Atmospheric Circulation Response to Short-Term Arctic Warming in an Idealized Model

Recent Arctic sea ice loss in fall has been posited to drive midlatitude circulation changes into winter and even spring. Past work has shown that sea ice loss can indeed trigger a weakening of the stratospheric polar vortex, which can lead to delayed surface weather changes. But the mechanisms of such changes and their relevant time scales have remained unclear. This study uses large ensembles of idealized GCM simulations to identify how and over what time scales the atmospheric circulation responds to short-term surface heat flux changes in high latitudes. The ensemble-mean response of the atmospheric circulation is approximately linear in the amplitude of the surface forcing. It is also insensitive to whether the forcing is zonally asymmetric or symmetric, that is, whether stationary waves are generated or not. The circulation response can be decomposed into a rapid thermal response and a slower dynamic adjustment. The adjustment arises through weakening of vertical wave activity fluxes from the troposphere into the stratosphere in response to polar warming, a mechanism that differs from sudden stratospheric warmings yet still results in a weakened stratospheric circulation. The stratospheric response is delayed and persists for about 2 months because the thermal response of the stratosphere is slow compared with that of the troposphere. The delayed stratospheric response feeds back onto the troposphere, but the tropospheric effects are weak compared with natural variability. The general pathway for the delayed response appears to be relatively independent of the atmospheric background state at the time of the anomalous surface forcing

Hydrogen content in doped and undoped BaPrO3 and BaCeO3 by cold neutron prompt-gamma activation analysis

Proton uptake in undoped and Y-doped BaPrO3 has been measured by cold neutron prompt-gamma activation analysis, and compared to the proton uptake in Gd-doped BaCeO3, as determined by the same technique. The conventional proton incorporation model of perovskites in which oxygen ion vacancies, generated by the introduction of the trivalent dopant onto the tetravalent perovskite site, are filled with hydroxyl groups upon exposure of the sample to H2O containing atmospheres, predicts that the proton concentration in such materials should be no greater than the dopant concentration. In contradiction to this model, the proton concentration in BaPr1–xYxO3 after humidification is as much as three times greater than the dopant concentration, and even undoped samples contain a high concentration of protons. Moreover, x-ray photoemission spectra suggest that the Pr oxidation state is lowered upon hydration. In contrast, BaCe0.9Y0.1O3 shows a typical hydrogen concentration, of close to 90% of the yttrium concentration. The results are interpreted in terms of the variable valence of Pr, which can become reduced from the 4+ to the 3+ oxidation state upon exposure to water, and effectively behaves as a self-dopant within the structure

Coupled atmosphere-ice-ocean dynamics in Dansgaard-Oeschger events

The Dansgaard-Oeschger events of the last ice age are among the best studied abrupt climate changes, yet a comprehensive explanation is still lacking. They are most pronounced in the North Atlantic, where they manifest as large temperature swings, on timescales of decades or shorter, between persistent cold (stadial) and warm (interstadial) conditions. This review examines evidence that Dansgaard-Oeschger events are an unforced or “spontaneous” oscillation of the coupled atmosphere-ice-ocean system comprising the North Atlantic, Nordic Seas and Arctic, collectively termed the Northern Seas. Insights from reanalysis data, climate model simulations, and idealized box model experiments point to the subpolar gyre as a key coupling region where vigorous wind systems encounter the southernmost extension of sea ice and the most variable currents of the North Atlantic, with connections to the deep ocean via convection. We argue that, under special conditions, these components can interact to produce Dansgaard-Oeschger events. Finding the sweet spot is a matter of understanding when the subpolar region enters a feedback loop whereby changes in wind forcing, sea ice cover, and ocean circulation amplify and sustain perturbations towards cold (ice-covered) or warm (ice-free) conditions. The resulting Dansgaard-Oeschger-like variability is seen in a handful of model simulations, including some “ugly duckling” pre-industrial simulations: these may be judged as undesirable at the outset, but ultimately show value in suggesting that current models include the necessary physics to produce abrupt climate transitions, but exhibit incorrect sensitivity to the boundary conditions. Still, glacial climates are hypothesized to favour larger, more persistent transitions due to differences in large-scale wind patterns. Simplified models and idealized experimental setups may provide a means to constrain how the critical processes act, both in isolation and in combination, to destabilize the subpolar North Atlantic.publishedVersio

Automated Lensing Learner: Automated Strong Lensing Identification with a Computer Vision Technique

Forthcoming surveys such as the Large Synoptic Survey Telescope (LSST) and Euclid necessitate automatic and efficient identification methods of strong lensing systems. We present a strong lensing identification approach that utilizes a feature extraction method from computer vision, the Histogram of Oriented Gradients (HOG), to capture edge patterns of arcs. We train a supervised classifier model on the HOG of mock strong galaxy-galaxy lens images similar to observations from the Hubble Space Telescope (HST) and LSST. We assess model performance with the area under the curve (AUC) of a Receiver Operating Characteristic (ROC) curve. Models trained on 10,000 lens and non-lens containing images images exhibit an AUC of 0.975 for an HST-like sample, 0.625 for one exposure of LSST, and 0.809 for 10-year mock LSST observations. Performance appears to continually improve with the training set size. Models trained on fewer images perform better in absence of the lens galaxy light. However, with larger training data sets, information from the lens galaxy actually improves model performance, indicating that HOG captures much of the morphological complexity of the arc finding problem. We test our classifier on data from the Sloan Lens ACS Survey and find that small scale image features reduces the efficiency of our trained model. However, these preliminary tests indicate that some parameterizations of HOG can compensate for differences between observed mock data. One example best-case parameterization results in an AUC of 0.6 in the F814 filter image with other parameterization results equivalent to random performance.Comment: 18 pages, 14 figures, summarizing results in figure

Connecting ocean heat transport changes from the mid-latitudes to the Arctic Ocean

Under greenhouse warming, climate models simulate a weakening of the Atlantic Meridional Overturning Circulation and the associated ocean heat transport at midlatitudes but an increase in the ocean heat transport to the Arctic Ocean. These opposing trends lead to what could appear to be a discrepancy in the reported ocean contribution to Arctic amplification. This study clarifies how ocean heat transport affects Arctic climate under strong greenhouse warming using a set of the 21st century simulations performed within the Coupled Model Intercomparison Project. The results suggest that a future reduction in subpolar ocean heat loss enhances ocean heat transport to the Arctic Ocean, driving an increase in Arctic Ocean heat content and contributing to the intermodel spread in Arctic amplification. The results caution against extrapolating the forced oceanic signal from the midlatitudes to the Arctic.publishedVersio

Suppressed eddy driving during southward excursions of the North Atlantic jet on synoptic to seasonal time scales

Jet streams shape midlatitude weather and climate. The North Atlantic jet is mainly eddy‐driven, with frequent north–south excursions on synoptic time scales arising from eddy forcings and feedbacks. There are, however, special periods during which the underlying dynamics appear to change—for example, winter 2009/2010, when the jet was persistently southward‐shifted, extremely zonal, and more thermally driven. This study shows evidence that the southern jet configuration exhibits altered dynamical behavior involving a shift in the balance of thermal and eddy‐driving processes, independent of timescale. Specifically, southern jets exhibit weaker eddy feedbacks and are associated with enhanced heating in the tropical Pacific. During winter 2009/2010, a remarkably frequent (66 days out of the 90‐day winter season) and persistent southern jet shaped the unusual seasonal signature. These results bridge the synoptic and climate perspectives of jet variability, with potential to help understand and reduce biases in regional climate variability as simulated by models.publishedVersio

Can we use ice sheet reconstructions to constrain meltwater for deglacial simulations?

Freshwater pulses from melting ice sheets are thought to be important for driving deglacial climate variability. This study investigates challenges in simulating and understanding deglacial climate evolution within this framework, with emphasis on uncertainties in the ocean overturning sensitivity to meltwater inputs. The response of an intermediate complexity model to a single Northern Hemisphere meltwater pulse is familiar: a weakening of the ocean overturning circulation in conjunction with an expansion of sea ice cover and a meridional temperature seesaw. Nonlinear processes are vital in shaping this response and are found to have a decisive influence when more complex scenarios with a history of pulses are involved. A meltwater history for the last deglaciation (21–9 ka) was computed from the ICE‐5G ice sheet reconstruction, and the meltwater was routed into the ocean through idealized ice sheet drainages. Forced with this meltwater history, model configurations with altered freshwater sensitivity produce a range of outcomes for the deglaciation, from those in which there is a complete collapse of the overturning circulation to those in which the overturning circulation weakens slightly. The different outcomes are interpreted in terms of the changing hysteresis behavior of the overturning circulation (i.e., non‐stationary freshwater sensitivity) as the background climate warms through the course of the deglaciation. The study illustrates that current uncertainties in model sensitivity are limiting in efforts to forward‐model deglacial climate variability. Furthermore, ice sheet reconstructions are shown to provide poor constraints on meltwater forcing for simulating the deglaciation.publishedVersio

The mechanisms that determine the response of the Northern Hemisphere’s stationary waves to North American Ice Sheets

Stationary waves describe the persistent meanders in the west–east flow of the extratropical atmosphere. Here, changes in stationary waves caused by ice sheets over North America are examined and the underlying mechanisms are discussed. Three experiment sets are presented showing the stationary wave response to the albedo or topography of ice sheets, as well as the albedo and topography in combination, as the forcings evolve from 21 to 6 ka. It is found that although the wintertime stationary waves have the largest amplitude, changes due to an ice sheet are equally large in summer and winter. In summer, ice sheet albedo is the dominant cause of changes: topography alone gives an opposite response to realistic ice sheets including albedo and topography. In winter, over the Atlantic, stationary wave changes are due to the ice sheet topography; over the Pacific, they are due to the persistence of summertime changes, mediated by changes in the ocean circulation. It is found that the response of stationary waves over the last deglaciation echoes the above conclusions, with no evidence of abrupt shifts in atmospheric circulation. The response linearly weakens as the albedo and height decrease from 21 to 10 ka. As potential applications, the seasonal cycle over Greenland is shown to be sensitive primarily to changes in summer climate caused by the stationary waves; the annual mean circulation over the North Pacific is found to result from summertime, albedo-forced, stationary wave effects persisting throughout the year because of ocean dynamics.publishedVersio

BioModels Database: An enhanced, curated and annotated resource for published quantitative kinetic models

Background: Quantitative models of biochemical and cellular systems are used to answer a variety of questions in the biological sciences. The number of published quantitative models is growing steadily thanks to increasing interest in the use of models as well as the development of improved software systems and the availability of better, cheaper computer hardware. To maximise the benefits of this growing body of models, the field needs centralised model repositories that will encourage, facilitate and promote model dissemination and reuse. Ideally, the models stored in these repositories should be extensively tested and encoded in community-supported and standardised formats. In addition, the models and their components should be cross-referenced with other resources in order to allow their unambiguous identification. Description: BioModels Database http://www.ebi.ac.uk/biomodels/ is aimed at addressing exactly these needs. It is a freely-accessible online resource for storing, viewing, retrieving, and analysing published, peer-reviewed quantitative models of biochemical and cellular systems. The structure and behaviour of each simulation model distributed by BioModels Database are thoroughly checked; in addition, model elements are annotated with terms from controlled vocabularies as well as linked to relevant data resources. Models can be examined online or downloaded in various formats. Reaction network diagrams generated from the models are also available in several formats. BioModels Database also provides features such as online simulation and the extraction of components from large scale models into smaller submodels. Finally, the system provides a range of web services that external software systems can use to access up-to-date data from the database. Conclusions: BioModels Database has become a recognised reference resource for systems biology. It is being used by the community in a variety of ways; for example, it is used to benchmark different simulation systems, and to study the clustering of models based upon their annotations. Model deposition to the database today is advised by several publishers of scientific journals. The models in BioModels Database are freely distributed and reusable; the underlying software infrastructure is also available from SourceForge https://sourceforge.net/projects/biomodels/ under the GNU General Public License