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AS3.4

EDI
Reducing uncertainty in regional climate responses to anthropogenic aerosol emissions

Anthropogenic aerosol plays a key role in driving climate anomalies over a range of spatial and temporal scales, both near the emission location and remotely through teleconnections. Aerosols can interact with radiation and clouds, directly and through absorption, microphysics and circulation, and thereby modify the surface and atmospheric energy balance, cloud dynamics and precipitation patterns, and the atmospheric and oceanic circulation. This session addresses progress in understanding the mechanisms and pathways by which aerosols affect regional climate features, overall, over the historical era, and in the near future. We encourage contributions on new model and observation-based approaches to investigate the effects of aerosols on regional decadal climate variability and extremes, tropical-extratropical interactions and teleconnections, and the interplay with modes of variability such as the NAO, AMV, and PDO. Focus studies on monsoon, midlatitude, and Arctic responses, extreme precipitation, circulation changes, daily variability, CMIP6 projections of high and low aerosol futures, and investigations using large ensemble simulations are welcome.

Convener: Laura Wilcox | Co-conveners: Sabine Undorf, Massimo Bollasina, Bjørn Samset
Presentations
| Wed, 25 May, 10:20–11:50 (CEST)
 
Room M2

Wed, 25 May, 10:20–11:50

Chairpersons: Laura Wilcox, Bjørn Samset, Sabine Undorf

10:20–10:22
Introduction

10:22–10:32
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EGU22-3788
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ECS
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solicited
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Andrew Williams et al.

The spatial pattern of anthropogenic aerosol has changed markedly over the historical period and is expected to continue evolving in the coming decades. Additionally, the global composition of anthropogenic aerosol is expected to become relatively more absorbing because policy measures often target sources of scattering and absorbing aerosols differently. However, despite these historical and future changes,  relatively little attention has been given to the potential climatic impacts of the evolving spatial pattern of absorbing aerosol.

In this talk, we will present results from a large ensemble of idealised aerosol absorption experiments with a state-of-the-art climate model to show that the global-mean effective radiative forcing (ERF) from absorbing aerosol strongly depends on their location, driven by rapid adjustments of clouds and circulation. Furthermore, by viewing absorbing aerosol as a localised diabatic heating source we will provide an explanation for this location-dependence of ERF in terms of simple atmospheric dynamics. We will also demonstrate how this approach can be used to understand the sensitivity of local and global precipitation to realistic and idealised changes in the spatial pattern of absorbing aerosol. 

Our results have implications for understanding the climatic impacts of regional aerosol absorption and demonstrate the utility of an ensemble approach to understanding the impacts of variations in the spatial pattern of aerosol.

How to cite: Williams, A., Stier, P., Dagan, G., and Watson-Parris, D.: Strong control of effective radiative forcing and precipitation by the spatial pattern of absorbing aerosol, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3788, https://doi.org/10.5194/egusphere-egu22-3788, 2022.

10:32–10:36
Questions for Andrew

10:36–10:43
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EGU22-1628
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Virtual presentation
Paul-Arthur Monerie et al.

An increase in European and North American anthropogenic aerosol emissions in the 1970s and 1980s led to a decrease in Sahel precipitation during the same time. Although significant, the effect of anthropogenic aerosols on Sahel precipitation is uncertain across a set of CMIP6 single-forcing simulations. However, understanding the cause of this uncertainty in simulated effects of anthropogenic aerosols on West African precipitation in CMIP6 models is difficult, largely due to the relatively small number of large-ensembles with single-forcing simulations. Here, we use a single-model ensemble that spans much of the range in anthropogenic aerosol effective radiative forcing from the CMIP5 and CMIP6 multi-model ensembles. The simulations are performed with HadGEM3-GC3.1 and the different forcings are achieved by scaling emissions in anthropogenic aerosols. We show that changes in anthropogenic aerosols have strong effects on the drivers of the West African monsoon, and on precipitation extremes. Further, we show that the magnitude and even the occurrence of the West African drought (1970s-1980s) strongly depend on the simulated effective anthropogenic aerosol radiative forcing in the model simulations. Our results show that a better understanding of the effects of anthropogenic aerosols on climate is necessary to improve predictions of decadal trends in Sahel precipitation. 

How to cite: Monerie, P.-A., Dittus, A., Wilcox, L. J., and Turner, A. G.: Uncertainty in effects of anthropogenic aerosols on Sahel precipitation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1628, https://doi.org/10.5194/egusphere-egu22-1628, 2022.

10:43–10:50
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EGU22-13284
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ECS
Jitendra Singh et al.

Anthropogenic aerosols (AERs) affect several aspects of the climate system across the world through radiative forcing and microphysical effects. These influences are particularly strong across South Asia, where AER concentrations are highest and further projected to increase in coming decades. Using large ensemble experiments from Earth system model, we examine how AERs shape the evolution of seasonal precipitation over South Asia inlate 20th century and 21st century climate in the presence of rising greenhouse gases (GHGs) concentrations. We find that AERs strongly reduce monsoon precipitation, moderately reduce post-monsoon precipitation, and negligibly influence pre-monsoon precipitation. Consequently, AERs delay the emergence of GHG-forced increases in precipitation by ~5 decades in the monsoon season and ~1 decade in the post-monsoon season. However, GHGs are projected to outpace the influence of AERs by mid 21st century, causing a steep intensification of monsoon and post-monsoon precipitation. We further show that local AERs have the strongest influence on precipitation in the monsoon and post monsoon seasons in the near-future (2020-2049). However, the contribution from remote AERs changes is also important in shaping the monsoon precipitation changes over northwestern South Asia. Further, the influence of local AERs monsoon precipitation remains stationary throughout the 21st century, indicating the insensitivity of relationship between local AOD and precipitation to the projected warming. A better understanding of aerosol-climate interactions and associated precipitation responses in is pertinent for policymakers to address the critical aspect of regional consequences over South Asia induced by externally forced climate change.

How to cite: Singh, J., Marvel, K., Cook, B., Rajaratnam, B., Persad, G., McDermid, S., and Singh, D.: Aerosols delay the emergence of greenhouse gas forcing on 21st century South Asian monsoon precipitation by several decades, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13284, https://doi.org/10.5194/egusphere-egu22-13284, 2022.

10:50–10:57
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EGU22-13286
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ECS
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On-site presentation
Amy Peace et al.

Increasing anthropogenic aerosol emissions have been attributed as the main driver of an observed southward shift in tropical precipitation between the 1950s and 1980s. In the near-term future, anthropogenic aerosol emissions will decline which could drive a northward shift in tropical precipitation over the coming decades. We use a perturbed parameter ensemble (PPE) of transient coupled-ocean atmosphere simulations that span a range of aerosol radiative forcing to investigate the role of aerosol radiative forcing uncertainty on tropical precipitation shifts in the 20th and 21st centuries. We find no relationship between the strength of the hemispheric contrast in pre-industrial to 1975 anthropogenic aerosol radiative forcing and tropical precipitation shifts during the 20th century. This result is in contrary to that from CMIP5, and we suspect internal variability plays a large role in why we do not see the expected relationship in our PPE. Tropical precipitation shifts are associated with major volcanic eruptions over the 20th century. However, we do find a relationship between the hemispheric contrast in pre-industrial to 2005 anthropogenic aerosol radiative forcing and the magnitude of future tropical precipitation shifts over 2006 to 2060 under scenario RCP8.5. Overall, our results suggest that reduction in aerosol radiative forcing uncertainty would improve projections of future precipitation shifts, but any predictive gains would be offset if future major volcanic eruptions temporarily shift tropical precipitation.

How to cite: Peace, A., Booth, B., Regayre, L., Carslaw, K., and Sexton, D.: Evaluating uncertainty in aerosol forcing of tropical precipitation shifts, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13286, https://doi.org/10.5194/egusphere-egu22-13286, 2022.

10:57–11:04
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EGU22-8985
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Virtual presentation
Toshihiko Takemura et al.

The Summary for Policymakers of the Working Group I of the 6th assessment report of the Intergovernmental Panel for Climate Change (IPCC) contained a diagram of the contribution of the global mean change in surface air temperature from the preindustrial to the present climate by the composition of the short-lived climate forces (SLCFs) including aerosols. However, it was estimated by a two-layer energy budget emulator with effective radiative forcing obtained from a model inter comparison project, AerChemMIP. Although the effects of total anthropogenic aerosols have been included in the past, present, and future simulations by climate models, it is essential to estimate and analyze climate change by composition of SLCFs using coupled atmosphere-ocean models in the next step. For example, the amount of temperature change varies significantly with CO2 concentration even when the reduced amount of anthropogenic SO2 emissions and then the instantaneous radiative forcing are the same (Takemura, 2020, doi:10.1038/s41598-020-78805-1). In this study, sensitivity experiments to reduce anthropogenic emissions of SO2, organic matter, and black carbon to zero for each of the 12 regions of the world are simulated using a coupled atmosphere-ocean aerosol model MIROC-SPRINTARS and the results are analyzed in comparison with the experiment under standard emissions. Similar experiments are conducted for biomass burning aerosols from several regions. In the simulations, well-mixed greenhouse gas concentrations are set in two patterns, 2015 and 2060 for SSP3-7.0. The same set of simulations using an atmospheric general circulation model with prescribed sea surface temperature and sea ice are conducted for calculating the effective radiative forcing and rapid adjustment due to each anthropogenic aerosol. This set of experiments also aims to generate scientific knowledge to explore the optimal path for emission reductions of SLCFs and use it in policy making. The project S-20 of the Ministry of the Environment of Japan is also conducting experiments to reduce emissions of SLCFs other than aerosols, as well as experiments to reduce SLCFs emissions using a global cloud-resolving model NICAM. Conducting similar experiments with other climate models and comparing them will enable us to better understand the climate impact of SLCFs with uncertainty.

How to cite: Takemura, T., Sudo, K., Goto, D., and Suzuki, K.: Simulation of climate change due to reducing emission of each anthropogenic aerosol component by region using a coupled atmosphere-ocean model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8985, https://doi.org/10.5194/egusphere-egu22-8985, 2022.

11:04–11:13
Discussion 1

11:13–11:20
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EGU22-12547
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ECS
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On-site presentation
Andrea Dittus et al.

In recent years, there has been increasing interest in how possible future climates at different stabilised, policy relevant global warming levels above pre-industrial might look like. Modelling groups are designing novel climate model simulations to investigate these questions and help answer important questions on the linearity of future climate change across warming levels, tipping points, and climate extremes, among others. A key question is how these projected changes are dependent on scenario choices, particularly the role of future anthropogenic aerosol emissions.

Here, we present the results of new “quasi-stable” climate model simulations with UKESM1.0. Six multi-century simulations have been run under fixed forcings, branching-off from ScenarioMIP simulations of the same model. These simulations explore a range of global warming levels, from approximately 1.5 to 5°C above pre-industrial. In addition, they also explore the role of different balances of forcings for achieving the same target warming level, in particular different combinations of greenhouse gas concentrations and anthropogenic aerosols. In this presentation, we describe how the climate evolves in each of these simulations. We focus on two key aspects:  1) differences between a more stable climate vs. transient climate change at the same warming level and 2) importance of scenario differences, in particular differences in anthropogenic aerosol emissions at the same warming level.

We discuss various aspects of how climate changes in each of the above simulations, including climate extremes, which arguably are one of the most important aspects to consider when assessing the socio-economic impacts of possible future climate conditions at different warming levels and under different scenarios.

How to cite: Dittus, A., Hawkins, E., Collins, M., and Sexton, D.: Climate at various global warming levels: importance of scenario differences , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12547, https://doi.org/10.5194/egusphere-egu22-12547, 2022.

11:20–11:27
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EGU22-7075
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ECS
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Virtual presentation
Kalle Nordling et al.

Regional climate change is what people will experience on their daily lives. However, the regional temperature changes in response to changing greenhouse gases and aerosols vary between current climate models. The origin of these inter-model differences is poorly understood . Here we relate temperature changes in response to different anthropogenic climate forcing agents to changes in atmospheric and oceanic energy fluxes.

We use climate model simulations forced by idealized perturbations in four major anthropogenic climate forcing agents (CO2, CH4, sulfate, and black carbon aerosols) from Precipitation Driver Response Model Intercomparison Project (PDRMIP) climate experiments for six climate models (CanESM2, HadGEM2-ES, NCAR-CESM1-CAM4, NorESM1, MIROC-SPRINTARS, GISS-E2). We decompose the regional temperature change to its  different energy budget components: change in longwave and shortwave fluxes under clear-sky and cloudy conditions, surface albedo changes, and oceanic and atmospheric energy transport. We also analyze the regional model-to-model temperature response spread due to each of these components.

The main physical processes driving global temperature responses vary between forcing agents, but for all forcing agents the model-to-model spread in temperature responses is dominated by differences in modeled changes in effective longwave clear-sky emissivity. Furthermore, in polar regions for all forcing agents, the differences in surface albedo change are a major contributor to temperature response and its spread between models. Our results provide valuable information on what is causing the spread between climate models’ response to various forcing agents. 

How to cite: Nordling, K., Korhonen, H., Räisänen, J., Partanen, A.-I., Samset, B., and Merikanto, J.: Understanding the surface temperature response and its uncertainty to CO2, CH4, black carbon, and sulfate, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7075, https://doi.org/10.5194/egusphere-egu22-7075, 2022.

11:27–11:34
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EGU22-12573
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ECS
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On-site presentation
Michael Lai et al.

Anthropogenic aerosols have been implicated as an important driver of North Atlantic variability. However, the exact mechanism of how aerosol affect the North Atlantic is not well understood and questions remain about the relative importance of aerosols compared to other forcings or internal variability. Therefore, to better understand how aerosols can drive the North Atlantic, we performed idealised experiments using a state-of-the-art coupled climate model (HadGEM3-GC3.1) by applying varying levels of North American and European anthropogenic sulphate aerosol emissions. Medium and (0.25° ocean, ~60km atmosphere) and low-resolution (1° , ~135km) versions of the model were used to assess how model differences may impact on the forced response. We show that the aerosol increases initially cool the North Atlantic SST by a combination of decreased surface shortwave radiation and increased turbulent heat loss. This surface cooling induces surface density anomalies and strengthening of the Atlantic Meridional Overturning Circulation (AMOC), leading to a lagged warming of the Subpolar North Atlantic. However, the AMOC response and subsequent warming is much stronger in the medium-resolution model, despite an overall stronger radiative forcing in the low-resolution model. We show evidence that this AMOC difference is consistent with differences in the sea ice response in a key region of the Subpolar North Atlantic. These results indicate that while surface temperature, sea ice and the AMOC are all sensitive to aerosol forcing in the HadGEM3-GC3.1 models, small regional differences between the model climatologies can significantly alter the pattern and magnitude of the large-scale response.  

How to cite: Lai, M., Robson, J., Wilcox, L., Sutton, R., and Dunstone, N.: Exploring the North Atlantic response to anthropogenic aerosols through idealised single-forcing experiments in a model at two different resolutions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12573, https://doi.org/10.5194/egusphere-egu22-12573, 2022.

11:34–11:41
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EGU22-9731
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Highlight
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Virtual presentation
Buwen Dong et al.

The Eurasian subtropical westerly jet (ESWJ) is a major feature of the summertime atmospheric circulation in the Northern Hemisphere. Here, we demonstrate that four reanalysis datasets show a robust and substantial weakening trend in the summer ESWJ over the last four decades, amounting to a total change of approximately 7%. This weakening has been linked to significant impacts on extreme weather in the northern hemisphere. Furthermore, we use climate model simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6) to identify the causes of the weakening trend. Our results strongly suggest that anthropogenic aerosols were likely the primary driver of the weakening ESWJ. In particular, warming over mid-high latitudes due to aerosol reductions in Europe, and cooling in the tropics and subtropics due to aerosol increases over South and East Asia acted to reduce the meridional temperature gradient at the surface and in the lower and middle troposphere, leading to reduced vertical shear of the zonal wind and a weaker westerly jet in the upper troposphere. Our results suggest that if, as expected, Asian anthropogenic aerosol precursor emissions decline in future, we should anticipate a renewed strengthening of the summer ESWJ.

How to cite: Dong, B., Sutton, R., Shaffrey, L., and Harvey, B.: Recent decadal weakening of the summer Eurasian westerly jet attributable to anthropogenic aerosol emissions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9731, https://doi.org/10.5194/egusphere-egu22-9731, 2022.

11:41–11:50
Discussion 2