Public Release: 

AGU journal highlights -- March 17, 2011

American Geophysical Union

The following highlights summarize research papers that have been recently published in Geophysical Research Letters (GRL), Journal of Geophysical Research-Atmospheres (JGR-D), Journal of Geophysical Research-Oceans (JGR-C) and Paleoceanography.

1. Huge ocean "Frisbees™" spin off Brazil's coast

As the North Brazil Current (NBC) moves northward along the northeastern coast of Brazil, it draws water from the South Equatorial Current and the freshwater outflow of freshwater from the Amazon River, providing warm, nutrient-rich water to areas north of the equator. Just northwest of Brazil, part of the NBC makes a hard right and flows east along the equator. But once in a while, the turn is especially sharp and the current loops around, pinching off an independently- traveling parcel of the warm water. This portion travels northwest with a clockwise rotation, moving through the ocean like a Frisbee™ travels through air.

These current rings have been known to exist for decades, but knowledge of their basic properties such as size, speed, depth, and rotation velocity is limited. Drawing on current profiles from both shipboard and stationary instruments, researchers Castelão and Johns describe the basic properties of 10 rings sampled between 1998 and 2000. The authors find that the rings are best described as solid, clockwise-rotating parcels of water enclosed within a band of lower-speed water that tends to shield them from the surrounding environment.

For many of the rings the sea surface height increases parabolically toward the center, reaching up to 38 centimeters (about 15 inches) above the surrounding ocean. The inner core can be more than 300 kilometers (205 miles) across and can have a maximum rotation speed over 1 meter/sec (3.3 feet/sec). Overall, the NBC rings seem to be bigger, faster, and taller than previous observations suggested.

Source:
Journal of Geophysical Research-Oceans, doi:10.1029/2010JC006575, 2011
http://dx.doi.org/10.1029/2010JC006575

Title: Sea surface structure of North Brazil Current rings derived from shipboard and moored acoustic Doppler current profiler observations

Authors: G. P. Castelão and W. E. Johns: Division of Meteorology and Physical Oceanography, Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, Florida, USA.


2. Martian weather report: ice and fog near surface

A pair of cameras mounted on the back of the Phoenix Mars Lander captured how laser light, emitted by the Lander's light detection and ranging (lidar) system, was scattered by water ice in the red planet's thin atmosphere. Moores et al. used the technique over four nights in 2008 to give the first detailed profile of the ice water content in the Martian near-surface atmosphere.

The authors find that the icy fog was thickest around 50 meters (164 feet) above the surface, with maximum concentration of 1.7 milligrams per cubic meter (0.000002 ounces per cubic foot). They also find that the fog was not uniform but tended to decrease in thickness toward the surface. As the Martian night wears on, the surface of the planet cools below the frost point and water vapor in the atmosphere gets deposited on the ground. As the atmosphere is mixed by turbulence, more water is brought to lower altitudes, adding to the growing frost layer.

Ice crystals also form in the air and precipitate to the ground from successively higher altitudes. The researchers estimate that by the time the Sun started to rise in the morning, 2.5 micrograms (0.000000088 ounces) of snow and frost would have coated the surface of Mars in the northerly region around the Phoenix Lander.

Source:
Geophysical Research Letters, doi:10.1029/2010GL046315, 2011
http://dx.doi.org/10.1029/2010GL046315

Title: Observations of near-surface fog at the Phoenix Mars landing site

Authors: John E. Moores, Léonce Komguem and James A. Whiteway: Centre for Research in Earth and Space Science, York University, Toronto, Ontario, Canada;

Mark T. Lemmon: Atmospheric Sciences Department Texas A&M University, College Station, Texas, USA;

Cameron Dickinson: MacDonald Dettwiler and Associates Space Missions, Brampton, Ontario, Canada;

Frank Daerden: Division of Planetary Aeronomy, Belgian Institute for Space Aeronomy, Brussels, Belgium.


3. Auroral radio wave emissions reach ground level

Electrons that contribute to Earth's colorful auroras radiate some of their energy in radio waves. Theories of the mechanism for emission of this radiation, known as auroral kilometric radiation (AKR), suggest that this radiation propagates away from Earth and cannot be detected at ground level. However, LaBelle and Anderson present the first evidence that AKR does penetrate to ground level. On three occasions in July 2004, they detected radio emissions that appeared to be AKR at the South Pole Station in Antarctica. They compare these emissions with AKR emissions detected simultaneously by the Geotail satellite and find that they have the same frequency structure, suggesting that the emissions detected at ground level were indeed AKR.

Source:
Geophysical Research Letters, doi:10.1029/2010GL046411, 2011
http://dx.doi.org/10.1029/2010GL046411

Title: Ground-level detection of auroral kilometric radiation

Authors: James LaBelle: Department of Physics and Astronomy, Dartmouth College, Hanover, New Hampshire, USA;

Roger R. Anderson: Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa, USA.


4. Stratospheric water vapor increase at Colorado site

Water vapor in the atmosphere is responsible for a significant portion of the greenhouse effect, and even small changes in the upper troposphere or lower stratosphere can have a large effect on climate. A new analysis of balloon-borne water vapor measurements using frost point hygrometers over Boulder, Colorado, shows that stratospheric water vapor has increased over the past 30 years. Hurst et al. break the long measurement record into four discrete time periods and determined the water vapor trends in each period for five 2-kilometer-thick stratospheric layers 16 km to 26 km above the ground.

They find that, on average, stratospheric water vapor increased by about 1 part per million by volume (27 percent) over the past 30 years, though there were many shorter-term variations in the record. Water vapor levels increased during 1980 to 1989 and 1990 to 2000, decreased from 2001 to 2005, and then increased again after 2005. The authors find that, at most, 30 percent of the observed water vapor increases can be attributed to greater amounts of methane oxidation in the stratosphere. The 2001 to 2005 decrease in midlatitude water vapor has been linked to observations of anomalously low tropopause temperatures in the tropics, but, to date, no connection between the observed water vapor increases and tropical tropopause temperatures has been found despite ongoing efforts.

Source:
Journal of Geophysical Research-Atmospheres, doi:10.1029/2010JD015065, 2011
http://dx.doi.org/10.1029/2010JD015065

Title: Stratospheric water vapor trends over Boulder, Colorado: Analysis of the 30 year Boulder record

Authors: Dale F. Hurst: Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA, Global Monitoring Division, NOAA Earth System Research Laboratory, Boulder, Colorado, USA;

Samuel J. Oltmans: Global Monitoring Division, NOAA Earth System Research Laboratory, Boulder, Colorado, USA;

Holger Vömel Meteorologisches Observatorium Lindenberg, Deutscher Wetterdienst, Lindenberg, Germany;

Karen H. Rosenlof: Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, Colorado, USA;

Sean M. Davis and Eric A. Ray: Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA, Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, Colorado, USA;

Emrys G. Hall and Allen F. Jordan: Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA, Global Monitoring Division, NOAA Earth System Research Laboratory, Boulder, Colorado, USA.


5. What can opals tell us about past climate?

New insight into the mechanisms that caused Earth's glacial periods to abruptly end could come from opal accumulations in sediment cores. Previous studies have shown that the most recent glacial period ended when the southern hemisphere's westerly winds intensified and shifted southward. This change led to increased upwelling that stirred up carbon dioxide in the Southern Ocean, leading to a rise in atmospheric carbon dioxide and warming through the greenhouse effect. The upwelled water from the Southern Ocean also would have traveled to the equatorial zones, where the added nutrient-rich water would have enabled increased production of opal shells by diatoms. These opal shells would then accumulate in the sediment.

To search for evidence that a change in Southern Hemisphere westerly winds also occurred during several earlier glacial terminations, Hayes et al. measure opal accumulations in sediment cores from the eastern and central equatorial Pacific Ocean. The core from the eastern equatorial Pacific covered the past three glacial terminations; the central Pacific core covered the past five deglaciations.

The results are mixed. The researchers find evidence of increased opal accumulation during some, but not all, of the glacial terminations. There is also evidence of increased opal accumulations at times not associated with glacial terminations. The researchers suggest that a combination of opal fluxes and other measurements could provide a better signature of the mechanism of deglaciation. They also suggest that it is possible that different glacial terminations occurred through different processes.

Source:
Paleoceanography, doi:10.1029/2010PA002008, 2011
http://dx.doi.org/10.1029/2010PA002008

Title: Opal accumulation rates in the equatorial Pacific and mechanisms of deglaciation

Authors: C. T. Hayes and R. F. Anderson: Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York, USA, Department of Earth and Environmental Sciences, Columbia University, New York, New York, USA;

M. Q. Fleisher: Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York, USA.


6. The rains that flooded Pakistan could have been predicted

In the middle of the week, during the heart of summer, at the onset of the 2010 monsoon season, the northern end of Pakistan was pounded by torrential rain. Some areas of the country were drowned in 300 mm (11.8 inches) of rain over four days, 10 times the seasonal average. The extreme rainfall caused widespread flooding, killing 2000 people and causing more than $40 billion in damage by the time it let up and the water withdrew. A new analysis by Webster et al. suggests that some of this damage might have been prevented, because the heavy rains that triggered the flooding could have been predicted.

Pakistan is no stranger to floods, having experienced at least 52 of them since the 1970s, but the researchers wanted to know if the 2010 rainfall was different. Drawing on precipitation data collected by rain gauges and satellite observations since 1981, the authors find that while the total precipitation experienced by Pakistan was only slightly above average, there were more periods of extreme rainfall in 2010 than in previous years, even when compared to other years that experienced flooding. Using a meteorological prediction model, the authors find that they would have been able to forecast these extreme events at least 8 days ahead of time.

The authors note that the heavy rain alone was not necessarily enough to cause the widespread devastation; other factors, such as deforestation, and low vegetation caused by a drought the previous year, would have contributed to increased river flow rates. Still, the researchers suggest that if an appropriate flood forecasting system had been in place to translate the precipitation forecasts to streamflow forecasts, and if the precipitation forecasts had been available in real time, then officials in the country could have taken steps to limit the damage and loss of life.

A Dec. 31, 2010 press release on this is available online at http://www.agu.org/news/press/pr_archives/2011/2011-04.shtml.

Source:
Geophysical Research Letters, doi:10.1029/2010GL046346, 2011
http://dx.doi.org/10.1029/2010GL046346

Title: Were the 2010 Pakistan floods predictable?

Authors: P. J. Webster, V. E. Toma, and H.‐M. Kim: School of Earth and Atmospheric Science, Georgia Institute of Technology, Atlanta, Georgia, USA.

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