AGU Journal Highlights—6 April 2009

Highlights, including authors and their institutions

The following highlights summarize research papers that have been published or are “in press” (accepted, but not yet published) in Geophysical Research Letters (GRL) or the Journal of Geophysical Research – Atmospheres (JGR-D).

Anyone may read the scientific abstract for any already-published paper (not papers “in press”) by clicking on the link provided at the end of each Highlight. You can also read the abstract by going to search options for AGU journals and inserting into the search engine the full doi (digital object identifier), e.g. 10.1029/2008GL036288. The doi is found at the end of each Highlight below.

Journalists and public information officers (PIOs) at educational or scientific institutions, who are registered with AGU, also may download papers cited in this release by clicking on the links below. Instructions for members of the news media, PIOs, and the public for downloading or ordering the full text of any research paper summarized below are available. Please note that papers not yet published (i.e. “in press”) are available only to journalists and public information officers.

Natural protector for Pacific reefs disputed
Pacemaker of major climate shifts revealed
How space dust vaporizes in the atmosphere
Cyclone locations affect cooling or warming of poles
Monitoring aerosols over the world’s oceans
Refined model mimics speeding ice streams
Modeling magnetic humps and dips around planets
Updated model offers aerosol insights

1. Natural protector for some Pacific reefs disputed

Widespread coral bleaching, in which reefs lose their colorful symbiotic algae, is predicted to occur in most tropical locations in the coming decades due to rising ocean temperatures. A previous study suggested that reefs in the warm region of the Pacific known as the Western Pacific Warm Pool (WPWP) are protected by a natural ocean “thermostat” that limits the maximum sea surface temperature in the region. This finding was based on the observation of abnormally low levels of coral bleaching in the WPWP compared with other tropical regions. Reexamining the data, van Hooidonk and Huber find no strong evidence for a thermostat or for abnormally low coral bleaching in the WPWP. The authors conclude that because there are very few observations of coral reefs (bleached or not) in the WPWP, bleaching events in this remote region are probably underreported, not anomalously low. They also point out that no physical basis for a thermostat has been convincingly identified, and no evidence of a thermostat exists in geological records. Therefore the authors argue that the concept of a thermostat in the WPWP should be discarded.

Title:

“Equivocal evidence for a thermostat and unusually low levels of coral bleaching in the Western Pacific Warm Pool”

Authors:

Ruben van Hooidonk and Matthew Huber: Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana, USA

Source:

Geophysical Research Letters (GRL) paper 10.1029/2008GL036288, 2009

2. Pacemaker of major climate shifts revealed

Major climate oscillations modes, including the North Atlantic Oscillation (NAO), the Pacific Decadal Oscillation (PDO), the El Niño-Southern Oscillation (ENSO), and the North Pacific Index (NPI), each involve different mechanisms and operate in different regions. Nonetheless, they can exert varying levels of influence on each other and can become synchronized. Understanding how these modes dynamically interact is crucial because they have significant effects on global climate variability. Using events seen in three climate simulations as well as observations, Wang et al. examine the details of how these oscillations influence each other. The authors note that during periods when the modes were synchronized, if the strength of coupling between different modes increased, then synchronization was destroyed and the climate shifted to a new state with significant global temperature changes. They find that one of these modes, the NAO, serves as a “pacemaker” controlling the others; climate shifts occur only when the NAO is strongly coupled with the North Pacific Ocean. These finding should help scientists better understand the mechanisms driving complex global climate variations.

Title:

“The pacemaker of major climate shifts”

Authors:

Geli Wang: Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
Kyle L. Swanson and Anastasios A. Tsonis: Department of Mathematical Sciences, Atmospheric Sciences Group, University of Wisconsin-Milwaukee, Milwaukee, WI 53201, USA

Source:

Geophysical Research Letters (GRL) paper 10.1029/2008GL036874, 2009, in press

3. How space dust vaporizes in the atmosphere

Each day, clouds of space dust—billions of tiny microgram-sized meteoroids—enter the atmosphere and vaporize, leaving behind various metal ions and molecules high in the atmosphere. Different elements in these micrometeoroids have been thought to vaporize at different rates, but this process, known as differential ablation, had not been directly observed, making it difficult to understand in detail why different particles are left in the atmosphere. Janches et al. make the first observation of differential ablation of micrometeoroids. They study two particular micrometeoroids from among the observations made using an Arecibo radar in Puerto Rico, which detects electrons released around the micrometeoroids as their various constituents evaporate. The authors apply a model to correlate radar signals with the moment at which a particular chemical is predicted to vaporize from the micrometeoroid. They find that the most volatile elements, sodium and potassium, evaporate first high in the atmosphere. Next, and at lower altitudes, metals including silicon, iron, and magnesium vaporize, followed by other elements in the meteoroid. This work indicates that differential vaporization is the main mechanism by which micrometeorites deposit their mass in the atmosphere.

Title:

“First observation of micrometeoroid differential ablation in the atmosphere”

Authors:

D. Janches: Colorado Research Associates, NorthWest Research Associates, Boulder, Colorado, USA
L. P. Dyrud: Center for Remote Sensing, Inc., Fairfax, Virginia, USA
S. L. Broadley and J. M. C. Plane: School of Chemistry, University of Leeds, Leeds, UK

Source:

Geophysical Research Letters (GRL) paper 10.1029/2009GL037389, 2009

4. Cyclone locations affect cooling or warming of poles

Tropical cyclones generate strong winds that mix the upper ocean. This mixing can affect the poleward transfer of oceanic heat from tropical latitudes, an effect critical to understanding the influence of tropical cyclones on Earth’s climate. Previous studies suggested that the latitudes where tropical cyclone–induced mixing occurs could affect the degree to which ocean heat transport is enhanced or decreased. To learn more, Jansen and Ferrari use a general circulation model to simulate the effects of cyclone-induced ocean mixing at various latitudes on poleward heat transfer. They find that if cyclone-induced ocean mixing occurs everywhere within 30 degrees of the equator, ocean heat transport toward the poles increases. However, if cyclone-induced mixing only occurs in the subtropical bands between 11 degrees and 30 degrees, where most cyclones actually occur, the circulation pattern changes such that the poleward heat transport out of the equatorial region decreases.

Title:

“Impact of the latitudinal distribution of tropical cyclones on ocean heat transport”

Authors:

Malte Jansen and Raffaele Ferrari: Program in Atmospheres, Oceans, and Climate, Massachusetts Institute of Technology, Cambridge, Massachusetts, USA/dd>

Source:

Geophysical Research Letters (GRL) paper 10.1029/2008GL036796, 2009

5. Monitoring aerosols over the world’s oceans

The world’s oceans produce a large amount of natural aerosols, such as vaporized sea salt, that can serve to scatter or absorb sunlight, changing the Earth’s reflective properties and influencing climate. Thus accurate knowledge of atmospheric aerosol properties over the oceans is important to global climate models. To help gain better spatial coverage over the oceans, Smirnov et al. suggest that aerosol monitoring efforts should include the Maritime Aerosol Network (MAN), which involves taking measurements using a handheld photometer from ships—commercial, government, recreational, scientific—to extend data collection to the vast regions of open ocean. A few cruises have already tested MAN instrumentation and found that observations near shore compare well with simultaneous measurements collected on land. The authors describe how the new database created from MAN would be updated and maintained; for context, they summarize historical and current efforts to calculate aerosol properties. They hope that MAN observational platforms can be used as a component to the Aerosol Robotic Network (AERONET), a worldwide network of land-based aerosol sensors.

Title:

“Maritime Aerosol Network as a component of Aerosol Robotic Network”

Authors:

A.Smirnov: Science Systems and Applications, Inc., Lanham, Maryland, U.S.A.; also at Biospheric Sciences Branch, NASA Goddard Space Flight Center, Greenbelt, Maryland, U.S.A.; and collaborators (for complete list see online abstract at link below.)

Source:

Journal of Geophysical Research-Atmospheres (JGR-D) paper 10.1029/2008JD011257, 2009

6. New tool differentiates man-made from natural nitrogen-oxide pollution

Nitrogen oxides in the atmosphere, which are produced by lightning, biomass burning, and soil outgassing, are converted into atmospheric nitrate through oxidation reactions. Nitrogen oxide, itself a pollutant, controls the production of ozone, which in turn is a greenhouse gas and a pollutant at ground levels. Atmospheric nitrate contributes to the load of atmospheric particulate matter and, along with sulfate, to acid rain. Despite efforts to regulate and monitor emissions, nitrogen oxide and atmospheric nitrate burdens in the atmosphere are increasing in many regions. To learn more, Morin et al. study the stable isotopic composition of nitrate within aerosol samples, collected along a shipborne transect, in the lower atmosphere over the Atlantic Ocean from 65 degrees South to 79 degrees North. They find that in nonpolar regions, nitrate derived from anthropogenically emitted nitrogen oxide had isotopic properties distinct from locations influenced by natural nitrogen oxide sources. Further, air masses exposed to snow-covered areas have low nitrogen isotopic ratios, showing that snowpack emissions of nitrogen oxide from upwind regions can have a significant effect on the local surface budget of reactive nitrogen.

Title:

“Comprehensive isotopic composition of atmospheric nitrate in the Atlantic Ocean boundary layer form 65 degrees South to 79 degrees North”

Authors:

S. Morin, J. Savarino, and F. Domine: Institut National des Sciences de l'Univers, CNRS, Grenoble, France; also at Laboratoire de Glaciologie et Géophysique de l'Environnement, Université Josef Fourier, Grenoble, France
M. M. Frey: Institut National des Sciences de l'Univers, CNRS, Grenoble, France; also at Laboratoire de Glaciologie et Géophysique de l'Environnement, Université Josef Fourier, Grenoble, France; now at British Antarctic Survey, Natural Environment Research Council, Cambridge, U.K.
H.-W. Jacobi: Institut National des Sciences de l'Univers, CNRS, Grenoble, France; also at Laboratoire de Glaciologie et Géophysique de l'Environnement, Université Josef Fourier, Grenoble, France; Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany
L. Kaleschke: ZMAW, Institute of Oceanography, University of Hamburg, Hamburg, Germany
J. M. F. Martins: Institut National des Sciences de l'Univers, CNRS, Grenoble, France; also at Laboratoire d'Étude des Transferts en Hydrologie et Environnement, Université Josef Fourier, Grenoble, France

Source:

Journal of Geophysical Research-Atmospheres (JGR-D) paper 10.1029/2008JD010696, 2009

7. Modeling magnetic humps and dips around planets

Structures in the form of local magnetic enhancements (humps) or depressions (dips) with little change in the magnetic field direction across them have been observed in planetary magnetosheaths (the region between the magnetopause of a planet and the shock front of the solar wind) and in the solar wind. Scientists have worked to interpret and model the development of these structures from the mirror mode instability, which arises out of imbalances in plasma pressure. However, the understanding of the processes involved has been incomplete. Various methods of modeling mirror modes have had some success but have not been able to reproduce all their features. Hellinger et al. present a new model for mirror mode dynamics that, by combining two previous approaches, reproduces the development of growing magnetic humps from tiny variations. The authors conclude that their results provide a step toward better understanding the origin of mirror mode structures and their evolution.

Title:

“Mirror instability: From quasi-linear diffusion to coherent structures”

Authors:

P. Hellinger: Institute of Atmospheric Physics and Astronomical Institute, AS CR, Prague, Czech Republic
E. A. Kuznetsov: P. N. Lebedev Physical Institute, Moscow, Russia and Space Research Institute, Moscow, Russia
T. Passot and P. L. Sulem: Observatoire de la Côte d'Azur, Université de Nice-Sophia Antipolis, CNRS, Nice, France
P. M. Trávnícěk: Institute of Atmospheric Physics and Astronomical Institute, AS CR, Prague, Czech Republic

Source:

Geophysical Research Letters (GRL) paper 10.1029/2008GL036805, 2009

8. Updated model offers aerosol insights

In 1989, the European Centre for Medium-Range Weather Forecasts (ECMWF) model became the first operational forecast model to begin including the effects of aerosols as part of its radiation transfer calculations. Since then, several changes have been made to the model, including a recent effort to include observations pertaining to greenhouse gases, reactive gases, and aerosols in its analysis system and subsequent forecasts. This new effort expanded ECMWF’s Integrated Forecast System (IFS) to include new prognostic variables for the mass of sea salt, dust, organic matter and black carbon, and sulfate aerosols, all of which can interact with both the dynamics and physics of the model, making the sources and skins of the different aerosols more tightly coupled to the motions of the atmosphere. Morcrette at al. review the recent model updates by comparing data on quantities such as retrieved optical depths against satellite and surface observations. They find that the ECMWF IFS provides a good description of the horizontal distribution and temporal variability of the main aerosol types. In particular, comparisons of model predictions with observations from dust plume events show good agreement.

Title:

“Aerosol analysis and forecast in the European Centre for Medium-Range Weather Forecasts Integrated Forecast System: Forward modeling”

Authors:

J.-J. Morcrette: European Centre for Medium-Range Weather Forecasts, Reading, U.K., and collaborators (for complete list see online abstract at link below.)

Source:

Journal of Geophysical Research-Atmospheres (JGR-D) paper 10.1029/2008JD011235, 2009