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Wilcox L.J.,National Center for Atmospheric Science Climate | Wilcox L.J.,University of Reading | Highwood E.J.,University of Reading | Dunstone N.J.,UK Met Office
Environmental Research Letters

Analysis of single forcing runs from CMIP5 (the fifth Coupled Model Intercomparison Project) simulations shows that the mid-twentieth century temperature hiatus, and the coincident decrease in precipitation, is likely to have been influenced strongly by anthropogenic aerosol forcing. Models that include a representation of the indirect effect of aerosol better reproduce inter-decadal variability in historical global-mean near-surface temperatures, particularly the cooling in the 1950s and 1960s, compared to models with representation of the aerosol direct effect only. Models with the indirect effect also show a more pronounced decrease in precipitation during this period, which is in better agreement with observations, and greater inter-decadal variability in the inter-hemispheric temperature difference. This study demonstrates the importance of representing aerosols, and their indirect effects, in general circulation models, and suggests that inter-model diversity in aerosol burden and representation of aerosol-cloud interaction can produce substantial variation in simulations of climate variability on multi-decadal timescales. © 2013 IOP Publishing Ltd. Source

Tegtmeier S.,Leibniz Institute of Marine Science | Kruger K.,Leibniz Institute of Marine Science | Quack B.,Leibniz Institute of Marine Science | Atlas E.L.,University of Miami | And 4 more authors.
Atmospheric Chemistry and Physics

Oceanic emissions of halogenated very short-lived substances (VSLS) are expected to contribute significantly to the stratospheric halogen loading and therefore to ozone depletion. The amount of VSLS transported into the stratosphere is estimated based on in-situ observations around the tropical tropopause layer (TTL) and on modeling studies which mostly use prescribed global emission scenarios to reproduce observed atmospheric concentrations. In addition to upper-air VSLS measurements, direct observations of oceanic VSLS emissions are available along ship cruise tracks. Here we use such in-situ observations of VSLS emissions from the West Pacific and tropical Atlantic together with an atmospheric Lagrangian transport model to estimate the direct contribution of bromoform (CHBr3), and dibromomethane (CH2Br 2) to the stratospheric bromine loading as well as their ozone depletion potential. Our emission-based estimates of VSLS profiles are compared to upper-air observations and thus link observed oceanic emissions and in situ TTL measurements. This comparison determines how VSLS emissions and transport in the cruise track regions contribute to global upper-air VSLS estimates. The West Pacific emission-based profiles and the global upper-air observations of CHBr3 show a relatively good agreement indicating that emissions from the West Pacific provide an average contribution to the global CHBr3 budget. The tropical Atlantic, although also being a CHBr3 source region, is of less importance for global upper-air CHBr3 estimates as revealed by the small emission-based abundances in the TTL. Western Pacific CH2Br2 emission-based estimates are considerably smaller than upper-air observations as a result of the relatively low sea-to-air flux found in the West Pacific. Together, CHBr3 and CH2Br2 emissions from the West Pacific are projected to contribute to the stratospheric bromine budget with 0.4 pptv Br on average and 2.3 pptv Br for cases of maximum emissions through product and source gas injection. These relatively low estimates reveal that the tropical West Pacific, although characterized by strong convective transport, might overall contribute less VSLS to the stratospheric bromine budget than other regions as a result of only low CH2Br2 and moderate CHBr3 oceanic emissions.© Author(s) 2012. Source

Abbatt J.P.D.,University of Toronto | Thomas J.L.,University of Versailles | Thomas J.L.,University of California at Los Angeles | Abrahamsson K.,Gothenburg University | And 13 more authors.
Atmospheric Chemistry and Physics

The role of ice in the formation of chemically active halogens in the environment requires a full understanding because of its role in atmospheric chemistry, including controlling the regional atmospheric oxidizing capacity in specific situations. In particular, ice and snow are important for facilitating multiphase oxidative chemistry and as media upon which marine algae live. This paper reviews the nature of environmental ice substrates that participate in halogen chemistry, describes the reactions that occur on such substrates, presents the field evidence for ice-mediated halogen activation, summarizes our best understanding of ice-halogen activation mechanisms, and describes the current state of modeling these processes at different scales. Given the rapid pace of developments in the field, this paper largely addresses advances made in the past five years, with emphasis given to the polar boundary layer. The integrative nature of this field is highlighted in the presentation of work from the molecular to the regional scale, with a focus on understanding fundamental processes. This is essential for developing realistic parameterizations and descriptions of these processes for inclusion in larger scale models that are used to determine their regional and global impacts. © 2012.Author(s). Source

Klingaman N.P.,National Center for Atmospheric Science Climate | Klingaman N.P.,University of Reading | Woolnough S.J.,National Center for Atmospheric Science Climate | Woolnough S.J.,University of Reading
International Journal of Climatology

Queensland experiences considerable inter-annual and decadal rainfall variability, which impacts water-resource management, agriculture and infrastructure. To understand the mechanisms by which large-scale atmospheric and coupled air-sea processes drive these variations, empirical orthogonal teleconnection (EOT) analysis is applied to 1900-2010 seasonal Queensland rainfall. Fields from observations and the 20th Century Reanalysis are regressed onto the EOT timeseries to associate the EOTs with large-scale drivers. In winter, spring and summer the leading, state-wide EOTs are highly correlated with the El Niño-Southern Oscillation (ENSO); the Inter-decadal Pacific Oscillation modulates the summer ENSO teleconnection. In autumn, the leading EOT is associated with locally driven, late-season monsoon variations, while ENSO affects only tropical northern Queensland. Examining EOTs beyond the first, southeastern Queensland and the Cape York peninsula emerge as regions of coherent rainfall variability. In the southeast, rainfall anomalies respond to the strength and moisture content of onshore easterlies, controlled by Tasman Sea blocking. The summer EOT associated with onshore flow and blocking has been negative since 1970, consistent with the observed decline in rainfall along the heavily populated coast. The southeastern Queensland EOTs show considerable multi-decadal variability, which is independent of large-scale drivers. Summer rainfall in Cape York is associated with tropical-cyclone activity. © 2012 Royal Meteorological Society. Source

Yang X.,National Center for Atmospheric Science Climate | Yang X.,University of Cambridge | Yang X.,British Antarctic Survey | Abraham N.L.,National Center for Atmospheric Science Climate | And 13 more authors.
Atmospheric Chemistry and Physics

Naturally produced very short-lived substances (VSLS) account for almost a quarter of the current stratospheric inorganic bromine, Bry. Following VSLS oxidation, bromine radicals (Br and BrO) can catalytically destroy ozone. The extent to which possible increases in surface emissions or transport of these VSLS bromocarbons to the stratosphere could counteract the effect of halogen reductions under the Montreal Protocol is an important policy question. Here, by using a chemistry-climate model, UM-UKCA, we investigate the impact of a hypothetical doubling (an increase of 5 ppt Bry) of VSLS bromocarbons on ozone and how the resulting ozone changes depend on the background concentrations of chlorine and bromine. Our model experiments indicate that for the 5 ppt increase in Bry from VSLS, the ozone decrease in the lowermost stratosphere of the Southern Hemisphere (SH) may reach up to 10% in the annual mean; the ozone decrease in the Northern Hemisphere (NH) is smaller (4-6%). The largest impact on the ozone column is found in the Antarctic spring. There is a significantly larger ozone decrease following the doubling of the VSLS burden under a high stratospheric chlorine background than under a low chlorine background, indicating the importance of the inter-halogen reactions. For example, the decline in the high-latitude, lower-stratospheric ozone concentration as a function of Bry is higher by about 30-40% when stratospheric Cly is ∼ 3 ppb (present day), compared with Cly of ∼ 0.8 ppb (a pre-industrial or projected future situation). Bromine will play an important role in the future ozone layer. However, even if bromine levels from natural VSLS were to increase significantly later this century, changes in the concentration of ozone will likely be dominated by the decrease in anthropogenic chlorine. Our calculation suggests that for a 5 ppt increase in Bry from VSLS, the Antarctic ozone hole recovery date could be delayed by approximately 6-8 years, depending on Cly levels. © 2014 Author(s). Source

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