AGU journal highlights - 31 March 2006

I. Highlights, including authors and their institutions

The following highlights summarize research papers in Geophysical Research Letters (GRL) and Paleoceanography (PA). 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. More intense solar activity expected

Sunspot activity, which cycles about every eleven years, correlates with the intensity of solar radiation: a large number of sunspots indicates a brighter sun. Past research has used data from the previous solar cycle to predict properties of the upcoming one. Dikpati et al. refined this idea through a dynamo-based method, in which polar fields of the previous three cycles combine and are sheared by solar differential rotation to produce fields of the new cycle. The authors show that their new approach correctly and independently described the relative peaks of the last eight solar cycles. They expect that the next solar cycle will have a 30–50 percent higher peak than the current cycle, in contrast to other recent predictions that used a precursor method to predict solar activity. With this forecasting technique, the authors say that solar flares and solar storms potentially will be more accurately anticipated, helping to mitigate adverse effects to power grids, satellite systems, and future space missions.

Title: Predicting the strength of solar cycle 24 using a flux-transport dynamo-based tool

Authors: Mausumi Dikpati, Giuliana de Toma, and Peter A. Gilman: High Altitude Observatory, National Center for Atmospheric Research, Boulder, Colorado, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL025221, 2006

2. Increasing greenhouse gases will strengthen the Antarctic Circumpolar Current

The Antarctic Circumpolar Current (ACC) connects all major ocean basins and exchanges heat, salt, and other tracers among them. Through an ensemble of 12 global climate models, Fyfe and Saenko showed that the ACC will likely be sensitive to climate changes induced by increased greenhouse gas concentrations. These models show a consistent strengthening and poleward shifting of zonal wind stress through the 20th and 21st centuries at sub-Antarctic latitudes, which in the models translated into strengthening and poleward shifting of the ACC. The strengthening of zonal wind stress in the models also strengthened the Ekman transport, the process by which windblown surface water drags the subsurface layers along its path. In the southern hemisphere, Ekman transport associated with zonal wind stress is deflected northward across the ACC by the Coriolis effect [the apparent deflection of moving objects, caused by Earth's rotation]; the authors note that strengthening this northward transport must be balanced by increasing southward transport in the deep ocean. The authors further suggest that the zonal wind stress changes seen in the simulations might affect the oceanic uptake of manmade carbon dioxide.

Title: Simulated changes in the extratropical Southern Hemisphere winds and currents

Authors: John C. Fyfe and Oleg A. Saenko: Canadian Centre for Climate Modelling and Analysis, Meteorological Service of Canada, Victoria, British Columbia, Canada.

Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL025332, 2006

3. Deep atmospheric convection enhances natural greenhouse gas feedbacks

Deep atmospheric convection occurs when air moves vertically through the atmosphere on timescales quicker than atmospheric circulation. Past studies have recognized that deep convection in the tropics is associated with cloud formations in the upper troposphere and is closely related to the variability of the upper tropospheric humidity. This variation in water vapor content is important because, as one of the dominant greenhouse gases in Earth's atmosphere, water vapor can have strong feedbacks on climate change. To study this, Su et al. collected simultaneous observations of upper tropospheric water vapor and cloud ice from the Microwave Limb Sounder on the Aura satellite. Their observations show that upper tropospheric water vapor increases as cloud ice water content increases. Additionally, when sea surface temperature exceeds around 300 Kelvin [30 degrees Celsius; 80 degrees Fahrenheit], upper tropospheric cloud ice associated with tropical deep convection increases sharply with increasing sea surface temperature. This moistening leads to an enhanced positive water vapor feedback, about three times larger than that in the absence of convection.

Title: Enhanced positive water vapor feedback associated with tropical deep convection: New evidence from Aura MLS

Authors: Hui Su: Skillstorm Government Services, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA; William G. Read, Jonathan H. Jiang, Joe W. Waters, Dong L. Wu, and Eric J. Fetzer: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL025505, 2006

4. Submarine Groundwater Discharge in Japan

Submarine groundwater discharge (SGD), which regionally affects coastal circulation and the transport of dissolved material (such as nutrients) from land to the ocean, can be divided into two categories: fresh SGD and recirculated saline SGD. One way to study submarine groundwater discharge uses a simple water balance equation that gives precipitation as a function of evaporation, surface runoff, groundwater discharge, and other variables. Taniguchi et al. used this method on two similarly sized basins near Kumamoto, Japan, that experience matching meteorological patterns but are different geologically. They then compared these values to that directly observed, using seepage meters installed throughout the basin. The authors found that the observed fresh SGD in both areas agrees relatively well with that expected by the water balance equation, and that fresh SGD decreased with distance from the coast. Recirculated saline SGD peaked both near the shore, due to waves, and offshore, due to tidal pumping. Disparities in fresh SGD values between the basins were likely a result of the aquifer's permeability and other hydrological and geological differences of the basins themselves.

Title: Evaluations of spatial distribution of submarine groundwater discharge

Authors: Makoto Taniguchi and Tomotoshi Ishitobi: Research Institute for Humanity and Nature, Kyoto, Japan; Jun Shimada and Naohiko Takamoto: Department of Earth Sciences, Kumamoto University, Kumamoto, Japan.

Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL025288, 2006

5. Infragravity energy is transferred to swell in the surfzone

Infragravity waves, which have periods between 200 and 20 seconds and form within the ocean as surface wind waves propagate into shallow water, are found through the deep and coastal oceans and are strongest near the shoreline, where they force circulation and aid in transporting sediment. Thomson et al. studied infragravity waves on the southern California shelf and found that the observed strong energy loss in the surfzone was the result of nonlinear energy transfers to higher frequency swell. The observations and model simulations showed that this surfzone energy loss is stronger as waves propagate during low tide, because the beach slope allows for more energy exchange than at high tide, resulting in a tidal modulation of seaward-radiated infragravity energy. According to the researchers' results, these energy transfers are similar over several kilometers [miles] of the California coast. This suggests that the tidal modulation of infragravity energy levels observed in deep ocean bottom pressure seismic records, used for tsunami detection, may be the result of nonlinear wave processes on the world's shorelines.

Title: Tidal modulation of infragravity waves via nonlinear energy losses in the surfzone.

Authors: Jim Thomson, Steve Elgar, and Britt Raubenheimer: Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA; T. H. C. Herbers: Naval Postgraduate School, Monterey, California, USA; R. T. Guza: Scripps Institution of Oceanography, La Jolla, California, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/2005GL0225514, 2006

6. Sea ice during the Last Glacial Maximum weakened deep water formation in the North Atlantic

The Last Glacial Maximum (about 14-22 thousand years ago) was characterized by significantly lower temperatures than today, with large continental areas covered by great ice sheets. With proxy data gathered from foraminifer [marine protozoan fossil] assemblages and ice core records, Yang et al. used a regional eddy permitting ocean model to cycle back what oceanic circulation would have been like during the Last Glacial Maximum, focusing on formation of the North Atlantic Deep Water and the pathway of the North Atlantic Current, the latter of which currently brings warm equatorial waters from the Gulf Stream to areas above the Artic Circle. Though early results from their models suggested that Last Glacial Maximum conditions strengthened the North Atlantic Deep Water and the North Atlantic Current due to stronger air-sea fluxes thought to be present during this time, sediments from ocean cores indicate that the opposite occurred. The authors then refined their model to include greater sea ice extents, which isolates the underlying ocean from wintertime atmospheric fluxes, curtailing North Atlantic Deep Water formation and weakening the North Atlantic Current.

Title: Sensitivity of the sub-polar north Atlantic to LGM surface forcing and sea-ice distribution in an eddy-permitting regional ocean model

Authors: Duo Yang, Paul G. Myers, and Andrew B. G. Bush: Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada.

Source: Paleoceanography (PA) paper 10.1029/2005PA001209, 2006

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).
2. Paste it into the second-from-left search box at http://www.agu.org/pubs/search_options.shtml and click "Go."
3. This will take you to the citation for the article, with a link marked "Abstract + Article."
4. Clicking on that link will take you to the paper's abstract, with a link to purchase the full text: "Print Version (Nonsubscribers may purchase for $9.00)."

The Highlights and the papers to which they refer are not under AGU embargo.

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