AGU journal highlights -- Jan. 2, 2007

Natural aerosols are known to have a cooling effect on Earth's climate, because they scatter incoming solar radiation. Volcanic eruptions provide a natural experiment to observe the time constants for the onset and decay of consequent radiative perturbations. Harries and Futyan studied atmospheric conditions following the 1991 eruption of Mt. Pinatubo in the Philippines. Using atmospheric records before and after the event, the authors analyzed the growth and decay of perturbations in atmospheric temperature, humidity, and radiative fields following the eruption. They were able to quantitatively distinguish between processes that respond quickly to the insertion of aerosols into the atmosphere, such as the shortwave and longwave fluxes at the top of the atmosphere, and those which evolve on slower timescales, such as changes in the humidity and temperature fields. The authors suggest that a valuable test of coupled climate models should be whether they correctly reproduce these response times after a volcanic eruption.

Title: On the stability of the Earth's radiative energy balance: Response to the Mt. Pinatubo eruption

Authors:
J. E. Harries: Space and Atmospheric Physics Group, Blackett Laboratory, Imperial College, London, United Kingdom;
J. M. Futyan: Department of Applied Mathematics and Applied Physics, and Institute for Space Studies, Columbia University, New York, New York, U.S.A.

Source:
Geophysical Research Letters (GRL) paper 10.1029/2006GL027457, 2006

2. Hydrographic control of the Indian Ocean by Antarctic climate

The tropical Indian Ocean, which modulates the Asian monsoons and the aridity of the east Africa and the Sahel region, exerts a controlling influence on large-scale regional climate. Further, hydrographic changes in the surface of the tropical Indian Ocean, which is linked by oceanic currents to the Atlantic and Pacific oceans, may also affect global heat and salt distribution and in turn, global climate. Kiefer et al. sought to investigate the millennial-scale variability of the western Indian Ocean's surface. Using paleoceanographic analyses of stable isotope and magnesium/calcium ratios in planktonic foraminifera, the authors demonstrate that tropical Indian Ocean temperatures resemble temperatures from Antarctic ice cores and show patterns in warm and cold events known to have existed in southern high latitudes. The authors speculate that the link between the Antarctic and the Indian Ocean could involve Subantarctic Mode Water, which forms north of the subpolar frontal zone and spreads northward into the Indian Ocean.

Title: Antarctic control on tropical Indian Ocean sea surface temperature and hydrography

Authors:
Thorsten Kiefer: The Godwin Laboratory for Paleoclimate Research, Department of Earth Sciences, University of Cambridge, Cambridge, United Kingdom; now at Past Global Changes (PAGES) International Project Office, Bern, Switzerland.
Nick McCave and Henry Elderfield: Godwin Laboratory for Paleoclimate Research, Department of Earth Sciences, University of Cambridge, Cambridge, United Kingdom.

Source:
Geophysical Research Letters (GRL) paper 10.1029/2006GL027097, 2006

3. Modeling large vortex action at small scales

Vortex action, frequently observed in the atmosphere and oceans, can exist at several spatial scales in a single system, ranging from synoptic-scale to small-scale. To understand how these scales of vortex action interact, Luo and Liu modeled a system of four coexisting scales--a subtropical high ridge, a major vortex, as well as meso- and small-scale vortices--with the goal of determining how small-scale interactions affect the behavior of major systems. Noting that the microenvironment of a major vortex consists of a group of small-scale vortices, the authors discovered that varying initial number of these small-scale systems causes three kinds of major vortex motion: the initial major vortex decays away with no new major vortex formation, a weak new major vortex forms during the initial major vortex decay, or a new major vortex forms via self-organization. In the latter case, the intensity and scale corresponds to the pre-existing vortex.

Title: Diversity of microenvironments and the complexity of vortex motion

Authors:
Zhexian Luo: School of Remote Sensing, Nanjing University of Information Science and Technology, Nanjing, China;
Chongjian Liu: State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing, China

Source:
Geophysical Research Letters (GRL) paper 10.1029/2006GL027765, 2006

4. Seasonal changes in the age and structure of dissolved organic carbon in Siberian rivers and streams

Northern latitude rivers contain the highest concentrations of dissolved organic carbon found in rivers worldwide and represent an important term in northern latitude carbon budgets. Several studies, using data collected during late summer and early fall, indicate that dissolved organic carbon in Arctic rivers originates from carbon deposited in the past few decades. Little information exists, however, on the seasonal pattern of the dissolved organic carbon's age in these regions. Neff et al. studied Siberian rivers and streams during 2003, their observation period including winter and the spring floods. Through radiocarbon measurements, they discovered that the bulk of dissolved organic carbon during winter and spring was modern. By September 2003, however, radiocarbon ages had increased, indicating that carbon from old deposits of soils was being mobilized. Based on their findings, the authors note that, while the annual flux of dissolved organic carbon is most affected by what happens early in the growing season, the late season increase in dissolved organic carbon's age indicates the vulnerability of old carbon stocks to leaching and carbon loss in this region.

Title: Seasonal changes in the age and structure of dissolved organic carbon in Siberian rivers and streams

Authors:
J. C. Neff and S. P Davydov: Geological Sciences and Environmental Studies, University of Colorado, Boulder, Colorado, U.S.A.
J. C. Finlay: Department of Ecology, Evolution and Behavior, University of Minnesota, St. Paul, Minnesota, U.S.A.;
S. A. Zimov and A. I. Davydova: North-East Scientific Station, Pacific Institute of Geography, Far East Branch, Russian Academy of Sciences, Cherskii, Russia;
E. A. G. Schuur: Department of Botany, University of Florida, Gainesville, Florida, U.S.A.

Source:
Geophysical Research Letters (GRL) paper 10.1029/2006GL028222, 2006

5. South China Sea throughflow affects heat and freshwater distributions in the tropical Indian and Pacific Oceans

The Indonesian Maritime Continent is of major climate importance. It is a region where interaction between the atmosphere and the ocean's mixed layer spawns the El Nino Southern Oscillation phenomenon and where small changes in sea surface temperature can result in significant changes in weather patterns across the Indo-Pacific basin. To study the sea surface temperature variability of this region, Qu et al. investigated the South China Sea throughflow, a heat and freshwater conveyor that involves inflow of cold, salty water surrounding the Philippines and outflow of warm, fresh water through other straits along the South China Sea, before being distributed to other bodies of water within the Indonesian Maritime Continent. Based on their study, the authors hypothesize that the South China Sea acts as a heat capacitor, storing heat in certain years and releasing it in others. Results from a high-resolution general circulation model support this hypothesis and the possibility that the South China Sea plays a key role in regulating sea surface temperature patterns in the region.

Title: South China Sea throughflow: A heat and freshwater conveyor

Authors:
Tangdong Qu and Yan Du: International Pacific Research Center, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii, U.S.A.;
Hideharu Sasaki: Japan Agency for Marine Earth Science and Technology (JAMSTEC), Yokohama, Japan.

Source:
Geophysical Research Letters (GRL) paper 10.1029/2006GL028350, 2006

Changes in the hydrologic cycle, including droughts and flooding, are among the most immediate projected consequences of global warming for human society. The relationship between climate change and hydrologic change is, however, poorly understood on a global scale. Previous research has suggested substantial correlations between tropical meteorology and solar variations over the last 8,000 years. Shindell et al. used a coupled ocean-atmosphere model forced by sustained multi-decadal irradiance increases. Their data show that warmer tropical temperatures alter the hydrologic cycle, enhancing precipitation in the tropics, while drying subtropical regions. This response mimics patterns seen in a wide variety of paleoclimate proxy records. The authors note that the shift in climate that they described likely affected past civilizations, including the Maya and ancestral Puebloans, who experienced drought coincident with increased irradiance during late medieval times. Because projections of 21st century climate change, driven by increased greenhouse gas concentrations, yields hydrologic changes through similar processes, the authors expect increased subtropical drought as climate warms.

Title: Solar and anthropogenic forcing of tropical hydrology

Authors:
Drew T. Shindell, Greg Faluvegi, Ron L. Miller, Gavin A. Schmidt, James E. Hansen, and Shan Sun: NASA Goddard Institute for Space Studies and Columbia University, New York, New York, U.S.A.

Source:
Geophysical Research Letters (GRL) paper 10.1029/2006GL027468, 2006

7. Microearthquakes and subglacial conditions

Natural seismic emissions from glaciers have been associated with fracture within rapidly deforming ice, with ice motion over rock beds, and with glacial surges; and analysis of these seismic events can also help determine subglacial conditions. Using new microearthquake data from an array of passive seismic recording stations on Rutford Ice Stream, West Antarctica, A. M. Smith investigated subglacial conditions in an area previously mapped with seismic reflection surveys and thus known to have subglacial sediments whose porosity varied along the surveys. In previous studies Smith hypothesized that where the ice was underlain by high porosity sediment, deformation of the sediment itself was responsible for the motion of the overriding ice, while over low-porosity sediments, the ice moved mainly by sliding. Smith's current analysis shows that areas underlain by low porosity sediments produce six times more seismic events than high porosity areas, confirming that sliding is much more prevalent over low porosity sediments. He suggests that monitoring microearthquakes can help map ice stream basal conditions over wide areas.

Title: Microearthquakes and subglacial conditions

Author:
A. M. Smith: British Antarctic Survey, Natural Environment Research Council, Cambridge, United Kingdom.

Source:
Geophysical Research Letters (GRL) paper 10.1029/2006GL028207, 2006

Contents

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