AGU journal highlights -- 1 October 2012

(Press-News.org) The following highlights summarize research papers that have been recently published in Geophysical Research Letters (GRL), Journal of Geophysical Research - Solid Earth (JGR-B), Journal of Geophysical Research - Oceans (JGR-C), and Water Resources Research (WRR).

In this release:

1. The pros and cons of trading water: A case study in Australia 2. Linking typhoon tracks with rainfall patterns and flood timing 3. Novel observations of currents and drag generated by a tsunami 4. More water stored along major rivers during El Nino years 5. Model suggests Earth is undergoing true polar wander 6. High-speed imagery captures new sea spray formation mechanism

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1. The pros and cons of trading water: A case study in Australia

Water is a commodity, and water rights can be freely traded in an open market. Proponents of the free market approach argue that it leads to the most efficient allocation of water resources, as it would for any other commodity. However, unlike some commodities, water is critical for human life, for many human activities, and for environmental resources. When such an essential commodity becomes scarce, as frequently happens in Australia, a land prone to sudden and dramatic droughts, severe problems can occur quickly. In Australia's Murray Darling Basin, the country's largest agricultural region, the government had historically controlled the distribution of water rights. However, under these controls, a selected few controlled a large share of the water. To resolve this problem of overallocation, a free market approach was put in place in the early 1990s.

Crase et al. summarize the advantages and possible pitfalls of the free market approach in the Murray Darling Basin. They suggest that making water rights available in an open market generally had positive outcomes for the region; the approach released the state controls, which allocated water inefficiently, and created a situation in which supply and demand dictate price, and farmers seem to respond efficiently.

However, the authors note that the free market approach could lead to speculation, in which some people who have little practical use for water rights hoard them to drive up the price, leaving less water available for others who might need it. In conclusion, the authors advocate the free market-based approach but caution that such a system also has the potential to create economic, social, environmental, and ecological problems.

Source: Water Resources Research, doi:10.1029/2012WR012140, 2012 http://dx.doi.org/10.1029/2012WR012140

Title: Enhancing agrienvironmental outcomes: Market-based approaches to water in Australia's Murray-Darling Basin

Authors: Lin Crase and Sue O'Keefe: Centre for Water Policy and Management, La Trobe University, Victoria, Australia; Yukio Kinoshita: Faculty of Agriculture, Iwate University, Morioka, Japan.

2. Linking typhoon tracks with rainfall patterns and flood timing

Being able to predict the timing and amount of flooding during and following a hurricane or typhoon would improve early warning and mitigation efforts. However, variable typhoon tracks and interaction between typhoons and varied landscapes make flood prediction challenging.

Huang et al. investigated the detailed distribution of typhoon-induced rainfall over a mesoscale mountainous watershed located in eastern Taiwan, a region that is regularly affected by severe typhoons-losses associated with typhoons in Taiwan can reach $500 million per year. The researchers analyzed high-resolution radar observations of 38 rainfall events during 2000-2010 in eastern Taiwan to study the relationship between typhoon track, rainfall patterns, and the timing of peak flooding.

On the basis of the statistical characteristics of these events, they identify three different types of rainfall patterns. They find that the different types of rainfall patterns are correlated with different typhoon tracks and linked flood peak times to the rainfall types and typhoon tracks. For instance, the peak flood time for one of their identified rainfall patterns - a pattern with an approximately north- northeast-south-southwest oriented rainfall belt across the downstream area of the watershed they studied - was 3 hours earlier than peak flood time for other rainfall patterns.

They suggest that the relationships are due to the typhoons' interaction with the mountainous landscape in the region. Their study could lead to improved real- time flood warning systems in Taiwan and other typhoon-prone regions.

Source: Water Resources Research, doi:10.1029/2011WR011508, 2012 http://dx.doi.org/10.1029/2011WR011508

Title: Linking typhoon tracks and spatial rainfall patterns for improving flood lead time predictions over a mesoscale mountainous watershed

Authors: Jr-Chuan Huang, Jun-Yi Lee, and Tsung-Yu Lee: Department of Geography, National Taiwan University, Taipei, Taiwan;

Cheng-Ku Yu and Lin-Wen Cheng: Department of Atmospheric Sciences, Chinese Culture University, Taipei, Taiwan;

Shuh-Ji Kao: Research Center for Environmental Changes, Academia Sinica, Taipei, Taiwan, and State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, China.

3. Novel observations of currents and drag generated by a tsunami

Tsunamis cause damage even after they have traveled thousands of kilometers from their sources, and much of the damage is through generation of local strong currents. Even though wave heights of tsunamis that have traveled long distances are no greater than those of local tides or waves, tsunamis modify currents, resulting in unusually strong pulses of mixing, transport, and seiching (standing waves in enclosed water bodies). Seiching is common and is the most destructive hazard, particularly along narrow bays and harbors.

In a new description of currents and sediment transport associated with a tsunami, Lacy et al. studied water height, currents, and suspended sediment concentrations in the Monterey Bay, California, where a tsunami set off by a magnitude 8.8 earthquake off the coast of Chile on 27 February 2010 arrived approximately 14 hours later, causing strong seiching. Studying vertical velocity profiles of the tsunami waves reaching the inner shelf at Monterey Bay, the authors show that the friction from the shelf bed vastly affected the tsunami currents, and the bed drag coefficient varied with time. The observation is contrary to assumptions behind the common practice of using depth-averaged models to simulate tsunamis in the near-shore region.

The authors find that in the 10-meter (32.8-foot) deep study sites in the Monterey Bay area, the largest tsunami wave vastly increased sediment transport toward the shore. On the other hand, the strong currents that lasted for several days following the seiche enhanced sediment transport along the shore. The tsunami generated shear stress strong enough to entrain the sandy seafloor of the study site. Such in- depth understanding of the vertical structure of tsunami currents, and their interaction with the seafloor, is critical for improving the prediction of the passage of tsunamis across the inner shelf.

Source: Journal of Geophysical Research-Oceans, doi:10.1029/2012JC007954, 2012 http://dx.doi.org/10.1029/2012JC007954

Title: Currents, drag, and sediment transport induced by a tsunami thawing

Authors: Jessica R. Lacy and David M. Rubin: U.S. Geological Survey, Pacific Science Center, Santa Cruz, California, USA;

Daniel Buscombe: School of Marine Science and Engineering, University of Plymouth, Plymouth, UK.

4. More water stored along major rivers during El Nino years

El Nino-Southern Oscillation (ENSO) - the semiperiodic climate event associated with warming sea surface temperatures off the coast of Peru - not only disrupts atmospheric circulations, dramatically altering weather patterns across the globe, but also may be determining the amount of fresh water stored on continents in tropical rainforests, according to a new study.

Phillips et al. used more than seven years of satellite-based observations of fresh water content of continents from the Gravity Recovery and Climate Experiment (GRACE) to show that between 2003 and 2010 the amount of fresh water stored on land (in terms of rain water, river water, lake, ground water, and snow) between the 15-degree north and 15-degree south latitudes depends on El Nino- Southern Oscillation. The authors also find that the effect of ENSO is strongest on land areas adjacent to the Pacific Ocean. The maximum increase in the amount of stored fresh water on land was observed during El Nino years along major river valleys in the Borneo region of southeast Asia, the Amazon, the Congo, and the Yangzte and Met Cong in China.

The authors suggest that these findings are not surprising given that meteorological conditions within 15 degrees north and 15 degrees south are sensitive to the location of the intertropical convergence zone (ITCZ) - a zone of low pressure that brings heavy convective rainfall along the tropics; its location is sensitive to the same ocean temperatures and atmospheric circulation patterns that ENSO disrupts. The authors feel confident that as more data become available, it would be possible to predict water storage in other regions of the world, particularly in the Arctic region, where their preliminary study suggests that ice loss could be associated with El Nino events.

Source: Geophysical Research Letters, doi:10.1029/2012GL052495, 2012 http://dx.doi.org/10.1029/2012GL052495

Title: The influence of ENSO on global terrestrial water storage using GRACE

Authors: T. Phillips: Department of Aerospace Engineering, University of Colorado at Boulder, Boulder, Colorado, USA, Colorado Center for Astrodynamics Research, University of Colorado at Boulder, Boulder, Colorado, USA, and Earth System Observation Center, University of Colorado at Boulder, Boulder, Colorado, USA;

R. S. Nerem: Department of Aerospace Engineering, University of Colorado at Boulder, Boulder, Colorado, USA, Colorado Center for Astrodynamics Research, University of Colorado at Boulder, Boulder, Colorado, USA, and Cooperative Institute for Research in Environmental Science, University of Colorado at Boulder, Boulder, Colorado, USA;

Baylor Fox-Kemper: Cooperative Institute for Research in Environmental Science, University of Colorado at Boulder, Boulder, Colorado, USA, and Department of Atmospheric and Ocean Sciences, University of Colorado at Boulder, Boulder, Colorado, USA;

J. S. Famiglietti: Center for Hydrologic Modeling, University of California, Irvine, California, USA, and Department of Earth System Science, University of California, Irvine, California, USA;

B. Rajagopalan: Cooperative Institute for Research in Environmental Science, University of Colorado at Boulder, Boulder, Colorado, USA, and Department of Civil, Environmental and Architectural Engineering, University of Colorado at Boulder, Boulder, Colorado, USA.

5. Model suggests Earth is undergoing true polar wander

At various points throughout Earth's history, the planet's solid exterior has drifted about in relation to the planetary rotation axis. This solid body drift, which is known as "true polar wander," results in a wholesale shift in the orientation of Earth's landmasses and is different from the motion of individual tectonic plates ("tectonic drift") or of the magnetic pole ("apparent polar wander").

Sorting out when, in which direction, and at what rate the Earth's solid exterior has rotated in this way depends on having a stable frame of reference to which observations of relative motion can be compared. To develop such a frame, researchers rely on hot spots, regions of recurrent volcanism that are known to produce long, largely linear island chains - such as the Hawaiian islands - as an overlying tectonic plate passes overhead. Hot spots are fed by magma plumes from the deep mantle and hence tend to be long-lived and relatively stable. Hot spots have long been used to understand the motion of tectonic plates.

Traditionally, scientists have treated hot spots as completely static features. But by allowing hot spots' positions to slowly drift, Doubrovine et al. produced a model of a stable reference frame that better matched observations of hot spot tracks - the path drawn by each hot spot's island chain. Based on their new reference frame, which they consider accurate for the past 120 million years, the authors identify four possible instances of true polar wander, including two in which the solid Earth traveled back and forth by nearly 9 degrees from 90 to 40 million years ago. Further, they suggest that for the past 40 million years the Earth's solid outer layers have been slowly rotating at a rate of 0.2 degrees every million years.

Source: Journal of Geophysical Research - Solid Earth, doi:10.1029/2011JB009072, 2012 http://dx.doi.org/10.1029/2011JB009072

Title: Absolute plate motions in a reference frame defined by moving hot spots in the Pacific, Atlantic and Indian oceans

Authors: Pavel V. Doubrovine: Physics of Geological Processes, University of Oslo, Norway and Center for Advanced Study, Norwegian Academy of Science and Letters, Oslo, Norway;

Bernhard Steinberger: Helmholtz Center Potsdam, GFZ German Research Center for Geosciences, Potsdam, Germany, Physics of Geological Processes, University of Oslo, Norway, and Center for Advanced Study, Norwegian Academy of Science and Letters, Oslo, Norway;

Trond H. Torsvik: Physics of Geological Processes, University of Oslo, Norway, Center for Advanced Study, Norwegian Academy of Science and Letters, Oslo, Norway, Center for Geodynamics, Geological Survey of Norway, and School of Geosciences, University of Witwatersrand, South Africa.

6. High-speed imagery captures new sea spray formation mechanism

When strong winds blow over ocean waves, small droplets of sea spray rise into the air, enhancing the exchange of heat, mass, and energy between the air and the sea. How effective sea spray is at mediating each of these dynamics depends on the rate at which droplets are created and the drop size distribution of the mist. Unfortunately, research has been limited by a dearth of observational evidence that could explain the details of sea spray generation, including understanding the drop size distribution or the effects of different wind speeds. Previous research with high-speed cameras aiming to capture the moment of drop formation was limited by camera resolutions too low to see all but the largest drops.

Updating this previous line of work with improved technology, Veron et al. measured the formation of freshwater sea spray in a wave tank that also had controllable wind. The authors tested three different wind speeds: 31.3, 41.2, and 47.1 meters per second (70, 92.2, and 105.3 miles per hour), which equates to tropical storm strength winds up to a category 1 hurricane on the Saffir-Simpson scale. They used two different cameras: a high-speed camera that captured 1,000 frames per second and allowed them to observe spray-forming dynamics, and a high-resolution camera that let them observe droplets as small as 140 micrometers (0.006 inches). The authors find a higher abundance of large drops, those above a millimeter (0.04 inches) in diameter, than is expected by theoretical work. Further, they also identify a novel spray formation mechanism whereby strong winds cause a thin sheet of water from the crest of a wave to fill up like a balloon and burst in a spray of droplets, a mechanism which they suggest may be applicable only for hurricane force winds.

Source: Geophysical Research Letters, doi:10.1029/2012GL052603, 2012 http://dx.doi.org/10.1029/2012GL052603

Title: Sea spray spume droplet production in high wind speeds

Authors: F. Veron, E. L. Harrison, and J. A. Mueller: School of Marine Science and Policy, University of Delaware, Newark, Delaware, USA;

C. Hopkins: School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia, USA.

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