hughes report that to reliably detect yearly changes, the current must be sampled more than once a week. Monitoring seasonal changes requires even more frequent samples. To reach this conclusion, they used sea level and air pressure data from the Faraday Antarctic monitoring station in combination with the output of a global ocean model. They sampled this data at various intervals and compared the calculated average with the average obtained using all the data. Their study means that sporadic or snapshot monitoring efforts including hydrographic sections, expendable probes, and even satellite altimetry will fail to capture true variability from year to year. What is needed, they say, are fixed gauges, moorings, or monitors fixed on the ocean bottom.

Title: On the sampling timescale required to reliably monitor interannual variability in the Antarctic circumpolar transport

Authors:
Michael P. Meredith and Chris W. Hughes, Proudman Oceanographic Laboratory, Liverpool, United Kingdom.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL022086, 2005.

7. Volcanic eruptions' effect on vegetation

Volcanic eruptions may reduce the amount of isotopic carbon in vegetation, according to an analysis of the effect of historical eruptions on oak trees. Ogle et al. report on the chronology of carbon-13 records seen in a pair of Irish oaks through several major eruptions in the 18th and 19th centuries. The authors studied meteorological data from observatories in northern Europe and found a significant reduction in the carbon content stored inside the trees, which they attribute to the effects of the eruptions. The researchers suggest that airborne material spewed into the atmosphere from the eruptions likely decreased the amount of sunlight reaching the trees, which in turn hindered leaf formation and reduced the amount of carbon-13 absorbed from the air. They note that such soot and ash could remain in the atmosphere for several years, where it could be transported around the globe and affect incoming solar radiation and affect plant growth worldwide.

Title: Paleovolcanic forcing of short-term dendroisotopic depletion: The effect of decreased solar intensity on Irish oak

Authors:
Neil Ogle, Chris S. M. Turney, Robert M. Kalin, Louise O'Donnell, Queen's University of Belfast, Belfast, United Kingdom;
C. John Butler, Armagh Observatory, Armagh, United Kingdom.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL021623, 2005.

8. Sprites caused by intracloud electricity?

Lightning-like sprite discharges captured on film during thunderstorms in Japan provide the first evidence that the unusual events may be generated by in-cloud electrical activity. Ohkubo et al. analyzed images from more than 20 of the short-lived occurrences, taken as part of a study of wintertime sprites in 2003-2004, and report that the high-altitude luminous flashes are commonly associated with simultaneous low-frequency discharges within the cloud. The authors suggest that extremely low frequency evidence from the sprite discharges, observed on high-resolution audio-video equipment and confirmed by multiple stations, indicates that the charge transfer that causes lightning may also play a vital role in generating the distinctive sprites. Previous research had suggested that sprites were generated after cloud-to-ground lightning strokes, though no theory has sufficiently explained their initiation or development. The researchers say that the long time delays often seen between a lightning stroke and the sprite formation could be explained by the time evolution of the intracloud lightning discharge.

Title: VLF/ELF sferic evidence for in-cloud discharge activity producing sprites

Authors:
Atsushi Ohkubo, H. Fukunishi, Y. Takahashi, T. Adachi, Tohoku University, Sendai, Japan.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL021943, 2005.

9. Overestimating gas hydrates in northern Gulf of Mexico

Marine sediments in the northern Gulf of Mexico are likely too warm and salty to hold the amount of gas hydrates originally thought to exist in the ocean floor there. Ruppel et al. present high-resolution geophysical and geochemical data indicating that published estimates of abundant gas hydrates in the region should be revised sharply downward. Such hydrates occur naturally in marine sediment and are known to concentrate gases such as methane in the sediments. Using core samples, heat flow measurements, and images of the shallow sediment, the authors analyzed the seafloor at sites proposed for ocean drilling in the Gulf. Their results suggest that high salinity and elevated temperatures above buried salt deposits and near seafloor mud volcanoes cause conditions that are unfavorable for the development of thick hydrate deposits. Instead, the researcher propose that hydrates in the northern Gulf of Mexico may be confined to smaller areas and much thinner zones than previously believed.

Title: Heat and salt inhibition of gas hydrate formation in the northern Gulf of Mexico

Authors:
Carolyn Ruppel, D. Lizarralde, Georgia Institute of Technology, Atlanta, Georgia, USA;
G. R. Dickens, D. G. Castellini, Rice University, Houston, Texas, USA;
W. Gilhooly, University of Virginia, Charlottesville, Virginia, USA.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL021909, 2005.

10. Greenhouse gases affect the Arctic Oscillation

The Arctic Oscillation is a climate pattern defined by unusual Arctic low pressure that involves stronger than normal winds circulating counterclockwise around the Arctic at a latitude about even with Moscow. It has a profound effect on the weather in the northern United States and northern Eurasia. The Arctic Oscillation historically alternates between phases when there is relatively high pressure at the poles and phases when the pressure is low. But over the last 30 years, there has consistently been lower pressure at the poles, causing higher than normal temperatures in the United States and northern Eurasia. Scientists have speculated that this trend could be caused by external factors, including changes in the ocean, changes to the snow pack, and an increase in greenhouse gases in the atmosphere. Yukimoto and Kodera report that the long-term trends are most likely due to an increase in greenhouse gases. They used a general circulation model to simulate the past century's Arctic Oscillations. They found that an increase in greenhouse gases best reproduces the historical changes in the Arctic Oscillation.

Title: Interdecadal Arctic Oscillation in twentieth century climate simulations viewed as internal variability and response to external forcing

Authors:
Seiji Yukimoto and Kunihiko Kodera, Meteorological Research Institute, Tsukuba, Japan.

Source: Geophysical Research Letters (GL) paper 10.1029/2004GL021870, 2005.

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