atmosphere

Article The Role of Samalas Mega Volcanic Eruption in European Summer Hydroclimate Change

Bin Liu 1 , Jian Liu 1,2,3,*, Liang Ning 1,3,4,5 , Weiyi Sun 1, Mi Yan 1,3, Chen Zhao 6, Kefan Chen 1 and Xiaoqing Wang 7

1 Key Laboratory for Virtual Geographic Environment, Ministry of Education, State Key Laboratory Cultivation Base of Geographical Environment Evolution of Jiangsu Province, Jiangsu Center for Collaborative Innovation in Geographical Information Resource Development and Application, School of Geography, Nanjing Normal University, Nanjing 210023, China; [email protected] (B.L.); [email protected] (L.N.); [email protected] (W.S.); [email protected] (M.Y.); [email protected] (K.C.) 2 Jiangsu Provincial Key Laboratory for Numerical Simulation of Large Scale Complex Systems, School of Mathematical Science, Nanjing Normal University, Nanjing 210023, China 3 Open Studio for the Simulation of Ocean-Climate-Isotope, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266237, China 4 Climate System Research Center, Department of Geosciences, University of Massachusetts, Amherst, MA 01003, USA 5 State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China 6 Institute of Natural Resources and Environmental Audits, Nanjing Audit University, Nanjing 211815, China; [email protected] 7 Hebei Provincial Weather Modification Office, Shijiazhuang 050021, China; [email protected] * Correspondence: [email protected]



Received: 17 August 2020; Accepted: 29 October 2020; Published: 2 November 2020


Abstract: In this study, the role of AD 1258 Samalas mega volcanic eruption in the summer hydroclimate change over Europe and the corresponding mechanisms are investigated through multi-member ensemble climate simulation experiments based on the Community Earth System Model (CESM). The results show that the CESM simulations are consistent with the reconstructed Palmer Drought Severity Index (PDSI) and the historical records of European climate. Europe experiences significant summer cooling in the first three years after the Samalas mega volcanic eruption, peaking at 3.61 C, 4.02 C, and 3.21 C in year 1 over the whole Europe, Southern Europe, and Northern − ◦ − ◦ − ◦ Europe, respectively. The summer surface air temperature (SAT, ◦C) changes over the European continent are mainly due to the direct weakening of shortwave solar radiation induced by volcanic aerosol. The summer precipitation over the European continent shows an obvious dipole distribution characteristic of north-south reverse phase. The precipitation increases up to 0.42 mm/d in year 1 over Southern Europe, while it decreases by 0.28 mm/d in year 1 over Northern Europe, respectively. − Both simulations and reconstructions show that the centers with the strongest increase in precipitation have always been located in the Balkans and Apennine peninsulas along the Mediterranean coast over Southern Europe, and the centers with the strongest precipitation reduction are mainly located in the British Isles and Scandinavia over northwestern Europe. The negative response of North Atlantic Oscillation (NAO) with significant positive sea level pressure (SLP) anomaly in the north and negative SLP anomaly in the south is excited in summer. The low tropospheric wind anomaly caused by the negative phase of NAO in summer affects the water vapor transport to Europe, resulting in the distribution pattern of summer precipitation in Europe, which is drying in the north and wetting in the south. The knowledge gained from this study is crucial to better understand and predict the potential impacts of single mega volcanic eruption on the future summer hydroclimate change in Europe.

Atmosphere 2020, 11, 1182; doi:10.3390/atmos11111182 www.mdpi.com/journal/atmosphere Atmosphere 2020, 11, 1182 2 of 17

Keywords: mega volcanic eruption; hydroclimate change; Europe; Samalas; climate simulation

1. Introduction Volcanic eruption is a major natural cause of interannual to multiannual climate change and has affected human society [1]. Understanding the climate response to volcanic eruptions is important for explaining past climate change [2] and predicting seasonal climate after future volcanic eruptions [3,4]. Especially, the mega volcanic eruption has potentially serious impacts on climate change and ecosystems, needing to attract more attention [5]. The AD 1258 Samalas mega volcanic eruption in Indonesia is the largest sulfur-rich volcanic eruption of the Common Era [5–9]. Sedimentological analyses of the deposits confirm the exceptional scale of this event, which had both an eruption magnitude and a volcanic explosion index of 7 [6]. Previous climate simulations suggested that the Samalas mega volcanic eruption and the following three large sulfur-rich volcanic eruptions in the 13th century triggered the Little Ice Age [10,11]. Recent research also showed that if other external forcings and new volcanic disturbances are excluded, it takes almost two decades for the global and hemispheric cooling caused by Samalas mega volcanic eruption to completely disappear [5]. Based on proxy records and historic evidence, previous studies suggested that the Samalas mega volcanic eruption has a great impact on climate and social economy in Europe [6,7,9,12,13]. Using historical documents, ice core data, and tree ring records, Guillet et al. [6] reconstructed the spatial and temporal climate response of Samalas volcanic eruption and found that Western Europe experienced strong cooling. The medieval chronicles highlight an unusually cold summer, with continuous rain, accompanied by devastating floods and poor harvests [12]. A large number of medieval European documents, including the ‘Monumenta Germaniae Historica,’ ‘Rerum Britannicarum Medii Aevi Scriptores,’ and ‘Recueil des historiens des Gaules et de la France,’ all reported that the cold, incessant rainfall and unusually high cloudiness in AD 1258 prevented crops and fruits from ripening [6]. The Norman ‘Notes of Coutances’ recorded: ‘There was no summer during summer. The weather was very rainy and cold at harvest time, neither the crop harvest nor the grape harvest were good. Grapes could not reach maturity; they were green, altered and in poor health’ [6]. The medieval chronicles in Northern Europe recorded the initial warming in the early winter of AD 1258 after the Samalas eruption, followed by extensive wet and cold climatic conditions in AD 1259, which may have affected crops and contributed to the beginning and severity of famines in some regions of the Northern Hemisphere at that time [7]. Archaeologists determined that the mass burial of thousands of medieval skeletons in London dates back to AD 1258 [14], which may be related to the disturbance in the Northern Hemisphere climate caused by the Samalas mega volcanic eruption [9]. The most serious socio-economic consequences reported at the time of the Samalas mega volcanic eruption came from England, where the famine caused by two consecutive years of poor harvests (AD 1256–1257), high prices, and speculation in AD 1258 may have killed about 15,000 people in London alone [14]. Several sources indicate that in AD 1258 and 1259, parts of Europe (Kingdom of France, Kingdom of England, Holy Roman Empire, Iberian Peninsula) experienced severe food shortages and survival crises [6]. However, the research about the Samalas eruption stated above is mainly based on proxy data and reconstructions. There is currently a lack of research on the response of the European summer climate to the Samalas mega volcanic eruption based on simulation. Further research is needed on the impact mechanisms of Samalas mega volcanic eruption on the European climate. Large volcanic eruption injects sulfur gases into the stratosphere, which will be converted into sulfate aerosols and cool the surface by blocking the incoming solar radiation, thus affecting the climate changes [15]. In Europe, the response of winter hydroclimate to volcanic eruption resembles a positive phase of the North Atlantic Oscillation (NAO) [16–18]. However, the impact of volcanic eruption on the summer hydrological climate over Europe is not fully understood, especially for the mega volcanic Atmosphere 2020, 11, 1182 3 of 17 eruption, such as Samalas. Reconstruction results show that larger volcanic eruptions are more likely to cause wetting and drying responses in Europe than smaller eruptions [19]. In addition, multi-proxy data show that the climate in Europe at the end of the 20th century and the early 21st century may be warmer than at any period over the past 500 years [20]. Therefore, it is very important to have an in-depth understanding of how stratospheric aerosols generated by the Samalas mega volcanic eruption change the hydrological conditions in Europe and potentially offset warming. What role does the Samalas mega volcanic eruption play in the summer hydroclimate changes in Europe? What are the differences in the response of the summer climate to the Samalas event in different sub-regions of Europe? What are the mechanisms behind these influences? To address these questions, we carried out multi-member ensemble climate simulation experiments on the AD 1258 Samalas mega volcanic eruption based on Community Earth System Model (CESM). Therefore, we will also offer a more comprehensive understanding of the historical impacts and mechanisms of Samalas mega volcanic eruption on European summer hydroclimate changes from the perspective of simulation research. The organizational structure of this paper is as follows: Section2 introduces the model and data. Section3 shows the detailed response and mechanisms of European summer hydroclimate to Samalas mega volcanic eruption. Section4 presents the main conclusions and discussion of this study.

2. Model and Data Description

2.1. Model and Experiments The fully-coupled Community Earth System Model 1.0.3 (CESM 1.0.3) version T31_g37 (equivalent to 3.75 3.75 ), which was developed by National Center for Atmospheric Research (NCAR), ◦ × ◦ was used to carry out the climate simulation experiments in this study. The model consisted of the Community Atmosphere Model version 4 (CAM4) [21], the Parallel Ocean Program version 2 (POP2) [22], the Community Land Model version 4 (CLM4) [23], and the Sea Ice Model version 4 (CICE4) [24]. The performance of CESM has been verified by numerous previous studies [25–29]. Firstly, we carried out a 2400-year control experiment (CTRL), which used fixed external forcing conditions of AD 1850, with the first 400 years as a spin-up run [27]. Then, the Samalas mega volcanic experiments (VOL), which contained 8-member ensemble simulations with a length of 20 years, that were performed using the 8 different initial conditions adopted from the CTRL. The volcanic forcing was the only changing external forcing in the VOL. Samalas volcanic forcing was added to the 11th model year of each VOL member, i.e., there was no volcanic disturbance in the first 10 model years of VOL, and the 11th model year in VOL was the Samalas mega volcanic eruption year. The volcanic forcing used to drive the VOL in this study was the reconstructed AD 1258 Samalas mega volcanic forcing based on the Ice-core Volcanic Index 2 [30]. In this study, the Samalas mega volcanic eruption was assumed to begin on April 1st. The eruption year and the first year following the Samalas mega volcanic eruption were named year 0 and year 1, respectively, with the same naming scheme for other years. The summer referred to June–August (JJA) in this study. That means the summer of year 0 referred to the JJA of the eruption year. The anomalies in each VOL experiment were calculated with respect to the 10 years before the Samalas mega volcanic eruption.

2.2. Reconstruction Data The ‘Old World Drought Atlas’ (OWDA), an annual tree-ring-based June–August reconstruction of self-calibrating Palmer Drought Severity Index (PDSI) over Europe and the Mediterranean Basin during the Common Era [31] was used as the hydrological index to validate the simulated summer hydroclimate changes in Europe. Instead of being based purely on precipitation, the PDSI was based upon a primitive water balance model. The basis of the index was the difference between the amount of precipitation required to retain a normal water-balance level and the amount of actual precipitation [32]. Atmosphere 2020, 11, 1182 4 of 17

The OWDA data used the PDSI unit, which used positive values to indicate wet conditions and negative values to indicate dry conditions.

3. Results

3.1. Summer Hydroclimate Change over Europe after Samalas Mega Volcanic Eruption

3.1.1. Comparison of Reconstruction and Simulation In order to validate the summer hydroclimate change in the historical period simulated by the CESM, we compared the ensemble mean precipitation simulation results in VOL with the reconstructed OWDA PDSI index (Figure1). We focused on the ensemble mean results of multi-member simulations when comparing with the reconstructed data. By averaging out the individual simulations, the internal variability was removed, and the impact of Samalas mega volcanic forcing was highlighted, which affects the comparison with reconstructions that are influenced by the combined effects of external forcing and internal variability. However, the magnitude of the Samalas mega volcanic eruption was extremely large, and its impact on the climate was also very strong. Compared with the internal variability, Samalas mega volcanic forcing was likely to dominate the summer hydroclimate changes in Europe after the eruption, at least in the short term. Furthermore, Zanchettin et al. [33] clarified the relative role of forcing uncertainties and initial-condition unknowns in spreading the climate response to volcanic eruptions, and they suggested that forcing uncertainties can overwhelm initial-condition spread in boreal summer. This means that it is relatively reasonable in this study to compare the dry and wet

Atmospherechanges in2020 European, 11, x FOR summerPEER REVIEW between ensemble mean simulation results and the reconstructed5 data.of 17

FigureFigure 1.1. TheThe standardizedstandardized time time series series anomalies anomalies of reconstructed of reconstructed ‘OldWorld ‘Old DroughtWorld Drought Atlas’ (OWDA) Atlas’ (OWDA)Palmer Drought Palmer SeverityDrought Index Severity (PDSI, Index blue (PDS line)I, and blue the line) simulated and the summer simulated (JJA) precipitationsummer (JJA) in precipitationvolcanic experiments in volcanic (VOL) experiments (red line). The(VOL) gray (r shadowed line). area The indicates gray shadow the uncertainty area indicates range of the the uncertainty2-sigma error range of the of PDSI the 2-sigma reconstruction. error of The the year PDSI 0 correspondsreconstruction to. the The eruption year 0 year.corresponds The anomalies to the eruptionare calculated year. withThe respectanomalies to theare 10 calculated years before with the respect Samalas to megathe 10 volcanic years before eruption. the Samalas mega volcanic eruption. The reconstructed JJA OWDA PDSI reflects soil moisture conditions and primarily represents 3.1.2.the warm-season Summer Precipitation hydroclimate Response over Europeto Sama [las31]. Mega As a Volcanic relative indexEruption of drought, over Europe OWDA PDSI has a high degree of comparability across a broad range of precipitation climatologies [32]. Therefore, the OWDAFigure 2 provides shows the a spatial reconstruction response of of hydroclimatic European precipitation variability after that the allows Samalas us to mega compare volcanic the eruption in VOL from year 0 to year 2. As can be seen from the ensemble mean results in the VOL (Figure 2), after the Samalas mega volcanic eruption, the summer precipitation over the European continent shows an obvious dipole distribution characteristic of the north-south reverse phase. There was a significant wetting response in year 0 and year 1 over Southern Europe (10° W–40° E, 35° N–50° N), especially along the Mediterranean coast, while a significant drying response was found over Northern Europe (10° W–40° E, 50° N–70° N). This spatial pattern of summer precipitation change still existed in year 2, but it was weakened. This summer precipitation spatial patterns were in agreement with Fischer et al. [34], Wegmann et al. [35], and Gao et al. [19], although their results also showed an averaged summer wetting response in northeast Europe after the tropical eruptions. Judging from the spatial distribution of precipitation anomaly, the wetting in year 0 and year 1 in Southern Europe was significant, especially the precipitation, which increased most in year 1. Although Northern Europe shows the characteristics of precipitation reduction during year 0–2, the Northern Europe land area only shows significant drought mainly in year 1. Interestingly, after the Samalas mega volcanic eruption, the simulation results of VOL show that the centers with the strongest increase in precipitation have always been located in the Balkans and Apennine peninsulas over Southern Europe, and the centers with the strongest precipitation reduction are mainly located in the British Isles and Scandinavia over northwestern Europe. The results drawn from the VOL simulations are in general agreement with proxy data and reconstructions. Based on seasonal paleoclimate reconstructions, Rao et al. [36] found that wet conditions occur in the eruption year and the following three years in the western Mediterranean. Conversely, northwestern Europe and the British Isles experienced dry conditions in response to volcanic eruptions. This good consistency once again indicates that the simulation results in VOL can well explain the characteristics and causes of summer hydroclimate changes in Europe after the Samalas event recorded in historical documents and reconstructions.

Atmosphere 2020, 11, 1182 5 of 17 simulated hydroclimate variability with the reconstructed dry and wet changes in Europe after Samalas mega volcanic eruption. In this study, we used JJA precipitation changes to represent the simulated summer hydroclimate changes in Europe. Due to the different units between the simulated precipitation and the reconstructed PDSI, we have standardized these two data for the convenience of comparison. As can be seen from Figure1, after the Samalas mega volcanic eruption, the fluctuation changes of precipitation in VOL and reconstructed PDSI index were relatively consistent. The simulation results were always within the range of two times the standard deviation of PDSI. The correlation coefficient between the standardized JJA precipitation anomaly in VOL and OWDA PDSI was 0.75 (at the 95% confidence level) over Europe (10◦ W–40◦ E, 35◦ N–70◦ N). Both reconstruction and simulations show that the hydrological changes in Europe were humid in the first two years after the Samalas mega volcanic eruption and then turned into drought. CESM has well simulated the dry and wet changes in Europe after the Samalas mega volcanic eruption.

3.1.2. Summer Precipitation Response to Samalas Mega Volcanic Eruption over Europe Figure2 shows the spatial response of European precipitation after the Samalas mega volcanic eruption in VOL from year 0 to year 2. As can be seen from the ensemble mean results in the VOL (Figure2), after the Samalas mega volcanic eruption, the summer precipitation over the European continent shows an obvious dipole distribution characteristic of the north-south reverse phase. There was a significant wetting response in year 0 and year 1 over Southern Europe (10◦ W–40◦ E, 35◦ N–50◦ N), especially along the Mediterranean coast, while a significant drying response was found over Northern Europe (10◦ W–40◦ E, 50◦ N–70◦ N). This spatial pattern of summer precipitation change still existed in year 2, but it was weakened. This summer precipitation spatial patterns were in agreement with Fischer et al. [34], Wegmann et al. [35], and Gao et al. [19], although their results also showed an averaged summer wetting response in northeast Europe after the tropical eruptions. Judging from the spatial distribution of precipitation anomaly, the wetting in year 0 and year 1 in Southern Europe was significant, especially the precipitation, which increased most in year 1. Although Northern Europe shows the characteristics of precipitation reduction during year 0–2, the Northern Europe land area only shows significant drought mainly in year 1. Interestingly, after the Samalas mega volcanic eruption, the simulation results of VOL show that the centers with the strongest increase in precipitation have always been located in the Balkans and Apennine peninsulas over Southern Europe, and the centers with the strongest precipitation reduction are mainly located in the British Isles and Scandinavia over northwestern Europe. The results drawn from the VOL simulations are in general agreement with proxy data and reconstructions. Based on seasonal paleoclimate reconstructions, Rao et al. [36] found that wet conditions occur in the eruption year and the following three years in the western Mediterranean. Conversely, northwestern Europe and the British Isles experienced dry conditions in response to volcanic eruptions. This good consistency once again indicates that the simulation results in VOL can well explain the characteristics and causes of summer hydroclimate changes in Europe after the Samalas event recorded in historical documents and reconstructions. In order to better quantify the summer precipitation response in different European regions to Samalas mega volcanic eruption, we divided Europe into two sub-regions: Southern Europe (10◦ W–40◦ E, 35◦ N–50◦ N) and Northern Europe (10◦ W–40◦ E, 50◦ N–70◦ N) according to the spatial distribution characteristics of precipitation changes after the Samalas event (Figure2), and calculated the average precipitation responses of the whole Europe and each sub-region (Figure3). Atmosphere 2020, 11, 1182 6 of 17 Atmosphere 2020, 11, x FOR PEER REVIEW 6 of 17

FigureFigure 2. The 2. Thespatial spatial pattern patternss of ofEuropean European summer summer (JJA) precipitationprecipitation (mm (mm/d)/d) anomalies anomalies after after the the SamalasSamalas mega mega volcanic volcanic eruption eruption in in VOL inin ( a()a year) year 0, (b0,) year(b) year 1, and 1, ( cand) year (c 2.) year The dots2. The denote dots areas denote areaswith with confidence confidence exceeding exceeding the 95%the 95% level. level. The anomaliesThe anomalies are calculated are calculated with respect with to respect the 10 yearsto the 10 yearsbefore before the the Samalas Samalas mega mega volcanic volcanic eruption. eruption. The year The 0 year corresponds 0 corresponds to the eruption to the eruption year. year.

In order to better quantify the summer precipitation response in different European regions to Samalas mega volcanic eruption, we divided Europe into two sub-regions: Southern Europe (10° W– 40° E, 35° N–50° N) and Northern Europe (10° W–40° E, 50° N–70° N) according to the spatial distribution characteristics of precipitation changes after the Samalas event (Figure 2), and calculated the average precipitation responses of the whole Europe and each sub-region (Figure 3).

Atmosphere 2020, 11, 1182 7 of 17 Atmosphere 2020, 11, x FOR PEER REVIEW 7 of 17

FigureFigure 3.3. The time time series series of of the the summer summer (JJA) (JJA) precipitation precipitation (mm/d) (mm/d) anomalies anomalies over over (a () athe) the whole whole of ofEurope, Europe, (b) ( bSouthern) Southern Europe, Europe, and and (c) (Northernc) Northern Europe Europe in VOL. in VOL. The The black black line line in ( ina), (reda), redline line in (b in), (andb), and blue blue line linein (c in) indicate (c) indicate the theensemble ensemble mean mean results. results. Other Other each each different different colorful colorful line lineis for is one for oneensemble ensemble member. member. The The anomalies anomalies are are calculated calculated wi withth respect respect to to the the 10 10 years years before the SamalasSamalas megamega volcanicvolcanic eruption.eruption. The black dots in (a), redred dotsdots inin ((bb),), andand blueblue dotsdots inin ((cc)) indicateindicate thatthat thethe anomaliesanomalies areare significantsignificant atat aa 95%95% confidenceconfidence levellevel basedbased onon thethe StudentStudent t-test.t-test. YearYear 00 correspondscorresponds toto thethe eruptioneruption year.year.

AsAs shownshown in in the the time time series series in Figure in Figure3, for the 3, wholefor the of Europe,whole of the Europe, ensemble the mean ensemble precipitation mean increasedprecipitation significantly increased insignificantly year 0 (peaking in year at 0 0.11 (pea mmking/d) at and0.11 yearmm/d) 1, reachingand year the1, reaching 95% confidence the 95% level,confidence and the level, European and the mean European precipitation mean precipitat anomalyion returned anomaly to areturned normal to state a normal in year state 2 (Figure in year3a). 2 Gao(Figure et al. 3a). [19 Gao] also et foundal. [19] similaralso found European similar precipitation European precipitation changes after changes tropical after volcanic tropical eruptions volcanic usingeruptions the reconstructedusing the reconstructed Old World Old Drought World Atlas Drought (OWDA) Atlas [(OWDA)31]. After [31]. the SamalasAfter the megaSamalas volcanic mega eruption,volcanic eruption, the precipitation the precipitation in Southern in Southern Europe increased Europe increased significantly significantly (at 95% confidence (at 95% confidence level) in thelevel) first in threethe first years three (year years 0–2) (year and 0–2) peaked and peaked with a with value a ofvalue about of about 0.42 mm 0.42/d mm/d in year in year 1 (Figure 1 (Figure3b). A3b). large A numberlarge number of medieval of medieval European European chronicles chro andnicles documents and documents also reported also thatreported the incessant that the rainfallincessant in ADrainfall 1258 in prevented AD 1258crops prevented and fruits crops from and ripening, fruits from resulting ripening, in severe resulting food in shortages severe food and survivalshortages crises and insurvival AD 1258 crises and 1259in AD in 1258 parts and of Europe 1259 in such parts as theof Europe Holy Roman such as Empire the Holy and IberianRoman PeninsulaEmpire and [6, 7Iberian,12]. In Peninsula contrast, the[6,7,12]. precipitation In contrast, in Northern the precipitation Europe decreasedin Northern significantly Europe decreased (at 95% confidencesignificantly level), (at 95% especially, confidence the droughtlevel), especially, was most the severe drought in year was 1 withmost the severe precipitation in year 1 anomalywith the peakingprecipitation at 0.28 anomaly mm/d peaking (Figure3 c).at − The0.28 di mm/dfferent (Figure precipitation 3c). The responses different ofprecipitation Northern and responses Southern of − EuropeNorthern to Samalasand Southern mega volcanic Europe eruptionto Samalas can explainmega volcanic the precipitation eruption changes can explain in the the whole precipitation of Europe tochanges some extent. in the whole In year of 0 andEurope year to 1, some the overall extent. wetting In year in 0 theand whole year 1, of the Europe overall was wetting due to in the the fact whole that theof Europe significant was precipitation due to the increasefact that in the Southern significan Europet precipitation was stronger increase than the in precipitation Southern Europe decrease was in Northernstronger Europe,than the which precipitation dominated decrease the increase in No ofrthern precipitation Europe, in which the whole dominated of Europe the in increase time series. of However,precipitation the in precipitation the whole of increase Europe in in Southern time series. Europe However, was almost the precipitation equivalent increase to the precipitation in Southern decreaseEurope was in Northern almost equivalent Europe in yearto the 2, whichprecipitation made the decrease change in of Northern spatial average Europe precipitation in year 2, which in the wholemade the of Europe change less of spatial obvious. average precipitation in the whole of Europe less obvious.

3.1.3.3.1.3. Summer TemperatureTemperature ResponseResponse toto SamalasSamalas MegaMega VolcanicVolcanic Eruption Eruption over over Europe Europe

TheThe ensembleensemble meanmean changeschanges ofof summersummer surfacesurface airair temperaturetemperature (SAT,(SAT, ◦°C)C) anomalyanomaly afterafter thethe SamalasSamalas megamega volcanic volcanic eruption eruption in VOLin VOL are shownare shown in Figure in Figure4. Europe 4. Europe experienced experienced significant significant cooling incooling the first in the three first years three (year years 0–2) (year after 0–2) the after Samalas the Samalas eruption eruption (Figure (Figure4a–c). 4a–c). These These results results were were in agreementin agreement with with Fischer Fischer et et al. al. [34 [34],], who who analyzed analyzed the the European European summer summer climatic climaticsignal signal followingfollowing 1515 majormajor tropicaltropical volcanicvolcanic eruptionseruptions overover thethe lastlast 500500 yearsyears basedbased onon multi-proxymulti-proxy reconstructions,reconstructions, andand suggestedsuggested thatthat thethe averageaverage influenceinfluence ofof 1515 majormajor volcanicvolcanic eruptionseruptions waswas aa significantsignificant EuropeanEuropean continentalcontinental scalescale summersummer coolingcooling duringduring thethe firstfirst andand secondsecond post-eruptionpost-eruption years.years. TheThe strongeststrongest andand highlyhighly significantsignificant coolingcooling signalsignal waswas foundfound in thethe summersummer of year 1 after the eruption (Figure4 4b).b). Spatially,Spatially, thethe sharpsharp coolingcooling responseresponse waswas concentratedconcentrated inin SouthernSouthern Europe,Europe, whichwhich waswas significantlysignificantly strongerstronger thanthan inin NorthernNorthern EuropeEurope inin yearyear 0.0. Then,Then, thethe entire EuropeanEuropean landland areaarea experiencedexperienced aa sharpsharp dropdrop inin temperaturetemperature inin yearyear 1.1. InIn thethe summersummer ofof yearyear 2,2, thethe coolingcooling overover thethe EuropeanEuropean continentcontinent waswas still significant, but it was obviously weaker than before, and the areas with the significant cooling retreated to the interior of Eastern Europe.

Atmosphere 2020, 11, x FOR PEER REVIEW 8 of 17

The time series of summer SAT anomaly over the whole of Europe, Southern Europe, and Northern Europe are shown in Figure 5. The whole of Europe and each sub-region show similar SAT variations, with significant cooling anomaly (at 95% confidence level) in the first three years after the Samalas eruption (Figure 5a–c). For the whole of Europe (Figure 5a), the summer cooling peaks during the eruption year and the first year after the eruption, which was in agreement with the European summer temperature response to the strong tropical volcanic events over the last millennium reported by Luterbacher et al. [37] using the Paleo Model Intercomparison Project Phase 3 (PMIP3) climate model simulations. The largest significant SAT reduction appears in year 1 over the whole of Europe, Southern Europe, and Northern Europe with values of −3.61 °C , −4.02 °C, and

−3.21 °CAtmosphere, respectively.2020, 11, 1182 From year 0 to year 2, the cooling in Southern Europe has 8 always of 17 been stronger than that in Northern Europe. Based on historical documents, ice core data, and tree ring records, previous studies also found that Europe experienced an unusually cold summer after the still significant, but it was obviously weaker than before, and the areas with the significant cooling Samalasretreated mega volcanic to the interior eruption of Eastern [6,7 Europe.,12].

Figure 4.Figure The 4. spatialThe spatial pattern patternss of of ensemble mean mean European European summer summer (JJA) surface (JJA) air surface temperature air (SAT, temperature (SAT, °C◦C)) anomalies anomalies after after the the Samalas Samalas mega mega volcanic volcanic eruption eruption in VOL in in (a )VOL year 0,in (b ()a year) year 1, and0, (b (c) year 2. 1, and (c) year 2. TheThe dotsdots denote denote areas areas with with confidence confidence exceeding exceeding the 95% the level. 95% The level. anomalies The areanomalies calculated are with calculated respect to the 10 years before the Samalas mega volcanic eruption. The year 0 corresponds to the with respect to the 10 years before the Samalas mega volcanic eruption. The year 0 corresponds to the eruption year. eruption year. The time series of summer SAT anomaly over the whole of Europe, Southern Europe, and Northern Europe are shown in Figure5. The whole of Europe and each sub-region show similar SAT variations, with significant cooling anomaly (at 95% confidence level) in the first three years after the Samalas eruption (Figure5a–c). For the whole of Europe (Figure5a), the summer cooling peaks during the eruption year and the first year after the eruption, which was in agreement with the European summer temperature response to the strong tropical volcanic events over the last millennium reported by Luterbacher et al. [37] using the Paleo Model Intercomparison Project Phase 3 (PMIP3) climate model simulations. The largest significant SAT reduction appears in year 1 over the whole of Europe, Southern Atmosphere 2020, 11, 1182 9 of 17

Europe, and Northern Europe with values of 3.61 C, 4.02 C, and 3.21 C, respectively. From year − ◦ − ◦ − ◦ 0 to year 2, the cooling in Southern Europe has always been stronger than that in Northern Europe. Based on historical documents, ice core data, and tree ring records, previous studies also found that EuropeAtmosphere experienced 2020, 11, x FOR an PEER unusually REVIEW cold summer after the Samalas mega volcanic eruption [6,7,129]. of 17

FigureFigure 5. 5.The The time time series series of of the the summer summer (JJA) (JJA) surface surface air air temperature temperature (SAT, (SAT,◦C) °C anomalies) anomalies over over (a) the (a) wholethe whole of Europe, of Europe, (b) Southern (b) Southern Europe, Europe, and ( andc) Northern (c) Northern Europe Europe in VOL. in VO TheL. black The black line in line (a), in red (a), line red inline (b), in and (b), blue and lineblue in line (c) in indicate (c) indicate the ensemble the ensemble mean mean results. results. Other Other each each different different colorful colorful line isline for is onefor ensembleone ensemble member. member. The anomalies The anomalies are calculated are calculated with respect with respect to the 10 to years the 10 before years the before Samalas the megaSamalas volcanic mega eruption.volcanic eruption. The black The dots black in (a dots), red in dots (a), inred (b dots), and in blue(b), and dots blue in (c )dots indicate in (c) that indicate the anomaliesthat the anomalies are significant are significant at a 95% confidence at a 95% levelconfiden basedce onlevel the based Student on t-test.the Student The year t-test. 0 corresponds The year 0 tocorresponds the eruption to year. the eruption year. 3.2. Mechanisms of European Summer Hydroclimate Changes after the Samalas Mega Volcanic Eruption 3.2. Mechanisms of European Summer Hydroclimate Changes after the Samalas Mega Volcanic Eruption The surface heat flux was analyzed to understand the mechanisms underlying the response of The surface heat flux was analyzed to understand the mechanisms underlying the response of the European summer hydroclimate changes to the Samalas mega volcanic eruption. The ensemble the European summer hydroclimate changes to the Samalas mega volcanic eruption. The ensemble means of surface net shortwave flux (FSNS), surface net longwave flux (FLNS), surface sensible heat means of surface net shortwave flux (FSNS), surface net longwave flux (FLNS), surface sensible heat flux (SHFLX), and surface latent heat flux (LHFLX) in VOL are shown in Figure6. The surface net flux (SHFLX), and surface latent heat flux (LHFLX) in VOL are shown in Figure 6. The surface net shortwave radiation over the European land area decreased significantly, with values of 27.62 W/m2, shortwave radiation over the European land area decreased significantly, with values− of −27.62 19.57 W/m2, and 0.58 W/m2 in year 0, year 1, and year 2, respectively (Figure6a–c). The spatial −W/m2, −19.57 W/m−2, and −0.58 W/m2 in year 0, year 1, and year 2, respectively (Figure 6a–c). The pattern variation of shortwave radiation was similar to that of temperature. The FSNS decrease in spatial pattern variation of shortwave radiation was similar to that of temperature. The FSNS Southern Europe was larger than that in Northern Europe in year 0, and it was significantly reduced in decrease in Southern Europe was larger than that in Northern Europe in year 0, and it was both Southern Europe and Northern Europe in year 1. Then, the FSNS decrease was weakened and significantly reduced in both Southern Europe and Northern Europe in year 1. Then, the FSNS retreated to Eastern Europe in year 2. The pattern correlation coefficients between the SAT (Figure4) decrease was weakened and retreated to Eastern Europe in year 2. The pattern correlation and FSNS (Figure6a–c) are 0.94, 0.86, and 0.39, respectively, in year 0, year 1, and year 2 over the coefficients between the SAT (Figure 4) and FSNS (Figure 6a–c) are 0.94, 0.86, and 0.39, respectively, European land area. This suggests that the summer SAT changes over the European continent are in year 0, year 1, and year 2 over the European land area. This suggests that the summer SAT largely due to the release of a large amount of sulfur dioxide into the stratosphere and its conversion changes over the European continent are largely due to the release of a large amount of sulfur into sulfate aerosol after the eruption. Volcanic aerosols weaken the shortwave solar radiation reaching dioxide into the stratosphere and its conversion into sulfate aerosol after the eruption. Volcanic the surface in a short period of time, resulting in a significant decrease in the net shortwave radiation aerosols weaken the shortwave solar radiation reaching the surface in a short period of time, on the surface of the European land area, which lasts for 2–3 years (Figure6a–c). The decrease of solar resulting in a significant decrease in the net shortwave radiation on the surface of the European radiation directly leads to the drop in temperature in year 0–2. Compared with shortwave radiation, land area, which lasts for 2–3 years (Figure 6a–c). The decrease of solar radiation directly leads to the the reduction of surface net longwave radiation (Figure6d–f) over the European continent was much drop in temperature in year 0–2. Compared with shortwave radiation, the reduction of surface net weaker, with a reduction of 4.74 W/m2, 2.56 W/m2, and 1.30 W/m2 in year 0, year 1, and year 2, longwave radiation (Figure −6d–f) over the− European continent was much weaker, with a reduction respectively, after the Samalas mega volcanic eruption. of −4.74 W/m2, −2.56 W/m2, and 1.30 W/m2 in year 0, year 1, and year 2, respectively, after the Samalas mega volcanic eruption. Following the mega volcanic eruption of Samalas, the sensible heat flux (Figure 6g–i) over the European land area mainly decreased, especially over Southern Europe, but did not decrease over the Atlantic Ocean, which indicates that the decrease of SAT on the land is larger than that on the Atlantic Ocean, thus leading to the decrease of the land-sea thermal contrast. From year 1 to year 2, SHFLX continued to decrease in Southern Europe, while it gradually showed a slight increase in Northern Europe (Figure 6h,i). This may be the potential reason for the spatial difference between the summer precipitation increase in Southern Europe and the precipitation decrease in Northern Europe after the Samalas volcanic eruption. Using a range of climate modeling results, Myhre et al. [38] also pointed out that over the historical period, the sensible heat at the surface was gradually reduced in the CMIP5 models, and hence contributes to an increase in precipitation. Furthermore, the increase in surface latent heat flux (Figure 6j–l) over the North Atlantic region along the

Atmosphere 2020, 11, x FOR PEER REVIEW 10 of 17

EuropeanAtmosphere 2020 coast,, 11, 1182especially in year 1 and year 2, indicated an increase in Atlantic evaporation, which10 of 17 may lead to an increase in precipitation in Southern Europe.

Figure 6. TheThe spatial spatial patterns patterns of ensemble mean ( a–c)) surface surface net net shortwave shortwave flux flux (FSNS, W/m W/m2),), ( (dd––ff)) surface net longwave flux flux (FLNS, W/m W/m2),), ( (gg––ii)) surface surface sensible sensible heat heat flux flux (SHFLX, W/m W/m2)) and and ( (jj––ll)) surface latentlatent heatheat flux flux (LHFLX, (LHFLX, W W/m/m2)2 anomalies) anomalies in summerin summer (JJA) (JJA) over over Europe Europe after after the Samalas the Samalas mega megavolcanic volcanic eruption eruption in VOL in from VOL year from 0 to year 2.0 Theto year dots 2. denote The dots areas denote with confidence areas with exceeding confidence the exceeding95% level. the The 95% anomalies level. The are anomalies calculated are with calculated respect to with the 10respect years to before the 10 the years Samalas before mega the Samalas volcanic megaeruption. volcanic The yeareruption. 0 corresponds The year to0 corresponds the eruption to year. the eruption year.

FigureFollowing 7 shows the mega the volcanicsea level eruptionpressure of (SLP) Samalas, anomalies the sensible in summer heat flux after (Figure the Samalas6g–i) over mega the volcanicEuropean eruption. land area The mainly ensemble decreased, mean especiallyresult shows over a Southerndipole-type Europe, response, but didwith not significant decrease overpositive the Atlanticpressure Ocean,anomaly which in the indicates north thatand the negative decrease pressure of SAT onanomaly the land in is the larger south. than This that onis thean obvious Atlantic negativeOcean, thus phase leading of NAO. to the These decrease anom ofalies the are land-sea similar thermal to the negative contrast. tropospheric From year 1 Arctic to year Oscillation 2, SHFLX patterncontinued in late to decrease winter and in Southernearly spring Europe, after whilethe Laki it graduallyeruption found showed in Zambri a slight et increase al. [39], in which Northern was manifestedEurope (Figure as 6anh,i). equatorward This may be shift the potential of the mid-la reasontitude for the tropospheric spatial di fference jet. NAO between is a thelarge-scale summer meridionalprecipitation oscillation increase in of Southern atmospheric Europe mass and between the precipitation the subtropi decreasecal high-pressure in Northern system Europe near after the AzoresSamalas and volcanic the subpolar eruption. low-pressure Using a range system of climate near modelingIceland [40]. results, It is Myhrewidely etconsidered al. [38] also to pointed be the mostout that important over the mid-latitude historical period, source the of sensible temperature heat at and the surfaceprecipitation was gradually changes reduced over Europe in the [41,42]. CMIP5 Previousmodels, andstudies hence [16–18,34] contributes have to shown an increase that a positive in precipitation. phase of Furthermore,NAO in boreal the winter increase will be in surfaceexcited afterlatent the heat large flux (Figuretropical6 j–l)volcanic over theeruption. North AtlanticHere, our region results along further the European show that coast, NAO’s especially negative in responseyear 1 and to yearthe large 2, indicated tropicalan eruption increase in insummer Atlantic is evaporation,totally different which from may that lead in winter. to an increase Based on in extratropicalprecipitation inNorth Southern Atlantic–European Europe. mean sea level pressure anomalies for 1881–2003, Folland et al. [43]Figure proposed7 shows the the summertime sea level pressure parallel (SLP) of th anomaliese winter NAO in summer known after as the the summer Samalas North mega volcanicAtlantic Oscillationeruption. The (SNAO). ensemble This mean SNAO result is showsdefined a dipole-typeas the first response, empirical with orthogonal significant function positive (EOF) pressure of observedanomaly insummertime the north and extratropical negative pressure North anomalyAtlantic pressure in the south. at mean This issea an level obvious and negative has a smaller phase spatialof NAO. extent These than anomalies the winter are NAO similar and to theis located negative farther tropospheric north [43]. Arctic Compared Oscillation with pattern Folland in et late al. winter[43], we and found early that spring the after SLP theanomalies Laki eruption in summer found inafter Zambri the Samalas et al. [39 ],mega which volcanic was manifested eruption aswas an moreequatorward like the shift typical of the definition mid-latitude of NAO-like tropospheric negative jet. NAO phase, is a large-scalewhich shows meridional that the oscillation meridional of atmospheric mass between the subtropical high-pressure system near the Azores and the subpolar

Atmosphere 2020, 11, 1182 11 of 17 low-pressure system near Iceland [40]. It is widely considered to be the most important mid-latitude source of temperature and precipitation changes over Europe [41,42]. Previous studies [16–18,34] have shown that a positive phase of NAO in boreal winter will be excited after the large tropical volcanic eruption. Here, our results further show that NAO’s negative response to the large tropical eruption in summer is totally different from that in winter. Based on extratropical North Atlantic–European mean sea level pressure anomalies for 1881–2003, Folland et al. [43] proposed the summertime parallel of the winter NAO known as the summer North Atlantic Oscillation (SNAO). This SNAO is defined as the first empirical orthogonal function (EOF) of observed summertime extratropical North Atlantic pressure at mean sea level and has a smaller spatial extent than the winter NAO and is located farther north [43]. Compared with Folland et al. [43], we found that the SLP anomalies in summer after the Samalas mega volcanic eruption was more like the typical definition of NAO-like negative phase, which shows that the meridional pressure gradient between Iceland and the Azores was weakened in summer (Figure7). In addition, during year 1–year 2, this summer NAO negative phase gradually coincided with the SNAO negative phase pattern in Folland et al. [43] (Figure7). Although the low-pressure center of the summer NAO was located further south in our results (Figure7), this may be due to the fact that the short-term impact intensity and spatial scale of the external forcing of Samalas mega volcanic eruption on summer NAO were much greater than the impact of the gradual increase of greenhouse gases. Combining reconstructions with ECHAM5.4 simulation results, Wegmann et al. [35] also found that volcanic-induced cooling leads the summer NAO toward a negative phase, which causes the southward movement of the storm track and enhances moisture advection toward southern Europe. In addition, the model simulated summer NAO in Wegmann et al. [35] was also located further south than the SNAO suggested by Folland et al. [43]. To further investigate the relationship between the summer hydroclimate over Europe and the circulation changes, the 850-hPa wind patterns associated with the negative NAO phase are also analyzed as shown in Figure7 (vector). In the eruption year (year 0), the 850-hPa wind anomalies are characterized by a strong cyclone over the Balkans and Apennine peninsulas near the Mediterranean Sea (Figure7a). Southwesterly airflow in the lower troposphere transports a large amount of warm and moist air from the Mediterranean to the Balkans, which is conducive to precipitation and makes Southern Europe near the Balkans form a precipitation increase center in year 0. There is also a cyclonic circulation over the Atlantic Ocean, but its location is to the west, which transports less warm and wet Atlantic air to the Iberian Peninsula in southwest Europe (Figure7a). At the same time, Northern Europe is controlled by a high-pressure anticyclone, resulting in the cold polar air to be blocked by the high pressure and brought to northern and central Europe, while warm air from the south cannot be transported to Northern Europe, which is not conducive to precipitation (Figure7a). In year 1, the surface latent heat flux over the Atlantic Ocean increases (Figure6k), indicating an increase in evaporation. At the same time, a strong low-pressure cyclonic circulation over the Atlantic Ocean extends eastward to the land area of Southern Europe, and southwest wind anomalies prevail in Southern Europe (Figure7b). This circulation change helps to transport a large amount of warm and moist air from the Atlantic Ocean to the Iberian Peninsula in southwest Europe and more moist air from the Mediterranean Sea to the Balkans and Apennine peninsulas, resulting in a significant increase in precipitation throughout Southern Europe. In addition, as can be seen from Figure7b, the southwest wind anomaly in the Mediterranean is stronger than that in the Atlantic Ocean, indicating that the Mediterranean Sea may be a more important source of water vapor for the increase of summer precipitation in Southern Europe than the Atlantic Ocean after the Samalas mega volcanic eruption. During the same period, Northern Europe is controlled by a strong high-pressure anticyclone, with anomalous strong northerly winds, and warm air could not be transported northward to Northern Europe, resulting in a significant reduction in precipitation there. In year 2, Southern Europe is basically controlled by low pressure, and the high pressure over Northern Europe weakens (Figure7c). The circulation anomalies in year 2 is similar to that in year 1, however, the northerly winds Atmosphere 2020, 11, x FOR PEER REVIEW 11 of 17 pressure gradient between Iceland and the Azores was weakened in summer (Figure 7). In addition, during year 1–year 2, this summer NAO negative phase gradually coincided with the SNAO negative phase pattern in Folland et al. [43] (Figure 7). Although the low-pressure center of the summer NAO was located further south in our results (Figure 7), this may be due to the fact that the short-term impact intensity and spatial scale of the external forcing of Samalas mega volcanic eruption on summer NAO were much greater than the impact of the gradual increase of greenhouse gases. Combining reconstructions with ECHAM5.4 simulation results, Wegmann et al. [35] Atmospherealso found2020, 11that, 1182 volcanic-induced cooling leads the summer NAO toward a negative12 of 17phase, which causes the southward movement of the storm track and enhances moisture advection toward southernover Northern Europe. EuropeIn addition, and the the southerly model windssimulated over SouthernsummerEurope NAO in are Wegmann both weakened et al. (Figure [35] was7c), also locatedwhich further contributes southto than the weakeningthe SNAO ofsuggested drought in by Northern Folland Europe et al. [43]. and wetness in Southern Europe.

FigureFigure 7. The 7. The spatial spatial patterns patterns of of the the ensemble ensemble mean seasea levellevel pressure pressure (SLP, (SLP, hPa, hPa, color color shading) shading) anomaliesanomalies and and 850-hPa 850-hPa wind (m(m/s,/s, vector) vector) anomalies anomalies in summer in summer (JJA) after (JJA) the Samalasafter the mega Samalas volcanic mega volcaniceruption eruption in VOL in (aVOL) year in 0, ((ba)) year year 1, and0, ( (bc) yearyear 2. 1, The and dots ( denotec) year areas 2. The with dots confidence denote exceeding areas with confidencethe 95% exceeding level. The the anomalies 95% level. are calculatedThe anomalies with respect are calculated to the 10 with years respect before theto the Samalas 10 years mega before the Samalasvolcanic eruption.mega volcanic The year eruption. 0 corresponds The ye toar the 0 corresponds eruption year. to the eruption year.

Sufficient water vapor supply provides favorable conditions for continuous precipitation. To further investigate the relationship between the summer hydroclimate over Europe and the The spatial patterns of the ensemble mean vertically integrated whole-level water vapor transport circulation changes, the 850-hPa wind patterns associated with the negative NAO phase are also and its divergence in summer are shown in Figure8. After the Samalas mega volcanic eruption, analyzedthe anomalous as shown convergence in Figure 7 center(vector). was In mainly the eruption distributed year in (year Southern 0), the Europe, 850-hPa especially wind anomalies over the are characterizedBalkans and by Apennine a strong peninsulas cyclone (Figure over8). Anomalousthe Balkans southerlies and overApennine the east ofpeninsulas this convergence near the Mediterraneancenter enhanced Sea the(Figure northward 7a). Southwesterly transport of water airf vaporlow toin Southernthe lower Europe, troposphere increasing transports precipitation a large amountthere of (Figure warm8). Anomalousand moist southerliesair from fromthe Mediterr the Mediterraneananean to carry the moreBalkans, water which vapor intois Southernconducive to precipitationEurope. Comparatively and makes Southern speaking, theEurope water near vapor the transported Balkans fromform the a precipitation Atlantic Ocean increase to the Iberian center in Peninsula is not as strong as that in the Mediterranean Sea (Figure8). In addition, Northern Europe is dominated by easterlies from the inland, and the anomalous divergence center is mainly distributed over this region, which suppresses precipitation there (Figure8). Atmosphere 2020, 11, 1182 13 of 17 Atmosphere 2020, 11, x FOR PEER REVIEW 13 of 17

Figure 8.Figure The 8.spatialThe spatial pattern patternss of the of the ensemble ensemble mean verticallyvertically integrated integrated whole-level whole water-levelvapor water vapor 5 2 transporttransport (100 kg/m/s, (100 kg /vector)m/s, vector) anomalies anomalies and and its divergencedivergence (10 −(10ekg-/m5 kg/m/s, color2/s, shading) color shading) anomalies anomalies in in summersummer (JJA) (JJA) after after the the Samalas Samalas megamega volcanic volcanic eruption eruption in VOL in in VOL (a) year in 0,(a (b) )year year 1,0, and (b) ( cyear) year 1, 2. and (c) The anomalies are calculated with respect to the 10 years before the Samalas mega volcanic eruption. year 2. TheThe yearanomalies 0 corresponds are calculated to the eruption with year. respect to the 10 years before the Samalas mega volcanic eruption. The year 0 corresponds to the eruption year. 4. Conclusions and Discussion 4. ConclusionIns this and study, Discussion the CESM multi-member ensemble climate simulations are used to analyze the role of AD 1258 Samalas mega volcanic eruption in the summer hydroclimate change over Europe In thisand study, understand the theCESM corresponding multi-member mechanisms ensemble behind climate that, which simulations intends toare provide used ato more analyze the role of ADcomprehensive 1258 Samalas understanding mega volcanic of the historicaleruption impact in the of summer Samalas megahydroclimate volcanic eruption change on over the Europe and understandsummer hydroclimate the corresponding change in Europe mechanisms from the behind perspective that, of simulation which intends research. to The provide major a more comprehensivefindings are understanding as follows. of the historical impact of Samalas mega volcanic eruption on the Both the reconstructed OWDA PDSI index and our simulations show that the hydrological changes summer hydroclimate change in Europe from the perspective of simulation research. The major in Europe were humid in the first two years after the Samalas mega volcanic eruption and then turned findingsinto are drought.as follows. CESM has well simulated the dry and wet changes in Europe after the Samalas eruption. Both theThe reconstructed whole Europe and OWDA each sub-region PDSI index experience and oursignificant simulations summer cooling show inthat the the first threehydrological changes yearsin Europe (year 0–2) were after humid the Samalas in the eruption. first two The years largest after significant the Samalas SAT reduction mega appearsvolcanic in yeareruption 1 and over the whole Europe, Southern Europe, and Northern Europe with values of 3.61 C, 4.02 C, then turned into drought. CESM has well simulated the dry and wet changes− in◦ Europe− ◦ after the Samalas eruption. The whole Europe and each sub-region experience significant summer cooling in the first three years (year 0–2) after the Samalas eruption. The largest significant SAT reduction appears in year 1 over the whole Europe, Southern Europe, and Northern Europe with values of −3.61 °C , −4.02 °C , and −3.21 °C , respectively. Previous reconstruction studies also found that Europe experienced an unusually cold summer after the Samalas mega volcanic eruption. The summer SAT changes over the European continent after the Samalas eruption is mainly due to the direct weakening of shortwave solar radiation-induced by Samalas volcanic aerosol. After the Samalas mega volcanic eruption, the summer precipitation over the European continent shows an obvious dipole distribution characteristic of the north-south reverse phase. The

Atmosphere 2020, 11, 1182 14 of 17 and 3.21 C, respectively. Previous reconstruction studies also found that Europe experienced an − ◦ unusually cold summer after the Samalas mega volcanic eruption. The summer SAT changes over the European continent after the Samalas eruption is mainly due to the direct weakening of shortwave solar radiation-induced by Samalas volcanic aerosol. After the Samalas mega volcanic eruption, the summer precipitation over the European continent shows an obvious dipole distribution characteristic of the north-south reverse phase. The precipitation increases up to 0.42 mm/d in year 1 over Southern Europe, while it decreases by 0.28 mm/d in year 1 − over Northern Europe. Both simulations and reconstructions show that the centers with the strongest increase in precipitation have always been located in the Balkans and Apennine peninsulas along the Mediterranean coast over Southern Europe, and the centers with the strongest precipitation reduction are mainly located in the British Isles and Scandinavia over northwestern Europe. Medieval European chronicles and documents also reported that the incessant rainfall in AD 1258 prevented crops and fruits from ripening, resulting in severe food shortages and survival crises in AD 1258 and 1259 in Southern Europe, such as Holy Roman Empire and Iberian Peninsula. The negative response of NAO with significant positive SLP anomaly in the north and negative SLP anomaly in the south is excited in summer after the Samalas mega volcanic eruption. The low tropospheric wind anomaly caused by the negative phase of NAO in summer affects the warm and moist air transport to Europe, resulting in the distribution pattern of summer precipitation in Europe, which is drying in the north and wetting in the south. Although the overall precipitation in Southern Europe increases after the Samalas mega volcanic eruption, humid summer in the Balkans and Apennine peninsulas are easier to maintain than those in the Iberian Peninsula over southwest Europe. Precipitation in southwest and southeast Europe comes from different sources of water vapor. Southwest Europe is due to anomalous southwesterlies that transport water vapor from the Atlantic Ocean to the land, while southeast Europe benefits from a steady supply of water vapor from the Mediterranean Sea. This implies that after the Samalas mega volcanic eruption, the Mediterranean Sea may be a more important source of water vapor for the increase of summer precipitation over Southern Europe than the Atlantic Ocean. In addition to the influence of summer NAO changes after the Samalas mega volcanic eruption, the hydroclimate response in Europe may be further complicated by the other influences of various modes of climate variability, such as the East Atlantic Pattern (EAP) and winter NAO teleconnection. Rao et al. [36] suggested that the impacts of volcanic eruption on spring-summer European hydroclimate are likely modulated by predisposing the EAP toward its negative phase. While the mechanisms for this are still uncertain, one possibility examined in model simulations suggests that its causes may originate in the tropics [36]. The role of these other variability modes in the summer hydroclimate changes in Europe after a single mega volcanic eruption (such as Samalas magnitude) will require further study. At present, little attention is paid to the summer hydroclimate change in Europe after the single mega volcanic eruption, especially the non-uniform precipitation changes between different sub-regions of Europe. Our results suggest that a single mega volcanic eruption (such as Samalas magnitude) can significantly change the summer hydroclimate in Europe. The knowledge gained from this research is crucial to better understand and predict the potential impacts of single mega volcanic eruption on future European summer hydroclimate changes. Our study has referenced significance for the implementation of stratospheric geoengineering in Europe.

Author Contributions: Conceptualization, B.L. and J.L.; methodology, B.L., J.L. and W.S.; software, J.L. and L.N.; validation, B.L., J.L., L.N., W.S., M.Y., C.Z., K.C., and X.W.; formal analysis, B.L., J.L., and C.Z.; investigation, B.L. and J.L.; resources, J.L.; data curation, J.L., L.N. and M.Y.; writing—original draft preparation, B.L. and J.L.; writing—review and editing, all; visualization, B.L. and C.Z.; supervision, J.L. and L.N.; project administration, J.L.; funding acquisition, J.L. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by “National Natural Science Foundation of China”, grant number 41971108; “National Key Research and Development Program of China”, grant number 2016YFA0600401; “Priority Academic Program Development of Jiangsu Higher Education Institutions”, grant number 164320H116; “Postgraduate Atmosphere 2020, 11, 1182 15 of 17

Research & Practice Innovation Program of Jiangsu Province”, grant number KYZZ16_0456; “Jiangsu Center for Collaborative Innovation in Geographical Information Resource Development and Application”. Acknowledgments: We thank two anonymous reviewers and editor for their valuable comments and suggestions. This research is jointly supported by the National Natural Science Foundation of China (Grant No. 41971108), the National Key Research and Development Program of China (Grant No. 2016YFA0600401), the Priority Academic Program Development of Jiangsu Higher Education Institutions (Grant No. 164320H116), the Postgraduate Research & Practice Innovation Program of Jiangsu Province (Grant No. KYZZ16_0456), and the Jiangsu Center for Collaborative Innovation in Geographical Information Resource Development and Application. Bin Liu acknowledges China Scholarship Council (201606860006) for the financial support. The reconstructed Samalas volcanic forcing was kindly provided by Chaochao Gao. The reconstructed Palmer Drought Severity Index was kindly provided by Edward R. Cook. Conflicts of Interest: The authors declare no conflict of interest.

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