AGU journal highlights -- Aug. 13, 2013

(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) and Journal of Geophysical Research-Atmospheres (JGR-D).

In this release:

1. An earthquake in Japan caused large waves in Norwegian fjords
2. Disposal of Marcellus Shale fracking waste caused earthquakes in Ohio
3. The Arctic is especially sensitive to black carbon emissions from within the region
4. A new metric to help understand Amazon rainforest precipitation
5. Detailed analysis shows clouds' effects on daily temperature
6. 2012 Great Plains drought not caused by climate change

Anyone may read the scientific abstract for any already-published paper by clicking on the link provided at the end of each Highlight. You can also read the abstract by going to http://onlinelibrary.wiley.com/ and inserting into the search engine the full doi (digital object identifier), e.g. doi:10.1002/grl.50534. The doi is found at the end of each Highlight below.

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1. An earthquake in Japan caused large waves in Norwegian fjords

Early on a winter morning a few years ago, many residents of western Norway who lived or worked along the shores of the nation's fjords were startled to see the calm morning waters suddenly begin to rise and fall. Starting at around 7:15 local time and continuing for nearly 3 hours, waves up to 1.5 meters (about 5 feet) high coursed through the previously still fjord waters. The scene was captured by security cameras and by people with cell phones, reported to local media, and investigated by a local newspaper. Drawing on this footage, and using a computational model and observations from a nearby seismic station, Bondevik et al. identify the cause of the waves—the powerful magnitude 9.0 Tohoku earthquake that hit off the coast of Japan half an hour earlier.

In closed or semi-enclosed bodies of water, seismic waves can trigger standing waves known as "seiches." Seiching had not been recorded in Norway's fjords since 1950. Scientists have traditionally thought that seiching is caused by seismic surface waves, but the authors find that the fjord seiching was initiated before the surface waves had arrived. Using seismic observations and a model for local fjord behavior, they find that in this case the seiching was triggered by S waves, which travel through Earth's body, and later was amplified by Love waves, which travel on Earth's surface. There are a lot of open questions surrounding the connection between earthquakes and seiching, but the authors' research supports the idea that not all earthquakes will cause seiching in all enclosed bodies of water. The occurrence of the Japanese earthquakeinduced seiches depended on the period and orientation of the seismic waves aligning with the natural frequency and orientation of the body of water.

Source: Geophysical Research Letters, doi:10.1002/grl.50639, 2013 http://onlinelibrary.wiley.com/doi/10.1002/grl.50639/abstract

Title: Norwegian seiches from the giant 2011 Tohoku earthquake

Authors: Stein Bondevik: Sogn og Fjordane University College, Faculty of Science, Sogndal, Norway;

Bjørn Gjevik: Department of Mathematics, University of Oslo, Blindern, Oslo;

Mathilde B. Sørensen: Department of Earth Science, University of Bergen, Allégaten 41, Bergen, Norway.

2. Disposal of Marcellus Shale fracking waste caused earthquakes in Ohio

Before January 2011, Youngstown, Ohio, had never had an earthquake since observations began in 1776. In December 2010, the Northstar 1 injection well came online, built to pump wastewater produced by hydraulic fracturing projects in Pennsylvania into storage deep underground. In the year that followed, seismometers in and around Youngstown recorded 109 earthquakes—the strongest of the set being a magnitude 3.9 earthquake on December 31, 2011.

In a new study analyzing the Youngstown earthquakes, Kim finds that the earthquakes' onset, cessation, and even temporary dips in activity were all tied to the activity at the Northstar 1 well. The first earthquake recorded in the city occurred 13 days after pumping began, and the tremors ceased shortly after the Ohio Department of Natural Resources shut down the well in December 2011. Also, the author finds that dips in earthquake activity correlated with Memorial Day, the Fourth of July, Labor Day, and Thanksgiving, as well as other periods when the injection at the well was temporarily stopped.

Further, the author finds that the earthquakes were centered in an ancient fault near the Northstar 1 well. The author suggests that the increase in pressure from the deep wastewater injection caused the existing fault to slip. Throughout the year, the earthquakes crept from east to west down the length of the fault away from the well—indicative of the earthquakes being caused by a traveling pressure front.

The author notes that of the 177 wastewater disposal wells of this size active in Ohio during 2011, only the Northstar 1 well was associated with such induced seismicity.

Source: Journal of Geophysical Research-Solid Earth, doi:10.1002/jgrb.50247, 2013 http://onlinelibrary.wiley.com/doi/10.1002/jgrb.50247/abstract

Title: Induced seismicity associated with fluid injection into a deep well in Youngstown, Ohio

Authors: Won-Young Kim: Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, USA.

3. The Arctic is especially sensitive to black carbon emissions from within the region

Black carbon, also known as soot, emitted from combustion of fuels and biomass burning, absorbs solar radiation in the atmosphere and is one of the major causes of global warming, after carbon dioxide emissions. When black carbon is deposited on snow and ice, the soot-covered snow or ice absorbs more sunlight, leading to surface warming. Due to the large amount of snow and ice in the Arctic—which has warmed twice as fast as the global average over the past century—the region is likely to be especially sensitive to black carbon.

To investigate how sensitive the Arctic is to black carbon emissions from within the Arctic compared to those transported from mid-latitudes, Sand et al. conducted experiments using a climate model that includes simulation of the effects of black carbon deposited on snow.

They find that most of the Arctic warming effect from black carbon is due to black carbon deposited on snow and ice, rather than in the atmosphere. Black carbon emitted within the Arctic is more likely to stay at low altitudes and thus to be deposited on the snow and ice there, whereas black carbon transported into the Arctic from mid-latitudes is more likely to remain at higher altitudes. Because of this, the Arctic surface temperature is almost 5 times more sensitive to black carbon emitted from within the Arctic than to emissions from mid-latitudes, the authors find.

They note that although there are currently few sources of black carbon emissions within the Arctic (the most dominant ones are oil and gas fields in northwestern Russia), that is likely to change as human activity in the region increases. Therefore, the authors believe there is a need to improve technologies for controlling black carbon emissions in the Arctic.

Source: Journal of Geophysical Research-Atmospheres, doi:10.1002/jgrd.50613, 2013 http://onlinelibrary.wiley.com/doi/10.1002/jgrd.50613/abstract

Title: Arctic surface temperature change to emissions of black carbon within Arctic or mid-latitudes

Authors: Maria Sand, Terje Koren Berntsen and Jón Egill Kristjánsson: Department of Geosciences, University of Oslo, Oslo, Norway;

Øyvind Seland: Norwegian Meteorological Institute, Oslo, Norway.

4. A new metric to help understand Amazon rainforest precipitation

In the Amazon rainforest, the chain of events that turns a small-scale process like a localized increase in evaporation into a towering storm cloud is long and twisted. To understand the complex dynamics that lead to precipitation, and to identify the relative importance of various processes, researchers need high temporal resolution, all-weather observations over many years. Such observations have traditionally been scarce for tropical continental environments, such as the Amazon, where logistics are difficult.

In recent years, however, Global Navigational Satellite System (GNSS) stations have provided a way to gather these measurements of atmospheric water vapor. In their study, Adams et al. use 3.5 years of observations from a GNSS meteorological station in Manaus, Brazil, to analyze the processes that turn localized dynamics into deep convective rainfall.

To identify which physical processes are most important in contributing to cloud formation, growth, and precipitation, the authors developed a new metric called the "water vapor convergence time scale." Moist air is more buoyant than dry, so understanding water vapor convergence is important to understanding the development of deep convective cloud formation. Using their metric derived from GNSS water vapor observations, the authors identify two main time scales relevant to Amazon convective storm formation.

Starting about 12 hours before precipitation onset, the authors find that localized evaporation is the most likely dominant factor in moistening the atmosphere. Then, about 4 hours before the onset of deep convective precipitation, water vapor convergence becomes dominant. This 4-hour period of strong water vapor convergence before heavy rainfall encompasses the transition from shallow to deep convection. This transition is a process during which small, scattered cumulus clouds grow into deep convective towers. The authors find that this 4-hour shallow-to-deep convection transition time scale is not dependent on the season, the intensity of the convective precipitation, or the time of day.

Source: Geophysical Research Letters, doi:10.1002/grl.50573, 2013 http://onlinelibrary.wiley.com/doi/10.1002/grl.50573/abstract

Title: GNSS Observations of Deep Convective Time Scales in the Amazon

Authors: D. K. Adams: Centro de Ciencias de la Atmósfera, Universidad Nacional Autónoma de México, D.F., México and Programa em Clima e Ambiente, Universidade do Estado do Amazonas, Manaus, Amazonas, Brazil;

Seth I. Gutman and Kirk L. Holub: Earth System Research Laboratory, National Atmospheric and Oceanic Administration, Boulder, Colorado, USA;

Dulcineide S. Pereira: Instituto Federal de Educação, Ciência e Tecnologia, Manaus, Amazonas, Brazil.

5. Detailed analysis shows clouds' effects on daily temperature

Clouds reflect some incoming sunlight, tending to cool Earth's surface, but they also trap some heat leaving Earth's surface, causing warming. These effects, known as cloud radiative forcing, play a key role in temperature variations on Earth's surface and thus are important for climate modeling.

However, the precise effects of cloud cover on the diurnal temperature cycle have not been well documented. Betts et al. provide a detailed analysis of a 40-year data set of hourly observations from 14 climate stations across the Canadian Prairies to determine how clouds affect daily maximum and minimum temperatures and the daily ranges of temperature and humidity.

They find that from April through October, maximum temperatures and diurnal ranges of temperature and relative humidity increase with decreasing cloud cover, while minimum temperature is almost independent of cloud cover. However, during winter months, both maximum and minimum temperature fall with decreasing cloud cover. The study could help improve modeling of the effects of cloud radiative forcing on Earth's surface temperature.

Source: Journal of Geophysical Research-Atmospheres, doi:10.1002/jgrd.50593, 2013 http://onlinelibrary.wiley.com/doi/10.1002/jgrd.50593/abstract

Title: Cloud radiative forcing of the diurnal cycle climate of the Canadian Prairies

Authors: Alan K. Betts: Atmospheric Research, Pittsford;

Raymond Desjardins and Devon Worth: Agriculture & Agri-Food Canada, Ottawa.

6. 2012 Great Plains drought not caused by climate change

From May to July 2012, the Great Plains region of the western United States faced a powerful and unpredicted drought. Following 7 months of normal rainfall, the drought was one of the largest deviations from seasonal precipitation rates seen in the region since observations began in 1895. When such extreme events take place today against the backdrop of ongoing global climate change, they raise questions about the relationship between climate change and natural disasters.

In a new modeling study, Kumar et al. use an ensemble of runs from an operational climate model, initialized with the observed conditions leading up to the 2012 Great Plains drought, to simulate the range of conditions that could have played out during the subsequent months. They find that the drought fell within the bounds of natural atmospheric variability. The strength of the drought, they suggest, was a consequence of the multiple complex nonlinear systems that make up the climate system and did not critically depend on the existence of a strong external forcing.

The authors note that their findings do not detract from the idea that climate change could enhance some extreme events. Rather, their research says that climate change was not a first-order forcing of the drought. They say that climate change and other pressures could still serve as "proximate causes," setting the stage for or enhancing, but not necessarily causing, extreme events. That the drought was driven by natural variability not requiring a steady background forcing, they say, will limit the predictability of similar future extreme events.

Source: Geophysical Research Letters, doi:10.1002/grl.50657, 2013 http://onlinelibrary.wiley.com/doi/10.1002/grl.50657/abstract

Title: Do Extreme Climate Events Require Extreme Forcings?

Authors: Arun Kumar and Mingyue Chen: NOAA Climate Prediction Center, College Park, Maryland, USA;

Martin Hoerling and Jon Eischeid: NOAA Earth System Research Laboratory, Boulder, Colorado, USA.

### Contact: Thomas Sumner
Phone (direct): +1 202 777 7516
E-mail: tsumner@agu.org

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