Pacific Ocean Forcing and Atmospheric Variability are the Dominant Causes of Spatially Widespread Droughts in the Contiguous United States

The contributions of oceanic and atmospheric variability to spatially widespread summer droughts in the contiguous United States (hereafter, pan-CONUS droughts) are investigated using 16-member ensembles of the Community Climate Model version 3 (CCM3) forced with observed sea surface temperatures (SSTs) from 1856 to 2012. The employed SST forcing fields are either (i) global or restricted to the (ii) tropical Pacific or (iii) tropical Atlantic to isolate the impacts of these two ocean regions on pan-CONUS droughts. Model results show that SST forcing of pan-CONUS droughts originates almost entirely from the tropical Pacific because of atmospheric highs from the northern Pacific to eastern North America established by La Nia conditions, with little contribution from the tropical Atlantic. Notably, in all three model configurations, internal atmospheric variability influences pan-CONUS drought occurrence by as much or more than the ocean forcing and can alone cause pan-CONUS droughts by establishing a dominant high centered over the US montane West. Similar results are found for the Community Atmosphere Model version 5 (CAM5). Model results are compared to the observational record, which supports model-inferred contributions to pan-CONUS droughts from La Nias and internal atmospheric variability. While there may be an additional association with warm Atlantic SSTs in the observational record, this association is ambiguous due to the limited number of observed pan-CONUS. The ambiguity thus opens the possibility that the observational results are limited by sampling over the 20th-century and not at odds with the suggested dominance of Pacific Ocean forcing in the model ensembles

Creation of Electron--Positron Wind in Gamma-Ray Bursts and Its Effect on the Early Afterglow Emission

We calculate the creation of electron--positron pairs in Gamma-Ray Bursts (GRBs) resulting from the collision between scattered and outward moving gamma-ray photons. The number of pairs exceeds the number of ambient medium electrons encountered by the GRB ejecta up to ~ 10^{16} cm from the center of explosion. The shock resulting from the interaction of the ejecta with the pair-wind may brighten the afterglow synchrotron emission during the first few minutes. Even without this effect, the peak intensity of the optical afterglow increases with the density of the surrounding medium. Therefore, observations of the optical flux at early times constrain the density of the circumburst medium. If the electron and magnetic field energies behind the forward shock sweeping-up the pair-wind and the circumburst medium are as inferred from fits to the broadband afterglow emission at 0.5-100 days, then the current upper limits on the optical counterpart emission, set by the ROTSE and LOTIS experiments, indicate that the circumburst medium within 0.01 pc is less dense than 100 cm^{-3} or, if a wind, corresponds to a progenitor mass-loss to wind speed ratio below 10^{-6} M_sun/yr/(1000 km/s).Comment: 9 pages, submitted to MNRAS in 200

Who is the new sheriff in town regulating boreal forest growth?

Climate change appears to be altering boreal forests. One recently observed symptom of these changes has been an apparent weakening of the positive relationship between high-latitude boreal tree growth and temperature at some sites (D'Arrigo et al 2008). This phenomenon is referred to as the 'divergence problem' or 'divergence effect' and is thought to reflect a non-linear relationship between temperature and tree growth, where recent warming has allowed other factors besides growing-season temperature to emerge as dominant regulators of annual growth rates

Projected increases in intensity, frequency, and terrestrial carbon costs of compound drought and aridity events.

Drought and atmospheric aridity pose large risks to ecosystem services and agricultural production. However, these factors are seldom assessed together as compound events, although they often occur simultaneously. Drought stress on terrestrial carbon uptake is characterized by soil moisture (SM) deficit and high vapor pressure deficit (VPD). We used in situ observations and 15 Earth system models to show that compound events with very high VPD and low SM occur more frequently than expected if these events were independent. These compound events are projected to become more frequent and more extreme and exert increasingly negative effects on continental productivity. Models project intensified negative effects of high VPD and low SM on vegetation productivity, with the intensification of SM exceeding those of VPD in the Northern Hemisphere. These results highlight the importance of compound extreme events and their threats for the capability of continents to act as a carbon sink

Climate-driven regime shifts in a mangrove-salt marsh ecotone over the past 250 years.

Climate change is driving the tropicalization of temperate ecosystems by shifting the range edges of numerous species poleward. Over the past few decades, mangroves have rapidly displaced salt marshes near multiple poleward mangrove range limits, including in northeast Florida. It is uncertain whether such mangrove expansions are due to anthropogenic climate change or natural climate variability. We combined historical accounts from books, personal journals, scientific articles, logbooks, photographs, and maps with climate data to show that the current ecotone between mangroves and salt marshes in northeast Florida has shifted between mangrove and salt marsh dominance at least 6 times between the late 1700s and 2017 due to decadal-scale fluctuations in the frequency and intensity of extreme cold events. Model projections of daily minimum temperature from 2000 through 2100 indicate an increase in annual minimum temperature by 0.5 °C/decade. Thus, although recent mangrove range expansion should indeed be placed into a broader historical context of an oscillating system, climate projections suggest that the recent trend may represent a more permanent regime shift due to the effects of climate change

The Circumstellar Environment of High Mass Protostellar Objects. III Evidence of Infall?

The results are presented of a molecular line survey to search for the spectral signature of infall towards 77 850 micron continuum sources believed to be candidate high mass protostellar objects. Up to six different transitions, HCO+ 1-0, 3-2 and 4-3, H2CO 2_12-1_11, N2H+ and H13CO+ 3-2, were observed towards each source. Towards the peak of the 850 micron emission, N2H+ was typically strong, with a peak antenna temperature of ~1.5K, with a typical linewidth of ~2km/s. The good agreement between the velocity and velocity width of the N2H+ and H13CO+ emission suggests that both species are tracing similar material in the sources. With respect to the velocity of the N2H+, there is a statistically significant excess of blue asymmetric line profiles in both the HCO+ 1-0 and H2CO transitions. This excess reaches levels similar to that seen towards samples of low mass protostars, and suggests that the material around these high mass sources is infalling. We identify 22 promising candidate infall sources which show at least one blue asymmetric line profile and no red asymmetric profiles. The infall velocity is estimated to be in the range of 0.1 km/s to 1 km/s with an implied mass accretion rate of between 2x10^{-4} Msol/yr and 10^{-3}Msol/yr.Comment: Accepted for publication in Astronomy and Astrophysics. Higher resolution versions of Figures 1 and 2 are available from http://www.jb.man.ac.uk/~gaf/Papers.htm

The G292.0+1.8 pulsar wind nebula in the mid-infrared

G292.0+1.8 is a Cas A-like supernova remnant that contains the young pulsar PSR J1124-5916 powering a compact torus-like pulsar wind nebula visible in X-rays. A likely counterpart to the nebula has been detected in the optical VRI bands. To confirm the counterpart candidate nature, we examined archival mid-infrared data obtained with the Spitzer Space Telescope. Broad-band images taken at 4.5, 8, 24, and 70 microns were analyzed and compared with available optical and X-ray data. The extended counterpart candidate is firmly detected in the 4.5 and 8 micron bands. It is brighter and more extended in the bands than in the optical, and its position and morphology agree well with the coordinates and morphology of the torus-like pulsar wind nebula in X-rays. The source is not visible in 24 and 70 micron images, which are dominated by bright emission from the remnant shell and filaments. We compiled the infrared fluxes of the nebula, which probably contains a contribution from an unresolved pulsar in its center, with the optical and X-ray data. The resulting unabsorbed multiwavelength spectrum is described by power laws of significantly steeper slope in the infrared-optical than in X-rays, implying a double-knee spectral break between the optical and X-rays. The 24 and 70 microns flux upper limits suggest a second break and a flatter spectrum at the long wavelength limit. These features are common to two other pulsar wind nebulae associated with the remnants B0540-69.3 and 3C 58 and observed in all three ranges. The position, morphology, and spectral properties of the detected source allow us to comfirm that it is the infrared-optical counterpart to both the pulsar and its wind nebula system in the G292.0+1.8 supernova remnant.Comment: 5 pages, 2 figure

Tunneling anisotropic magnetoresistance in multilayer-(Co/Pt)/AlOx/Pt structures

We report observations of tunneling anisotropic magnetoresitance (TAMR) in vertical tunnel devices with a ferromagnetic multilayer-(Co/Pt) electrode and a non-magnetic Pt counter-electrode separated by an AlOx barrier. In stacks with the ferromagnetic electrode terminated by a Co film the TAMR magnitude saturates at 0.15% beyond which it shows only weak dependence on the magnetic field strength, bias voltage, and temperature. For ferromagnetic electrodes terminated by two monolayers of Pt we observe order(s) of magnitude enhancement of the TAMR and a strong dependence on field, temperature and bias. Discussion of experiments is based on relativistic ab initio calculations of magnetization orientation dependent densities of states of Co and Co/Pt model systems.Comment: 4 pages, 5 figures, to be published in Phys. Rev. Let