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  • 1
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    Observational evidence of strengthening of the Brewer-Dobson circulation since 1980 (2015)
    Wiley
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    Publication Date: 2015-09-22
    Description: The change of the Brewer-Dobson circulation (BDC) over the period of 1980-2009 is examined through a combined analysis of satellite Microwave Sounding Unit (MSU/AMSU) lower-stratospheric temperatures ( T LS ), ERA-Interim reanalysis data, and observed estimates of changes in ozone, water vapor, well-mixed greenhouse gases, and stratospheric aerosols. The MSU/AMSU-observed tropical T LS trend is first empirically separated into a dynamic component associated with the BDC changes and a radiative component due to the atmospheric composition changes. The derived change in the dynamic component suggests that the annual mean BDC has accelerated in the last thirty years (at the 90% confidence interval), with most of the change coming from the Southern Hemisphere. The annual mean Northern Hemisphere contribution to the acceleration is not statistically significant. The radiative component of tropical T LS trends is independently checked using observed changes in stratospheric composition. It is shown that the changes in ozone, stratospheric aerosols, well-mixed greenhouse gases, and water vapor make important contributions to the radiative component of tropical T LS trends. Despite large uncertainties in lower-stratospheric cooling associated with uncertainties in observed ozone and water vapor changes, this derived radiative component agrees with the empirically-inferred radiative component, both in terms of its average value and small seasonal dependence. By establishing a relationship between tropical residual vertical velocity at 70 hPa and T LS , we show that the relative strengthening of the annual mean BDC is about 2.1% per decade for 1980-2009, supporting the results from state-of-the-art chemistry-climate model simulations.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 2
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    Identifying weekly cycles in meteorological variables: The importance of an appropriate statistical analysis (2012)
    Wiley
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    Publication Date: 2012-07-04
    Description: Weekly cycles in several meteorological parameters have been previously reported. Yet the extent to which these cycles are caused by anthropogenic activity remains unclear. Some of the complications associated with establishing this link are discussed here. Specifically, we highlight and quantify some common errors that have been made in the application of statistical techniques to this problem. Some errors, including the inappropriate use of the Student t test, have been significant enough to affect the conclusions of previous studies. A resampling technique that can properly account for both temporal and spatial correlation is evaluated and is shown to be accurate for determining the statistical significance of weekly cycles at the station level and for evaluating total field significance. We demonstrate that this resampling approach performs comparably to a Fourier analysis that evaluates the significance of the power at a seven-day period. Regardless of the analysis technique used, an understanding of the behavior of and uncertainties associated with the statistical analysis is critical to arriving at a justifiable conclusion regarding a human influence on weekly cycles and for putting results in context with other studies. We also discuss some general errors that can be made in weekly cycle analysis. These include selection of an analysis region after identifying where weekly cycles are significant, acceptance of a physical explanation for the hypothesized link that has not been properly tested given its large number of degrees of freedom, and ignoring the correlation among meteorological parameters.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
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  • 3
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    Separating signal and noise in atmospheric temperature changes: The importance of timescale (2011)
    Wiley
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    Publication Date: 2011-11-18
    Description: We compare global-scale changes in satellite estimates of the temperature of the lower troposphere (TLT) with model simulations of forced and unforced TLT changes. While previous work has focused on a single period of record, we select analysis timescales ranging from 10 to 32 years, and then compare all possible observed TLT trends on each timescale with corresponding multi-model distributions of forced and unforced trends. We use observed estimates of the signal component of TLT changes and model estimates of climate noise to calculate timescale-dependent signal-to-noise ratios (S/N). These ratios are small (less than 1) on the 10-year timescale, increasing to more than 3.9 for 32-year trends. This large change in S/N is primarily due to a decrease in the amplitude of internally generated variability with increasing trend length. Because of the pronounced effect of interannual noise on decadal trends, a multi-model ensemble of anthropogenically-forced simulations displays many 10-year periods with little warming. A single decade of observational TLT data is therefore inadequate for identifying a slowly evolving anthropogenic warming signal. Our results show that temperature records of at least 17 years in length are required for identifying human effects on global-mean tropospheric temperature.
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    Topics: Geosciences , Physics
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  • 4
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    An assessment of changing ozone loss rates at South Pole: Twenty-five years of ozonesonde measurements (2011)
    Wiley
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    Publication Date: 2011-11-18
    Description: In 2010, 25 years of regular, year-round ozone soundings at South Pole station, Antarctica, were completed. These measurements provide unique information about the seasonality, trends, and variability of ozone depletion in the polar stratosphere at high vertical resolution. Here, we focus on the observed loss rates, and their changes since the measurement series began. The fastest loss rates occur between the end of August and end of September between 50 hPa and 30 hPa. Loss rates at these pressure levels increased by approximately 40% from the late 1980s to the late 1990s and have remained stable within estimated uncertainties since then. To estimate the time frame when a reduction in ozone loss rates will be observable outside the range of dynamical variability at the South Pole, we scale the estimated loss rates to the future projected concentrations of equivalent effective stratospheric chlorine (EESC). If a linear relationship between ozone loss rates and EESC is assumed, we project that a change in lower stratospheric ozone loss rates at South Pole station will be first detectable in the 2017–2021 time period.
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    Topics: Geosciences , Physics
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  • 5
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    Changes in the polar vortex: Effects on Antarctic total ozone observations at various stations (2011)
    Wiley
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    Publication Date: 2011-01-12
    Description: October mean total column ozone data from four Antarctic stations form the basis for understanding the evolution of the ozone hole since 1960. While these stations show similar emergence of the ozone hole from 1960 to 1980, the records are divergent in the last two decades. The effects of long-term changes in vortex shape and location are considered by gridding the measurements by equivalent latitude. A clear eastward shift of the mean position of the vortex in October with time is revealed, which changes the fraction of ozone measurements taken inside/outside the vortex for stations in the vortex collar region. After including only those measurements made inside the vortex, ozone behavior in the last two decades at the four stations is very similar. This suggests that dynamical influence must be considered when interpreting and intercomparing ozone measurements from Antarctic stations for detecting ozone recovery and ozone-related changes in Antarctic climate.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 6
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    The Contribution of Anthropogenic SO2 Emissions to the Asian Tropopause Aerosol Layer (2014)
    Wiley
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    Publication Date: 2014-01-17
    Description: [1]  Recent observations reveal a seasonally occurring layer of aerosol located from 0 ∘ to 100 ∘ E, 20 ∘ to 45 ∘ N and extending vertically from about 13 km to 18 km; this has been termed the Asian Tropopause Aerosol Layer (ATAL) and its existence is closely associated with the Asian summer monsoon circulation. Observational studies argue that the ATAL is a recent phenomenon, as the layer was not observed in the satellite record prior to 1998. This suggests that the ATAL may be of anthropogenic origin associated with increased emissions, most notably sulfur dioxide (SO 2 ), from increased industrial activity in China and India starting during the same time. Here we test the hypothesis that SO 2 emitted from Asia led to the formation of the ATAL using an aerosol microphysical model coupled to a global chemistry climate model. This is the first modeling study to specifically examine the ATAL and its possible origin. From our results, we conclude that the ATAL is most likely due to anthropogenic emissions, but its source cannot solely be attributed to emissions from Asia. Specifically, the results indicate that Chinese and Indian emissions contribute ∼ 30% of the sulfate aerosol extinction in the ATAL during volcanically quiescent periods. We also show that even small volcanic eruptions preclude our ability to make any conclusions about the existence of the ATAL before 1998 with observations alone.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 7
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    Total volcanic stratospheric aerosol optical depths and implications for global climate change (2014)
    Wiley
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    Publication Date: 2014-11-01
    Description: Understanding the cooling effect of recent volcanoes is of particular interest in the context of the post-2000 slowing of the rate of global warming. Satellite observations of aerosol optical depth (AOD) above 15 km have demonstrated that small-magnitude volcanic eruptions substantially perturb incoming solar radiation. Here we use lidar, AERONET and balloon-borne observations to provide evidence that currently available satellite databases neglect substantial amounts of volcanic aerosol between the tropopause and 15 km at mid to high latitudes, and therefore underestimate total radiative forcing resulting from the recent eruptions. Incorporating these estimates into a simple climate model, we determine the global volcanic aerosol forcing since 2000 to be −0.19 ± 0.09 Wm −2 . This translates into an estimated global cooling of 0.05 to 0.12 °C. We conclude that recent volcanic events are responsible for more post-2000 cooling than is implied by satellite databases that neglect volcanic aerosol effects below 15 km.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 8
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    Modeling the climate impact of Southern Hemisphere ozone depletion: The importance of the ozone dataset (2014)
    Wiley
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    Publication Date: 2014-11-19
    Description: The ozone hole is an important driver of recent Southern Hemisphere (SH) climate change, and capturing these changes is a goal of climate modeling. Most climate models are driven by offline ozone datasets. Previous studies have shown that there is a substantial range in estimates of SH ozone depletion, but the implications of this range have not been examined systematically. We use a climate model to evaluate the difference between using the ozone forcing (SPARC) used by many IPCC Fifth Assessment Report (CMIP5) models and one at the upper end of the observed depletion estimates (BDBP). In the stratosphere, we find that austral spring/summer polar cap cooling, geopotential height decreases, and zonal wind increases in the BDBP simulations are all doubled compared to the SPARC simulations, while tropospheric responses are 20-100% larger. These results are important for studies attempting to diagnose the climate fingerprints of ozone depletion.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 9
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    Sensitivity of inorganic aerosol radiative effects to US emissions (2017)
    Wiley
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    Publication Date: 2017-05-23
    Description: Between 2005 and 2012, US emissions of nitrogen oxides (NO x ) and sulfur dioxide (SO 2 ) decreased by 42% and 62%, respectively. These species, as well as ammonia (NH 3 ), are precursors of inorganic fine aerosols, which scatter incoming shortwave radiation and thus affect climate. Scaling aerosol concentrations to emissions, as might be done for near-term climate projections, neglects nonlinear chemical interactions. To estimate the magnitude of these nonlinearities, we conduct a suite of simulations with a chemical transport model and an off-line radiative transfer model. We find that the direct radiative effect (DRE) over the North American domain decreases by 59 and 160 mW m −2 in winter and summer, respectively, between 2005 and 2012. The sensitivities of DRE to NO x and SO 2 emissions increase, by 11% and 21% in summer, while sensitivity to NH 3 emissions decreases. The wintertime sensitivity of DRE to NO x emissions is small in 2005 but is 5 times as large in 2012. Scaling radiative effects from 2005 to 2012 based on 2005 sensitivities overestimates the magnitude of the DRE of 7% and 6% of the US-attributable DRE in January and July, respectively. The difference between the changes in DRE and the changes in sensitivity suggests that scaling to SO 2 emissions alone has so far been an accurate approximation but it may not be in the near future. These values represent the level of accuracy that can be expected in adjusting aerosol radiative effects in climate models without chemistry.
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    Topics: Geosciences , Physics
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  • 10
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    Simulation of Polar Stratospheric Clouds in the Specified Dynamics version of the Whole Atmosphere Community Climate Model (2013)
    Wiley
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    Publication Date: 2013-04-18
    Description: [1]  We evaluate the simulation of Polar Stratospheric Clouds (PSCs) in the Specified Dynamics version of the Whole Atmosphere Community Climate Model (SD-WACCM) for the Antarctic winter 2005. In this model, PSCs are assumed to form instantaneously at a prescribed supersaturation, with a prescribed size distribution and number density. We use satellite observations of the Antarctic winter 2005 of nitric acid, water vapor, and PSCs to test and improve this PSC parameterization. CALIOP observations since 2006 show that in both hemispheres the dominant PSC type throughout the entire polar winter is a mixture of Nitric Acid Trihydrate (NAT) and Supercooled Ternary Solutions (STS) droplets, but typical assumptions about PSC formation in the model at a given supersaturation do not produce such a population of particles, and lead to earlier removal of HNO 3 from the gas-phase compared to observations. In our new PSC scheme, the formation of mixed PSCs is forced by only allowing a fraction of total available HNO 3 to freeze to NAT and the remaining part to form STS. With this approach, a mixture of both is present throughout the winter, in agreement with observations. This approach yields good agreement with observations in terms of temperature dependent removal of gas-phase HNO 3 and irreversible denitrification. In addition to nitric acid containing PSCs, we also investigate ice PSCs. We show that the choice of required saturation ratio of water vapor for ice formation can significantly improve the calculated vertical distribution of water vapor, and is required to produce good agreement with observations.
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    Topics: Geosciences , Physics
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