33 research outputs found
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Local and remote impacts of aerosol species on Indian summer monsoon rainfall in a GCM
The HadGEM2 AGCM is used to determine the most important anthropogenic aerosols in the Indian monsoon using experiments in which observed trends in individual aerosol species are imposed. Sulphur dioxide (SD) emissions are shown to impact rainfall more strongly than black carbon (BC) aerosols, causing reduced rainfall especially over northern India. Significant perturbations due to BC are not noted until its emissions are scaled up in a sensitivity test, resulting in rainfall increases over northern India due to the Elevated Heat Pump mechanism, enhancing convection during the pre-monsoon and bringing forward the monsoon onset. Secondly, the impact of anthropogenic aerosols is compared to that of increasing greenhouse-gas concentrations and observed sea-surface temperature (SST) warming. The tropospheric temperature gradient driving the monsoon shows weakening when forced
by either SD or imposed SST trends. However the observed SST trend is dominated by warming in the deep tropics; when the component of SST trend related to aerosol emissions is removed, further warming is found in the extratropical northern hemisphere that tends to offset monsoon weakening. This suggests caution is needed when using SST forcing as a proxy for greenhouse warming. Finally, aerosol
emissions are decomposed into those from the Indian region and those elsewhere, in pairs of experiments with SD and BC. Both local and remote aerosol emissions are found to lead to rainfall changes over India; for SD, remote aerosols contribute around 75% of the rainfall decrease over India, while for BC the remote forcing is even more
dominant
Stratospheric dynamics and midlatitude jets under geoengineering with space mirrors, and sulfate and titania aerosols
Copyright © 2015 The AuthorsThe impact on the dynamics of the stratosphere of three approaches to geoengineering by solar radiation management is investigated using idealized simulations of a global climate model. The approaches are geoengineering with sulfate aerosols, titania aerosols, and reduction in total solar irradiance (representing mirrors placed in space). If it were possible to use stratospheric aerosols to counterbalance the surface warming produced by a quadrupling of atmospheric carbon dioxide concentrations, tropical lower stratospheric radiative heating would drive a thermal wind response which would intensify the stratospheric polar vortices. In the Northern Hemisphere this intensification results in strong dynamical cooling of the polar stratosphere. Northern Hemisphere stratospheric sudden warming events become rare (one and two in 65 years for sulfate and titania, respectively). The intensification of the polar vortices results in a poleward shift of the tropospheric midlatitude jets in winter. The aerosol radiative heating enhances the tropical upwelling in the lower stratosphere, influencing the strength of the Brewer-Dobson circulation. In contrast, solar dimming does not produce heating of the tropical lower stratosphere, and so there is little intensification of the polar vortex and no enhanced tropical upwelling. The dynamical response to titania aerosol is qualitatively similar to the response to sulfate.Natural Environment Research Counci
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A case study of the radiative effect of aerosols over Europe: EUCAARI-LONGREX
The radiative effect of anthropogenic aerosols over Europe during the 2008 European Integrated Project on Aerosol Cloud Climate and Air Quality Interactions Long Range Experiment (EUCAARI-LONGREX) campaign has been calculated using measurements collected by the Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 aircraft and radiative transfer modelling. The aircraft sampled anthropogenically perturbed air masses across north-western Europe under anticyclonic conditions with aerosol optical depths ranging from 0.047 to 0.357. For one specially designed “radiative closure” flight, simulated irradiances have been compared to radiation measurements for a case of aged European aerosol in order to explore the validity of model assumptions and the degree of radiative closure that can be attained given the spatial and temporal variability of the observations and their measurement uncertainties. Secondly, the diurnally averaged aerosol radiative effect throughout EUCAARI-LONGREX has been calculated. The surface radiative effect ranged between −3.9 and −22.8 W m−2 (mean −11 ± 5 W m−2), whilst top-of-the-atmosphere (TOA) values were between −2.1 and −12.0 W m−2 (mean −5 ± 3 W m−2). We have quantified the uncertainties in our calculations due to the way in which aerosols and other parameters are represented in a radiative transfer model. The largest uncertainty in the aerosol radiative effect at both the surface and the TOA comes from the spectral resolution of the information used in the radiative transfer model (∼ 17 %) and the aerosol description (composition and size distribution) used in the Mie calculations of the aerosol optical properties included in the radiative transfer model (∼ 7 %). The aerosol radiative effect at the TOA is also highly sensitive to the surface albedo (∼ 12 %)
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Impacts of recent decadal changes in Asian aerosols on the East Asian summer monsoon: roles of aerosol–radiation and aerosol–cloud interactions
Anthropogenic aerosols (AA) can affect cloud and precipitation through aerosol–radiation interaction (ARI) and aerosol–cloud interaction (ACI). Over the past few decades, anthropogenic aerosol emissions have exhibited remarkable changes in the magnitude and in spatial pattern. The most significant changes are the increased emissions over both South Asia and East Asia. In this study, the atmospheric component of a state-of-the-art climate model that includes eight species of tropospheric aerosols, coupled to a multi-level mixed-layer ocean model, has been used to investigate the impacts of Asian anthropogenic aerosol precursor emission changes from 1970s to 2000s on large scale circulation and precipitation in boreal summer over East Asia. Results reveal significant changes in circulation and clouds over East Asia and over the tropical and western North Pacific (WNP). Increased Asian AA emissions lead to anomalous cyclonic circulation over the Maritime continent (MC) and anomalous anticyclonic circulation over the WNP, resulting in anomalous moisture transport convergence over the MC and therefore increased precipitation. They also lead to anomalous moisture flux divergence over both the WNP and large land areas of East Asia, especially over northern China, and therefore decreased precipitation there. These large scale circulation anomalies over the adjacent oceans are related to aerosol change induced ocean feedbacks, predominantly through ACI. It is the slow responses over the adjacent oceans (e.g., SST changes) through coupled atmosphere–ocean interaction in pre-monsoon seasons and summer that shape the changes of the East Asian summer monsoon and local precipitation. The results in this study suggest that increased Asian AA emissions from 1970s to 2000s may have played an important role for the observed southward shift of the Pacific intertropical convergence zone and precipitation belt, weakening of East Asian summer monsoon and reduced precipitation over northern China in East Asia during the latter half of the twentieth century
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Attribution of recent trends in temperature extremes over China: role of changes in anthropogenic aerosol emissions over Asia
Observations indicate large changes in temperature extremes over China during the last four decades, exhibiting as significant increases in the amplitude and frequency of hot extremes and decreases in the amplitude and frequency of cold extremes. An ensemble of transient experiments with a fully coupled atmosphere-ocean model HadGEM3-GC2, including both anthropogenic forcing and natural forcing, successfully reproduces the spatial pattern and magnitude of observed historical trends in both hot and cold extremes. The model simulated trends in temperature extremes primarily come from the positive trends in clear sky longwave radiation, which is mainly due to the increases in greenhouse gases (GHGs). An ensemble of sensitivity experiments with Asian anthropogenic aerosol (AA) emissions fixed at their 1970s levels tends to overestimate the trends in temperature extremes, indicating that local AA emission changes have moderated the trends in these temperature extremes over China. The recent increases in Asian AA drive cooling trends over China by inducing negative clear sky shortwave radiation directly through the aerosol-radiation interaction, which partly offsets the strong warming effect by GHG changes. The cooling trends induced by Asian AA changes are weaker over Northern China during summer, which is due to the warming effect by positive shortwave cloud radiative effect through the AA-induced atmosphere-cloud feedback. This accounts for the observed north-south gradients of the historical trends in some temperature extremes over China, highlighting the importance of local Asian AA emission changes on spatial heterogeneity of trends in temperature extremes
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Mechanisms for a remote response to Asian aerosol emissions in boreal winter
Asian emissions of anthropogenic aerosols have increased rapidly since 1980, with half of the increase since the pre-industrial era occurring in this period. Transient experiments with the HadGEM3-GC2 coupled model were designed to isolate the impact of Asian aerosols on global climate. In boreal winter, it is found that this increase has resulted in local circulation changes, which in turn have driven increases in temperature and decreases in precipitation over China, alongside an intensification of the offshore monsoon flow. Over India, the opposite response is found, with decreasing temperatures and increasing precipitation. The dominant feature of the local circulation changes is an increase in low-level convergence, ascent, and precipitation over the Maritime continent, which forms part of a tropical-Pacific-wide La-Nina-like response.
HadGEM3-GC2 also simulates pronounced far-field responses. A decreased meridional temperature gradient in the North Pacific leads to a positive-Pacific-North-American circulation pattern, with associated temperature anomalies over the North Pacific and North America. An anomalous anticyclonic circulation over the North Atlantic, and an anomalous cyclonic circulation over the Mediterranean drive advection of cold air into Europe, causing cooling in this region. Using a steady-state primitive equation model, LUMA, we demonstrate that these far-field midlatitude response arise primarily as a result of Rossby waves generated over China, rather than in the Equatorial Pacific
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Weakened tropical circulation and reduced precipitation in response to geoengineering
Copyright
© 2014 IOP Publishing LtdContent from this work may be used under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 licence (http://creativecommons.org/licenses/by-nc-sa/3.0/). Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Open Access journalGeoengineering by injection of reflective aerosols into the stratosphere has been proposed as a way to counteract the warming effect of greenhouse gases by reducing the intensity of solar radiation reaching the surface. Here, climate model simulations are used to examine the effect of geoengineering on the tropical overturning circulation. The strength of the circulation is related to the atmospheric static stability and has implications for tropical rainfall. The tropical circulation is projected to weaken under anthropogenic global warming. Geoengineering with stratospheric sulfate aerosol does not mitigate this weakening of the circulation. This response is due to a fast adjustment of the troposphere to radiative heating from the aerosol layer. This effect is not captured when geoengineering is modelled as a reduction in total solar irradiance, suggesting caution is required when interpreting model results from solar dimming experiments as analogues for stratospheric aerosol geoengineering.Natural Environment Research Counci
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A risk-based framework for assessing the effectiveness of stratospheric aerosol geoengineering
Open Access journalCopyright: © 2014 Ferraro et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.Geoengineering by stratospheric aerosol injection has been proposed as a policy response to warming from human emissions of greenhouse gases, but it may produce unequal regional impacts. We present a simple, intuitive risk-based framework for classifying these impacts according to whether geoengineering increases or decreases the risk of substantial climate change, with further classification by the level of existing risk from climate change from increasing carbon dioxide concentrations. This framework is applied to two climate model simulations of geoengineering counterbalancing the surface warming produced by a quadrupling of carbon dioxide concentrations, with one using a layer of sulphate aerosol in the lower stratosphere, and the other a reduction in total solar irradiance. The solar dimming model simulation shows less regional inequality of impacts compared with the aerosol geoengineering simulation. In the solar dimming simulation, 10% of the Earth's surface area, containing 10% of its population and 11% of its gross domestic product, experiences greater risk of substantial precipitation changes under geoengineering than under enhanced carbon dioxide concentrations. In the aerosol geoengineering simulation the increased risk of substantial precipitation change is experienced by 42% of Earth's surface area, containing 36% of its population and 60% of its gross domestic product.Natural Environment Research Council (NERC