The following highlights summarize research papers that have been published or are "in press" (accepted, but not yet published) in Geophysical Research Letters (GRL).
1. Decreased solar magnetic flux forecasts cosmic ray boost
Observations by the European Space Agency/NASA Ulysses mission have shown that the radial component of the magnetic field multiplied by the square of the distance from the Sun is independent of solar latitude and is a measure of the open magnetic flux in the heliosphere. Smith and Balogh find that the open flux during the ongoing solar activity minimum has reached the lowest level since the space age began. This decrease coincides with a factor of 2 decrease in the Sun's polar cap magnetic field as measured by ground-based magnetographs. These observations provide further evidence of the close connection between the open heliospheric flux and the polar cap field. An accompanying decrease is also reported in the level of magnetic fluctuations measured by Ulysses in the fast solar wind from the polar cap. Because these fluctuations oppose the access of galactic cosmic rays to the polar caps, the cosmic ray flux throughout the heliosphere is expected to reach unusually high levels.
Title: Decrease in heliospheric magnetic flux in this solar minimum: Recent Ulysses magnetic field observations
Authors: Edward J. Smith: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, U.S.A.;
Andre Balogh: Blackett Laboratory, Imperial College, London, U.K.
Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL035345, 2008; http://dx.doi.org/10.1029/2008GL035345
2. Do iceberg scrapes cause glacial quakes?
During the past decade, Jakobshavn Isbræ and several other glaciers draining the Greenland Ice Sheet have dramatically thinned, accelerated, and retreated, in some cases doubling their iceberg calving rates. Although ice discharge accounts for roughly two thirds of mass lost by the Greenland Ice Sheet, calving behavior, stability at the glacier's terminus, and related changes in glacier motion remain poorly understood. To learn more, Amundson et al. study the recent loss of Jakobshavn Isbræ's extensive floating ice tongue. Through cameras that recorded calving events, a tide gauge, a seismometer, and optical and GPS surveys that monitored iceberg and glacier motion, the authors find that calving occurs primarily in summer from a grounded terminus, involves detachment and overturning of icebergs on short timescales, and produces long-lasting and far-reaching ocean waves and seismic signals. Although calving increases near-terminus glacier velocities, seismic signals are not seen to be caused by episodic rapid glacial slip, contradicting the originally proposed "glacial earthquake" mechanism. Rather, the authors hypothesize that earthquakes are caused by icebergs scraping the fjord bottom during calving.
Title: Glacier, fjord, and seismic response to recent large calving events, Jakobshavn Isbræ, Greenland
Authors: J. M. Amundson, M. Truffer, M. West, and R. J. Motyka: Geophysical Institute, University of Alaska Fairbanks, Fairbanks, Alaska, U.S.A.;
M. P. Lüthi: Versuchanstalt für Wasserbau: Hydrologie und Glaziologie, ETH-Zürich, Zürich, Switzerland;
M. Fahnestock: Institute for the Study of the Earth, Oceans and Space, University of New Hampshire, Durham, New Hampshire, U.S.A.
Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL035281, 2008; http://dx.doi.org/10.1029/2008GL035281
3. Renewed growth of atmospheric methane
Methane, an important greenhouse gas, plays a significant role in ozone layer chemistry. Its average concentration in the atmosphere is determined largely by a balance between free emission from the surface (from outgassing wetlands and anthropogenic activity) and destruction by hydroxyl free radicals in the troposphere. Previous measurements of atmospheric methane show persistent increases in concentrations through the latter part of the twentieth century, followed by a period of little change since 1999. However, networks show increasing methane concentrations starting near the beginning of 2007. Using a simple model of atmospheric chemistry and transport, Rigby et al. investigate what caused this rise. Through simulating the behavior of hydroxyl free radicals, the authors conclude that if the annual mean hydroxyl free radical concentrations did not change, a substantial increase in methane emissions was required simultaneously in both hemispheres between 2006 and 2007 to explain current observations. However, if a small drop in the hydroxyl radical concentration occurred, as is implied by observations, the methane emissions increase is more strongly biased to the Northern Hemisphere.
Title: Renewed growth of atmospheric methane
Authors: M. Rigby, R. G. Prinn, and J. Huang: Center for Global Change Science, Massachusetts Institute of Technology, Cambridge Massachusetts, U.S.A.;
P. J. Fraser, R. L. Langenfelds, L. P Steele, and P. B. Krummel: Centre for Australian Weather and Climate Research, CSIRO Marine and Atmospheric Research, Aspendale, Victoria, Australia;
P. G. Simmonds and S. O'Doherty: School of Chemistry, University of Bristol, Bristol, U.K.;
D. M. Cunnold and H. J. Wang: School of Earth and Atmospheric Sciences, Georgia Institute of Technology Atlanta, Georgia, U.S.A.;
R. F. Weiss, P. K. Salameh, C. M. Harth, and J Mühle: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, U.S.A.;
L. W. Porter: Australian Government Bureau of Meteorology, Melbourne, Victoria, Australia; now deceased.
Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL036037, 2008; http://dx.doi.org/10.1029/2008GL036037
4. Improving measurements of ocean surface velocity from space
Advanced synthetic aperture radar (ASAR) signals collected by the European Space Agency's Envisat help scientists map surface currents in the world oceans. As demonstrated, ASAR is a valuable source of high-resolution information on the line-of-sight roughness and velocity of the moving ocean surface. The velocity is estimated from careful examination and analysis of residual Doppler frequency shifts extracted from the ASAR scenes at a spatial resolution of about 5 to 10 kilometers (3 to 6 miles). Using this methodology, Johannessen et al. consistently derive both kinematic and dynamic properties of the ocean surface roughness. The authors compare the observations with advanced simulations as well as altimeter-based surface current measurements, finding good agreement to motivate further research to integrate the derived characteristics of ocean surface motion into broader models. This can help scientists learn more about mesoscale and submesoscale upper ocean dynamics, coupled physical-biogeochemical processes, and air-sea interactions in intense current regimes.
Title: Direct ocean surface velocity measurements from space: Improved quantitative interpretation of Envisat ASAR observations
Authors: J. A. Johannessen: Nansen Environmental and Remote Sensing Center, Bergen, Norway; also at Geophysical Institute, University of Bergen, Bergen, Norway.
B. Chapron: Institute Francais de Recherche pour l'Exploitation de la Mer, Plouzané, France;
F. Collard and A. Mouche: BOOST Technologies, Brest, France; now at CLS-Direction of Radar Applications, Plouzané, France;
V. Kudryavtsev: Nansen International Environmental and Remote Sensing Center, St. Petersburg, Russia; also at Nansen Environmental and Remote Sensing Center, Bergen, Norway; also at Marine Hydrophysical Institute, Sebastopol, Ukraine;
D. Akimkov: Nansen International Environmental and Remote Sensing Center, St. Petersburg, Russia; now deceased;
K.-F. Dagestad: Nansen Environmental and Remote Sensing Center, Bergen, Norway.
Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL035709, 2008; http://dx.doi.org/10.1029/2008GL035709
5. Global model reproduces behavior of two real cyclones
Atmospheric models that resolve clouds have greatly contributed to understanding local and regional climate; excessive computational needs have in the past allowed these models to be run only over limited areas. The increasing capability of high-end computers now allows numerical simulations with horizontal resolutions high enough to resolve cloud systems in a global model. Fudeyasu et al. analyze initial results from the global Nonhydrostatic Icosahedral Atmospheric Model (NICAM), developed by Japanese scientists. In their study, NICAM simulation successfully reproduces the life cycles of two real tropical cyclones that formed in the Indian Ocean in 2006's austral summer. Initialized with atmospheric conditions that were present a few weeks before the cyclones formed, the model captures the timing of formation, motions, and overall evolution of the observed cyclones. The successful simulation is attributed to the realistic representation of the large-scale circulation and the embedded convective vortices. Thus, NICAM provides high temporal and spatial resolution data sets for detailed studies of tropical cyclone genesis and evolution, potentially ushering in a new era for weather and climate predictions.
Title: Global cloud-system-resolving model NICAM successfully simulated the lifecycles of two real tropical cyclones
Authors: Hironori Fudeyasu and Yuqing Wang: International Pacific Research Center, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii, U.S.A.;
Masaki Satoh: Center for Climate System Research, University of Tokyo, Kashiwa, Japan; also at Frontier Research Center for Global Change, Japan Agency for Marine Earth Science and Technology, Yokohama, Japan;
Tomoe Nasuno and Hiroaki Miura: Frontier Research Center for Global Change, Japan Agency for Marine Earth Science and Technology, Yokohama, Japan;
Wataru Yanase: Center for Climate System Research, University of Tokyo, Kashiwa, Japan.
Source: Geophysical Research Letters (GRL) paper 10.1029/2008GL036003, 2008; http://dx.doi.org/10.1029/2008GL036003
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Journal
Geophysical Research Letters