AGU journal highlights -- 2 July 2013

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

In this release:

Past decade saw unprecedented warming in the deep ocean Hurricanes could increase over western Europe as climate warms Space traffic may be cause of increase in polar mesospheric clouds Tropical storm Sandy was a one-in-700 year event German records from 1920s show long-term ocean warming Identifying slow slip events with GNSS

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1. Past decade saw unprecedented warming in the deep ocean

From 1975 on, the global surface ocean has shown a pronounced-though wavering-warming trend. Starting in 2004, however, that warming seemed to stall. Researchers measuring the Earth's total energy budget-the balance of sunlight streaming in compared to the amount of light and heat leaving from the top of the atmosphere-saw that the planet was still holding on to more heat than it was letting out. But with that energy not going into warming the surface ocean-a traditionally important energy sink-scientists weren't sure where it went. It became known, in some circles, as a case of "missing heat."

Through a reanalysis of global ocean heat content measurements, Balmaseda et al. find the missing heat. The authors show that though the upper ocean waters, from the surface to 700 meters (2,300 feet) depth, showed no warming from 2004 to 2008, the waters from 700 to 2000 meters (2,300 to 6,500 feet) were warming at an unprecedented rate. They find that during the past decade, of the excess energy trapped by the anthropogenic greenhouse effect that has gone into warming the ocean, 30 percent of it has contributed to warming the deep ocean.

The authors also find that throughout the observational record the warming of the surface ocean has stalled before, because of large volcanic eruptions or swings of the El Niño-Southern Oscillation. They also note that changes in surface wind patterns are an important factor in driving ocean heat content from the surface layers to the deep ocean.

Source: Geophysical Research Letters, doi:10.1002/grl.50382, 2013 http://onlinelibrary.wiley.com/doi/10.1002/grl.50382/abstract

Title: Distinctive climate signals in reanalysis of global ocean heat content

Authors: Magdalena A. Balmaseda and Erland Källén: European Centre for Medium Range Weather Forecasts, Reading, United Kingdom;

Kevin E. Trenberth: National Center for Atmospheric Research, Boulder, Colorado, United States.

2. Hurricanes could increase over western Europe as climate warms

Damaging hurricanes are familiar along the North American east coast but are relatively rare in western Europe. That could change as Earth's climate warms over the next century, according to a new study. Western European coastal areas do occasionally experience hurricane force storms in the current climate, but these occur mainly in winter and are formed not as tropical cyclones but by the midlatitude atmospheric baroclinic instability, which is driven by the north-south atmospheric temperature gradient.

Currently, most hurricanes begin in the western tropical Atlantic, where sea surface temperatures often rise above the threshold needed for formation of cyclones; the eastern tropical Atlantic is not currently warm enough to generate cyclones.

However, using a high-resolution global climate model, Haarsma et al. show that as sea surface temperatures in the Atlantic Ocean rise over the next century, the tropical cyclone breeding ground will extend northward and eastward. This will lead to the formation of more hurricanes that are on a path to hit western Europe. Although they will make a transition from a tropical to a hybrid storm, like Sandy, they will arrive there with exceptional strength. The authors' simulations indicate that the number of potentially damaging hurricanes during the August through October season over western Europe could increase more than fourfold by the end of the century.

Source: Geophysical Research Letters, doi:10.1002/grl.50360, 2013 http://onlinelibrary.wiley.com/doi/10.1002/grl.50360/abstract

Title: More hurricanes to hit western Europe due to global warming

Authors: Reindert J. Haarsma, Wilco Hazeleger, Camiel Severijns, Hylke de Vries, Andreas Sterl, Richard Bintanja, Geert Jan van Oldenborgh, and Henk W. van den Brink: Royal Netherlands Meteorological Institute (KNMI), De Bilt, The Netherlands.

3. Space traffic may be cause of increase in polar mesospheric clouds

A recent increase in polar mesospheric clouds could be due to a recent increase in space traffic, a new study suggests. Polar mesospheric clouds are diffuse collections of water ice crystals in the mesosphere near the poles at altitudes of about 80 kilometers (50 miles). The number and brightness of polar mesospheric ice clouds is expected to decrease when the incoming flux of solar ultraviolet radiation increases. Increases in solar radiation both heat and dry out the atmosphere slightly, leading to a decrease in ice cloud formation.

In the past 2 years, the solar ultraviolet flux has increased, but the occurrence of polar mesospheric clouds has actually increased, rather than decreasing as expected. Siskind et al. used observations from NASA's Aeronomy of Ice in the Mesosphere (AIM) satellite to quantify this recent increase in the number and brightness of polar mesospheric clouds. They suggest that water from spacecraft exhaust could contribute to formation of polar mesospheric clouds. They also note that an increase in the amount of space traffic in the past 2 years coincides with the unusual increase in polar mesospheric clouds. Their preliminary estimate of the amount of water released from this space traffic is consistent with the hypothesis that the increase in these clouds is due to space traffic.

Source: Geophysical Research Letters, doi:10.1002/grl.50540, 2013 http://onlinelibrary.wiley.com/doi/10.1002/grl.50540/abstract

Title: Recent observations of high mass density polar mesospheric clouds: A link to space traffic?

Authors: David E. Siskind and Michael H. Stevens: Space Science Division, Naval Research Laboratory, Washington, DC, USA;

Mark E. Hervig: GATS Inc., Driggs, Idaho, USA;

Cora E. Randall: University of Colorado, Boulder, Colorado, USA.

4. Tropical storm Sandy was a one-in-700 year event

On 29 October 2012 tropical storm Sandy slammed into the New Jersey shoreline, bringing wind and water that killed more than 100 people and caused tens of billions of dollars in damage. Though its wind speeds were only equivalent to those of a low-level hurricane, Sandy caused record-breaking flooding in New Jersey, New York, and elsewhere. In lower Manhattan water levels hit 4.28 meters (14.04 feet) above the mean low water level-the highest flood waters in the region since sensors were installed in 1920.

One of the drivers behind Sandy's extreme storm surge was the unusual angle Sandy took as it hit the New Jersey coast. Most tropical cyclones in the North Atlantic sweep up the coast on a northward or northeastward track. Sandy, on the other hand, drove into New Jersey travelling toward the northwest-the only tropical cyclone in the historical record to do so. With this near-perpendicular approach, Sandy's onshore winds had more time to drive a wall of water onto one coastal region, rather than moving along a swath of coastline.

Using information of tropical cyclone tracks for the whole North Atlantic from 1950 to 2010 Hall and Sobel calculate the odds that a similar storm-a category 1 or higher hurricane with an approach angle to New Jersey at least as close to perpendicular as Sandy-could happen again. According to the authors' statistical model, the occurrence rate of a Sandy-style storm is 0.0014 per year, meaning that if future hurricane activity matches the recent past we should expect a storm like Sandy on average about once every 700 years.

The fact that Sandy happened, the authors say, either means that New York and New Jersey were very unlucky, or that climate change has increased the probability of a Sandy-like storm beyond what they find with their steady-climate statistical model.

Source: Geophysical Research Letters, doi:10.1002/grl.50395, 2013 http://onlinelibrary.wiley.com/doi/10.1002/grl.50395/abstract

Title: On the Impact Angle of Hurricane Sandy's New Jersey Landfall

Authors: Timothy M. Hall: NASA Goddard Institute for Space Studies, New York, New York;

Adam H. Sobel: Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York.

5. German records from 1920s show long-term ocean warming

Over past centuries, the crews of ships regularly measured some basic properties of the waters through which they sailed, such as temperature and salinity. These historical observations have proven to be important for climate modelers who are trying to validate their work. In recent years, the importance of the deep ocean as a sink for the extra energy trapped by anthropogenic climate change has come to the fore. Unfortunately, the vast majority of deep ocean observations don't start until the 1980s. From 1925 to 1927, however, the German research vessel Meteor criss-crossed the Atlantic Ocean on an expedition that collected a uniquely thorough record of oceanographic properties for the full depth of the ocean basin. The ship made 13 coast-to-coast sweeps from the Southern Ocean to the tropical North Atlantic, providing a set of observations that were largely unmatched until the 1990s.

Working with this historical data set, Gouretski et al. identified long-term temperature and salinity trends for the entire water column of the Atlantic. They find that during the twentieth century, the upper 2,000 meters (6,500 feet) of water warmed by 0.272 degrees Celsius (0.49 degrees Fahrenheit) and became saltier by 0.030 per mil. Half of the heat content increase took place in the upper 400 meters (1,300 feet) of water, and three quarters took place in the upper 700 meters (2,300 feet). The water below 2,000 meters (6,500 feet), however, became slightly cooler and fresher. The authors calculated that a reduction in density of the ocean waters due to changes in temperature and salinity would have resulted in 3.7 centimeters (1.46 inches) of sea level rise over the 80 years following the Meteor's expedition. The authors note that the calculated warming trend aligns with modeled representations of anthropogenic warming.

Source: Geophysical Research Letters, doi:10.1002/grl.50503, 2013 http://onlinelibrary.wiley.com/doi/10.1002/grl.50503/abstract

Title: Revisiting the Meteor 1925?1927 hydrographic dataset reveals centennial full- depth changes in the Atlantic Ocean

Authors: Viktor Gouretski: KlimaCampus, Hamburg University, Hamburg, Germany;

Johann H. Jungclaus and Helmuth Haak: Max-Planck-Institute for Meteorology, Hamburg, Germany.

6. Identifying slow slip events with GNSS

Slow slip events (SSEs), in which tectonic plate interfaces slip slowly and generate seismic rumbling, have been observed in many subduction zones around the world. These events can provide insight into the accumulation and release of seismic stress, potentially giving scientists information on the processes generating megathrust quakes.

In southwest Japan, megathrust earthquakes tend to occur along the Nankai Trough, where the Philippine Sea plate subducts beneath the Amurian plate. SSEs have previously been observed along the Nankai Trough using seismological and geodetic instruments. Now, Nishimura et al. show that SSEs can also be identified remotely using Global Navigation Satellite System (GNSS) data.

They were able to successfully detect more than 150 short term SSEs with moment magnitudes ranging from 5.5 to 6.3 that occurred along the Nankai Trough between 1996 and 2012. Comparing the SSEs identified with GNSS with those identified from tiltmeter data, they find that both methods may have missed some short term SSEs. They note that GNSS is better for detecting large SSEs, while tiltmeters are better for detecting small ones.

Source: Journal of Geophysical Research-Solid Earth, doi:10.1002/jgrb.50222 http://onlinelibrary.wiley.com/doi/10.1002/jgrb.50222/abstract

Title: Detection of short-term slow slip events along the Nankai Trough, southwest Japan, using GNSS data

Authors: Takuya Nishimura: Geography and Crustal Dynamics Research Center, Geospatial Information Authority of Japan, Tsukuba, Japan, now at Disaster Prevention Research Institute, Kyoto University, Uji, Kyoto, Japan;

Takanori Matsuzawa: National Research Institute for Earth Science and Disaster Prevention, Tsukuba, Japan;

Kazushige Obara: Earthquake Research Institute, University of Tokyo, Tokyo, Japan.

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