Fast responses on pre-industrial climate from present-day aerosols in a CMIP6 multi-model study

. In this work, we use Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations from 10 Earth System Models (ESMs) and General Circulation Models (GCMs) to study the fast climate responses on pre-industrial climate, due to present-day aerosols. All models carried out two sets of simulations; a control experiment with all forcings set to the year 1850 20 and a perturbation experiment with all forcings identical to the control, except for aerosols with precursor emissions set to the year 2014. In response to the pattern of all aerosols effective radiative forcing (ERF), the fast temperature responses are characterised by cooling over the continental areas, especially in the Northern Hemisphere, with the largest cooling over East Asia and India, sulfate being the dominant aerosol surface temperature driver for present-day emissions. In the Arctic there is a warming signal for winter in the ensemble mean of fast temperature responses,

a characteristic dipole pattern with intensification of the Icelandic Low and an anticyclonic anomaly over Southeastern Europe, inducing warm air advection towards the northern polar latitudes in winter.

Introduction
Aerosols interact directly with radiation through scattering and absorption (Haywood and Boucher, 2000) as well as with clouds by acting as cloud condensation nuclei (CCN) and ice nuclei (IN), affecting the Earth's radiation budget and 5 climate (Lohmann and Feichter, 2005), while this impact can be much stronger on a regional scale (Ramanathan and Feng, 2009). On a global scale, aerosols have an inhomogeneous spatial distribution, due to their relatively short lifetime, closely following the patterns of regional emission sources. As a consequence, aerosols have a larger geographical variation in radiative forcing than CO2, with the pattern and spatial gradients of their forcing affecting global and regional temperature responses as well as the hydrologic cycle and precipitation patterns . In general, absorbing aerosols, like 10 black carbon, tend to warm the climate and stabilize the atmosphere, while sulfate aerosols tend to cool the climate (Bond et al., 2013), but the aerosol induced circulation changes influence the spatial patterns of temperature and precipitation response to the regional aerosol forcing, while aerosol-cloud interactions complicate further these responses (Baker et al., 2015;Boucher et al., 2013;Rosenfeld et al., 2014b). While the local influence of aerosols close to their emmision sources has been clearly seen in a number of studies (Bartlett et al., 2018;Ramanathan and Feng, 2009;Sarangi et al., 2018;15 Thornhill et al., 2018;Zhang et al., 2018), their impact can extend beyond their emission regions via fast and slow climate responses (Andrews et al., 2010;Boucher et al., 2013;Kvalevåg et al., 2013). Reduction in sulphur emissions in China was found to lead to increases in temperature in much of the US, northern Eurasia, and the Arctic . Removal of U.S. anthropogenic SO2 emissions showed robust patterns of temperature responses over land, with increases in temperature for most of the Northern Hemisphere land regions and the strongest response towards the Arctic (Conley et al., 20 2018;Shindell et al., 2015). Other recent model studies indicate an amplification of the temperature response towards the Arctic due to local and remote aerosol forcing (Stjern et al., 2017;Westervelt et al., 2018;Stjern et al., 2019). Furthermore, model perturbation simulations with increasing SO2 in Europe, North America, East Asia and South Asia, showed a consistent cooling almost everywhere over the Northern Hemisphere with the Arctic revealing the largest temperature response in all experiments (Lewinschal et al., 2019). The investigation of temperature and precipitation responses to single-25 species forcings in different latitudinal bands showed that the influence of remote forcings on certain regions can often outweigh and even have an opposite sign to the influence of local forcings (Shindell et al., 2012).
The Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) has clarified the importance of distinguishing instantaneous radiative forcing and fast responses (through rapid atmospheric adjustments which modify the radiative budget indirectly) from slow responses through feedbacks (affecting climate variables that are mediated by a 30 change in surface temperature) (Boucher et al., 2013). Τhe dual fast response (or rapid adjustment) and slow response framework has been verified across a range of recent global model studies (Baker et al., 2015;Richardson., https://doi.org/10.5194/acp-2019Richardson., https://doi.org/10.5194/acp- -1201 Preprint. Discussion started: 3 February 2020 c Author(s) 2020. CC BY 4.0 License. 2015; Samset et al., 2016Samset et al., , 2018. Rapid adjustments affect cloud cover and other components of the climate system and thereby alter the global radiation budget indirectly within a few weeks, much faster than responses of the ocean to forcing . Earlier studies have found that rapid adjustments have been important for global precipitation changes (Andrews et al. 2010;Kvalevåg et al. 2013) and regional temperature changes, but generally, the zonal means of slow precipitation and temperature responses are stronger than the fast responses (Samset et al., 2016;Lewinschal et al., 2019;5 Baker et al., 2015;Stjern et al., 2017;Voigt et al., 2017). Under the framework of the Precipitation Driver Response Model Intercomparison Project (PDRMIP), multiple model results indicate that the global fast precipitation response to regional aerosol forcing scales with global atmospheric absorption, and the slow precipitation response scales with global surface temperature response . Another recent PDRMIP multi-model study showed that unlike other drivers of climate change, the response of temperature and cloud profiles to the black carbon (BC) forcing is dominated by rapid 10 adjustments causing weak surface temperature response to increased BC concentrations (Stjern et al., 2017). While some aspects of the regional variation in precipitation and temperature predicted by climate models appear robust, there is still a large degree of inter-model differences unaccounted for, because of uncertainties involved in the related modeling aspects, such as representation of aerosols, their vertical distribution and radiative properties, parameterizations of aerosol removal processes including both wet and dry removal as well as aerosol-cloud interactions (Rosenfeld et al., 2014a;Shindell et al., 15 2015;Wilcox et al., 2015).
A forcing that accounts for rapid adjustments is termed as the effective radiative forcing (ERF) and conceptually represents the change in the net top of the atmosphere (TOA) radiative flux after allowing for atmospheric temperatures, water vapour and clouds to adjust, but with global mean surface temperature or a portion of surface conditions unchanged. A standard method to investigate the fast responses in climate simulations to forcing from aerosols or other short lived climate 20 forcers (SLCFs) is by fixing sea surface temperatures (SSTs) and sea ice cover (SIC) at climatological values, allowing all other parts of the system to respond until reaching steady state (Hansen et al., 2005). In this way, the climate response to a forcing agent in the fixed SST simulations is without any ocean response to climate change and therefore only weakly coupled to feedback processes through land surface responses .
Here, we present a first analysis of the fast responses on pre-industrial climate due to present-day aerosols in a 25 multi-model study based on simulations with 10 CMIP6 models. Section 2 presents the data used and the methodology applied in this study. In Section 3 the key results of this study are presented and discussed, while, finally, in Section 4 the main conclusions are summarized.

Data and methodology
In this work, we use CMIP6 simulations from 10 different models, namely CanESM5, CESM2, CNRM-CM6-1, CNRM-30 ESM2-1, GISS-E2-1-G, IPSL-CM6A-LR, MIROC6, MRI-ESM2-0, NorESM2-LM and UKESM1-0-LL. The aforementioned simulations were implemented within the framework of the Aerosol Chemistry Model Intercomparison Project (AerChemMIP), https://doi.org /10.5194/acp-2019-1201 Preprint. Discussion started: 3 February 2020 c Author(s) 2020. CC BY 4.0 License. which is endorsed by CMIP6 and aims at quantifying the impacts of aerosols and chemically reactive gases on climate and air quality (Collins et al., 2017). All models carried out two sets of simulations considering both aerosol-radiation and aerosolcloud interactions: the piClim-control (with all forcings set to the year 1850 using aerosol precursor emissions of 1850) and the piClim-aer (again with all forcings set to 1850 but using aerosol precursor emissions of the year 2014). All simulations covering at least a period of 30 years in total using fixed climatological average sea surface temperatures (SSTs) and sea ice 5 distributions corresponding the year 1850. Furthermore, concentrations of well mixed greenhouse gases (WMGHGs), emissions of ozone precursors and ozone depleting halocarbons, solar irradiance forcing and land use are also set to the year 1850. The year 1850 is considered here as a pre-industrial period although it could be also assigned as an early industrial period. The perturbation experiments (e.g. piClim-aer) are run similarly for the 30 years period following the control experiments (piClim-control), using the same control SST and sea ice, but with emissions for aerosol precursors set to present-10 day (2014) levels. It has to be noted that only 1 realization is analyzed for each model (see Table 1).
By subtracting the piClim-control simulations from the piClim-aer simulations the fast responses of pre-industrial climate to present-day aerosols are estimated since SST and sea ice distributions are fixed in the simulations. In this work, we examine the effect of aerosols on: 1) net radiative flux (shortwave and longwave) at the top of the atmosphere (TOA) which manifests the ERF, 2) surface air temperature, 3) precipitation and 4) atmospheric circulation (wind and geopotential height at 15 850 hPa). As the horizontal resolution ranges between the different models (from 0.  Table 1. Taking into consideration that the perturbation experiments to the control simulation are based on emissions for 30 aerosol precursors set to present-day (2014), Figure 1 shows the annual SO2 and BC emissions for 2014 used in piClim-aer simulations as well as the differences from their respective emissions for year 1850 used in piClim-control simulations. Figure   1 is based on the emissions used in CNRM-ESM2-1, but the emissions are similar for the rest of the models used here, indicating that the largest present-day sources of SO2 are over East Asia, India, North America and Europe, while for BC over https://doi.org/10.5194/acp-2019-1201 Preprint. Discussion started: 3 February 2020 c Author(s) 2020. CC BY 4.0 License.
East Asia, India and Africa. The differences between piClim-aer and piClim-control in SO2 emissions are peaking over East Asia, India, North America and Europe, while for BC the emissions peak over East Asia, India and spot regions in Africa. The differences between piClim-aer and piClim-control in BC emissions are very low over Europe and North America.

Results and discussion
3.1 Changes in net radiative flux at TOA 5 The difference between piClim-aer and piClim-control simulations in the TOA radiative flux including both the shortwave (SW) and longwave (LW) was calculated for each one of the models to estimate the total aerosol ERF following Forster et al. (2016). The ensemble mean of the aerosol ERF from the 10 models is shown in Figure 2 on an annual basis as well as for the boreal winter/austral summer period including the months December, January, February (DJF) and for the boreal summer/austral winter period including the months June, July, August (JJA). The mean ERF values (global, Northern 10 Hemisphere and South Hemisphere) for each model on an annual basis, DJF and JJA are shown in Table 2   In this study, the piClim-SO2, piClim-BC and piClim-OC simulations were not available for all the participating models to decompose their respective ERF responses. Nevertheless, the available piClim-SO2, piClim-BC and piClim-OC

Near surface temperature changes
The fast temperature responses on pre-industrial climate due to present day aerosols are illustrated in Figure 5 with the differences between piClim-aer and piClim-control in near surface temperature for the ensemble of the 10 models on an 10 annual basis as well as for DJF and JJA, separately. The mean fast temperature response values (global, Northern Hemisphere and South Hemisphere) for each model on an annual basis, DJF and JJA are shown in Table 2 mean surface heating of 0.5-1.1°C, with sulfate aerosols being the dominant surface air temperature driver for the presentday emissions . Another multi-model study indicated a global mean surface temperature increase of 0.7 °C in response to the reduction in SO2, with the zonal mean temperature changes increasing with latitude up to a value of around 2.5 °C at the North Pole (Baker et al., 2015). In a recent modelling study it was shown that removing SO2 emissions from any of the main emission regions in the northern-hemisphere (North America, Europe, East and South Asia) results in 25 significant warming across the northern hemisphere with a preferred spatial pattern, yet a varying magnitude . Simulated surface temperature changes due to the removal of U.S. anthropogenic SO2 emissions revealed robust patterns of temperature responses over land, with increases in temperature for most of the Northern Hemisphere land regions and the strongest response towards the Arctic (Conley et al., 2018;Shindell et al., 2015).
The fast temperature responses for each one of the models on annual basis are illustrated in Figure 6, while Figure  30 S3 and Figure S4 of the supplementary material show the respective patterns for, DJF and JJA. Most models show continental cooling on an annual basis with a robust feature of cooling over East Asia ( Figure 6). However, there are regional https://doi.org/10.5194/acp-2019-1201 Preprint. Discussion started: 3 February 2020 c Author(s) 2020. CC BY 4.0 License. differences among the models in the pattern of induced fast temperature responses over Europe, North America and Africa ( Figure 6). The continental cooling in the Northern Hemisphere becomes stronger in JJA but still we note regional differences in the pattern of fast temperature responses over Europe, North America and Africa ( Figure S4). The available piClim-SO2, piClim-BC and piClim-OC simulations of 3 models (CNRM-ESM2-1, MRI-ESM2-0 and NorESM2) show that the patterns of temperature differences between piClim-SO2 and piClim-control resemble the patterns of the differences 5 between piClim-aer and piClim-control (Figure S10 versus Figure 6). This is in line with previous multi-model studies showing that sulfates are the dominant aerosol surface temperature driver for the present-day emissions (Baker et al., 2015;Samset et al., 2018).
It is interesting to note the slight warming seen in the Arctic on the annual basis ( Siberia and part of the Arctic which could induce adiabatic heating of the subsiding air. Furthermore, there is a characteristic dipole pattern with intensification of the Icelandic Low (cyclonic anomaly) and an anticyclonic anomaly over Southeastern Europe inducing warm air advection towards the northern polar latitudes. Although sea-ice is fixed in these simulations, snow and ice over land can change from the Arctic warming, thus activating albedo feedbacks which could further amplify the warming signal. 20 Several models show this slight warming in regions of the Arctic on an annual basis, with CESM2 and NorESM2 revealing the largest warming signal ( Figure 6). This feature is stronger and more robust among the models during DJF ( Figure S3) implying the role of circulation changes rather than ERF as a plausible cause. For example, for CESM2 and NorESM2 there is no positive ERF to account for the Arctic warming ( Figure S1) but the DJF circulation anomalies ( Figure   S7) reveal a cyclonic (lower GH) anomaly over Europe which in association with an anticyclonic anomaly over Siberia 25 induces a warm advection at the eastern side of the cyclonic anomaly (or western side of the anticyclonic anomaly) towards the polar regions. Furthermore, the available piClim-SO2, piClim-BC and piClim-OC simulations for NorESM2 ( Figure S10) show that the pattern of Arctic warming seen from the temperature differences between piClim-aer and piClim-control ( Figure 6) is similar to the pattern of temperature differences between piClim-SO2 and piClim-control and not to either piClim-BC or piClim-OC ( Figure S10). So, the perturbation experiment with present-day BC emissions cannot justify this 30 warming in NorESM2. This warming signal in the Arctic in response to present-day cooling aerosols is also seen in a PDRMIP multi-model study from the pattern of fast temperature responses (with fixed SST) in perturbation experiments with a five-fold increase in SO4 over Asia or Europe (see Figure 2 in  Asia showed a consistent cooling almost everywhere in the Northern Hemisphere, with the Arctic exhibiting the largest temperature response in all experiments but these results were considering both slow and fast temperature responses (Lewinschal et al., 2019). There are also single-model studies (Sand et al., 2013) and multi-model studies (Stjern et al., 2017) indicating relatively large responses in the Arctic to BC perturbation, but with particularly large inter-model range. 10 The fast precipitation responses on pre-industrial climate due to present day aerosols are illustrated in Figure 7 with the differences between piClim-aer and piClim-control in precipitation for the ensemble of the 10 models on an annual basis as well as for DJF and JJA. Similarly, Figure 9 shows mutli-model study, it was shown that in response to an idealized anthropogenic aerosol, fast and slow ITCZ shifts oppose each other with the slow ITCZ southward shift dominating over the small fast northward ITCZ shift (Voigt et al., 2017). The small fast ITCZ northward shift differs from our results but in the study by Voigt et al. (2017) only aerosol-radiation interactions were considered. Allen and Ajoku (2016) reported that the increase in aerosols over the twentieth century has led to contraction of the northern tropical belt, thereby offsetting part of the widening associated with the increase in GHGs. 5

Precipitation and circulation changes
These processes partially also explain the southward shift of the NH tropical edge from the 1950s to the 1980s (Allen et al., 2014;Brönnimann et al., 2015) and the severe drought in the Sahel that peaked in the mid-1980s (Rotstayn and Lohmann, 2002;Undorf et al., 2018b).  The above-mentioned signals for the annual basis become stronger during the monsoon season JJA (Figure 9c) with the GH and wind vectors anomalies implying a southward shift of the ITCZ and a weakening of the Indian and East Asian monsoon systems. It should be also noted that this is a rather robust feature for all models in JJA ( Figure S8), verifying that 25 the overall negative radiative forcing of aerosols over Northern Hemisphere shifts southward the ITCZ and weakens the Asian monsoon systems. This is a characteristic and robust feature among most of the models utilized in our study, which is associated with a southward shift of the ITCZ and weakening of Indian and East Asian monsoon systems, verifying that the overall negative radiative forcing of aerosols over the Northern Hemisphere shifts southward the ITCZ and weakens the Asian monsoon systems. Summer monsoons rainfall is caused by the faster solar heating of subtropical land compared to the 30 adjacent oceans, which causes convergence and rising of the moist marine air over land. Therefore, the dimming weakens the monsoon flow and precipitation. This has been noted in several previous studies. The response to Asian and European SO2 emissions leads to cooling of East Asia and a weakening of the East Asian summer monsoon with decrease of precipitation over East Asia, and an increase to the south and over the Western North Pacific (Dong et al., 2016). Bartlett et https://doi.org/10.5194/acp-2019Bartlett et https://doi.org/10.5194/acp- -1201 Preprint. Discussion started: 3 February 2020 c Author(s) 2020. CC BY 4.0 License. al., (2018) also show that increased sulfate aerosols in SO2 emission sensitivity experiments under RCP2.6 will lead to surface cooling and weakening of the East Asian monsoon circulation. These processes explain the observed decrease of southeast Asian Monsoon precipitation during the second half of the 20 th century (Bollasina et al., 2011;Krishnan et al., 2016;Lau and Kim, 2017;Lin et al., 2018;Sanap, 2015;Takahashi et al., 2018;Undorf et al., 2018b).

On an annual basis it is characteristic that the dipole pattern of precipitation decreases over East
Another feature in the fast precipitation responses is the relative drying over Africa (southward of Sahel) on an 5 annual basis (Figure 7a), which is apparently a robust signal in all models (Figure 8) (Figure 9b).
An interesting feature in the wind vector and geopotential height (GH) differences at 850 hPa between piClim-aer and piClim-control is the anticyclonic anomaly over northern Siberia and part of the Arctic and a characteristic dipole pattern, with intensification of Icelandic Low (cyclonic anomaly) and an anticyclonic anomaly over Southeastern Europe, inducing warm air advection towards the northern polar latitudes in DJF (Figure 9b). The intensification of the Icelandic 30 Low (cyclonic anomaly) is also apparent in the annual basis ( Figure 9a) and in JJA (Figure 9c) and can be noted in the majority of models (although with spatial shifts) for the annual analysis (Figure 10), for DJF ( Figure S7) and for JJA ( Figure   S8). The pattern of aerosol induced circulation changes in DJF looks similar with the pattern of geopotential height changes at 500 hPa in simulations with predominantly scattering aerosols and opposite in simulations with predominantly absorbing https://doi.org/10.5194/acp-2019-1201 Preprint. Discussion started: 3 February 2020 c Author(s) 2020. CC BY 4.0 License. aerosols (see Figure 12 in Allen and Sherwood, 2011). The deepening of the Icelandic Low is also apparent in the the pattern of changes in sea level pressure in perturbation experiments with a five-fold increase in SO4 over Asia or Europe in a PDRMIP multi-model study (see Figure 7 from . Furthermore, the circulation changes in piClim-aer simulations with the intensification of the Icelandic Low are in line with aerosol induced circulation changes in CMIP6 hist simulations (with opposite sign) in response to aerosol reductions from 1990 to 2020 (Allen et al., in preparation 2019). 5 Hence, it appears that CMIP6 simulations are suggesting a deepening of the Icelandic Low in response to an increase in aerosols and a weakening in response to aerosol decreases. Figure 9b indicates also a slight weakening of the Aleutian Low in DJF but this is not a robust feature for all models ( Figure S7).

Conclusions
In this work, we use the CMIP6 simulations from 10 different models (CanESM5, CESM2, CNRM-CM6-1, The available piClim-SO2, piClim-BC and piClim-OC simulations of 3 models show that sulfates are the dominant aerosol surface temperature driver for the present-day emissions. In the northern polar latitudes, there is a warming signal on an annual basis (up to 0.25 o C) and for DJF (up to 0.45 25 o C) but the model range is large. This Arctic warming signal in DJF is not justified by the regional ERF signal but is presumably linked to aerosol induced circulation changes causing adiabatic heating of the subsiding air over northern Siberia and part of the Arctic, as well as warm air advection from Europe towards the northern polar latitudes. NorESM2 is one of the models showing a strong warming in the Arctic, but the perturbation experiment with present day BC emissions cannot justify this warming. Instead, the pattern of Arctic warming seen from the temperature differences between piClim-aer and 30 piClim-control is resembled by the perturbation experiment with present day SO2 emissions.
The largest fast precipitation responses are seen in the tropical belt regions, generally characterized by reduction over the continental regions. The zonal mean of fast precipitation responses on an annual basis shows overall small reductions over the Northern Hemisphere, but the characteristic feature is the appearance of the larger changes in the tropical belt with a dipole decrease-increase pattern in response to a southward shift of the ITCZ.
The zonal mean precipitation changes in boreal summer show a shift from -0.13 mm/day at 30 o N to 0.04 mm/day 5 at 15 o N, which can be largely justified by the dipole pattern of precipitation decrease over East Asia and increase over southern India, the Bay of Bengal and South China Sea. This is a characteristic and robust feature among most of the models utilized in this study, verifying that the overall negative radiative forcing of aerosols over the Northern Hemisphere shifts the ITCZ southward and weakens the Asian monsoon systems. Summer monsoons rainfall is caused by the faster solar heating of subtropical land compared to the adjacent oceans, which causes convergence and rising of the moist marine air over land. 10 Therefore, the dimming weakens the monsoon flow and precipitation.
It is also noticed that most models in this study yield a drying signal in Africa, shifting from Sahel in boreal summer JJA to southern Africa in austral summer DJF, linked to a weakening of the West African and Southeast African monsoon systems, respectively. Furthermore, we note a drying signal in America, shifting from Central America in boreal summer JJA to South America in austral summer DJF, which is also associated with circulation changes inducing a weakening of the 15 North American and South American Monsoon winds.
An interesting feature in aerosol induced circulation changes is the characteristic dipole pattern with intensification of the Icelandic Low (cyclonic anomaly) and an anticyclonic anomaly over Southeastern Europe, inducing warm air advection towards the northern polar latitudes in DJF. It appears that the deepening of the Icelandic Low in response to an increase in aerosols is a robust feature in the simulations. 20

Figure 9:
Differences between piClim-aer and piClim-control in geopotential height (m) and wind vectors at the 850 hPa pressure level for the ensemble of 10 models on an annual basis (a). for DJF (b) and for JJA (c). The dot shading indicates areas in which the differences are statistically significant at the 95% confidence level. 5 https://doi.org/10.5194/acp-2019-1201 Preprint. Discussion started: 3 February 2020 c Author(s) 2020. CC BY 4.0 License. Figure 10. Annual differences between piClim-aer and piClim-control in geopotential height (gpm) and wind vectors at the 850 hPa pressure level for each one of the models used for the ensemble. The dot shading indicates areas in which the differences are statistically significant at the 95% confidence level. Areas with surface pressure lower than 850 hPa are masked with grey shade. 5 https://doi.org/10.5194/acp-2019-1201 Preprint. Discussion started: 3 February 2020 c Author(s) 2020. CC BY 4.0 License.