AS3.4

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.

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 20^th century and 21^st 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 21^st 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 21^st 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.

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.

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.

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.