Response of the East Asian summer monsoon to large volcanic eruptions during the last millennium

Chin Sci Bull (2014) 59(31):44129 DOI 101007/s11434-014-0404-5 Article csbscichinacom wwwspringercom/scp Atmospheric Science Response of the East Asian summer monsoon to large volcanic eruptions during the last millennium Wenmin Man Tianjun Zhou Received: 15 November 2013 / Accepted: 1 April 2014 / Published online: 20 May 2014 Ó Science China Press and Springer-Verlag Berlin Heidelberg 2014 Abstract The responses of the East Asian summer monsoon (EASM) to large volcanic eruptions were analyzed using a millennial simulation with the FGOALS-gl climate system model The model was driven by both natural (solar irradiance, volcanic eruptions) and anthropogenic (greenhouse gases, sulfate aerosols) forcing agents The results showed cooling anomalies after large volcanic eruptions almost on a global scale The cooling over the continental region is stronger than that over the ocean The precipitation generally decreases in the tropical and subtropical regions in the first summer after large volcanic eruptions Cooling with amplitudes up to -03 C is seen over eastern China in the first summer after large volcanic eruptions The East Asian continent is dominated by northeasterly wind anomalies and the corresponding summer rainfall exhibits a coherent reduction over the whole of eastern China An analysis of the surface heat flux suggested the reduction in summer precipitation over eastern China can be attributed to a decrease of moisture vapor over the tropical oceans, and the weakening of the EASM may be attributed to the reduced landsea thermal contrast after large volcanic eruptions Keywords Large volcanic eruptions Global response East Asian summer monsoon Climate system model Millennial simulation W Man T Zhou (&) State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China e-mail: zhoutj@lasgiapaccn 1 Introduction Volcanic forcing has long been recognized as one of the important natural factors affecting climate variability over short time scales [13] Comparison of observations with simulations from an energy balance climate model showed that as much as 41 %64 % of pre-anthropogenic decadalscale temperature variations were due to changes in solar irradiance and volcanic forcing [4] Volcanic activity affects global climate through the radiative impacts of atmospheric sulfate aerosols injected into the stratosphere by volcanic eruptions The direct radiative cooling effect is remarkably robust following major tropical eruptions For example, the eruption of Mount Pinatubo in the Philippines in June of 1991 resulted in a significant decrease in solar heating due to the aerosol particles released [57], which led to a global cooling of the lower troposphere [8] Previous studies have indicated that the volcanic signal is also detectable in precipitation, although it is not as obvious as that in temperature [912] Negative precipitation anomalies are seen in many regions after the large volcanic eruptions, such as the tropics [13], and central and eastern parts of North America [14] Volcanic eruptions have also been found to play an important role in summer precipitation changes over eastern China [1517] Xu [15] found that North China experienced droughts and the middle and lower Yangtze River Valley saw floods in the wake of three colossal volcanic eruptions Shen et al [17] suggested that large volcanic eruptions might lead to exceptional drought over eastern China, and that coherent droughts over eastern China are associated with large low-latitude volcanic eruptions Climate models are useful tools for understanding the climate effects of volcanism Whether or not the volcanic signals observed in proxy and observational data are also detectable in simulations, and the mechanisms underlying the response of

4124 Chin Sci Bull (2014) 59(31):44129 summer temperature and precipitation to large volcanic eruptions, need to be further investigated The aims of the present study were to examine the responses of the East Asian summer monsoon (EASM) to large volcanic eruptions during the last millennium by using a millennial simulation with the coupled climate system model developed by the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG) at the Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences The effect of volcanism in the model was applied as a negative deviation from the solar radiation, which was represented by converting the aerosol optical depth to changes in downward shortwave radiative forcing at the tropopause, using the relationship discussed in Sato et al [18] The latitudinal dependence of volcanic aerosol, as well as the stratospheric warming by volcanic eruptions, was not taken into account in this study, although more recent simulations have incorporated the effects of volcanism in a more sophisticated manner Therefore, the results presented here are a basic guide toward the understanding of volcanic eruption effects on global and regional climate change More simulations driven by monthly and latitudinal volcanic forcing data are necessary for revealing more realistic effects of volcanic aerosol on global and regional climate The main motivation behind the study was to answer the following questions: (1) What are the responses of global summer temperature and precipitation to large volcanic eruptions? (2) What are the responses of the EASM to large volcanic eruptions? (3) What is the dominant reason for the monsoon and precipitation changes over East Asia after large volcanic eruptions? 2 Model and data description 21 Model description The coupled model used in this study was the fast version of the LASG/IAP s Flexible Global Ocean-Atmosphere-Land System (FGOALS) model, which employs a low resolution version of the Grid Atmospheric Model of the IAP/LASG (GAMIL) as its atmospheric component and is thus referred to as FGOALS-gl (the l of FGOALS-gl standing for low resolution ) [19] The low resolution version of GAMIL has a 72940 weighted equal-area mesh, which corresponds to approximately a 5 (lon)945 (lat) grid, with 26 vertical levels in a sigma coordinate The physical package of GAMIL is the same as CAM2 [20] The oceanic component of FGO- ALS is the LASG/IAP s Climate Ocean Model (LICOM), which employs a horizontal resolution of 10 910, and there are 30 levels in the vertical direction [21] In addition, both the land and sea ice components of the model are derived from the National Center for Atmospheric Research Community Climate System Model (NCAR CCSM2) [22, 23] The four components are coupled together using the NCAR CCSM2 coupler There is no correction in the heat and freshwater fluxes exchanged at the interfaces among the atmosphere, ocean, sea ice, and land during coupled integrations For more details of FGOALS-gl, the reader is referred to Zhou et al [19] 22 Experimental design and forcing data First, a 100-year control run was performed under AD 1000 external forcing conditions Starting from the oceanic initial conditions derived from a 500-year spin-up integration for present-day conditions, the simulation spanning the time AD 1000 to 1999 was conducted using the climate system model FGOALS-gl The model was driven by reconstructions of natural forcing (solar radiation and volcanic aerosols) and anthropogenic forcing (greenhouse gas emissions and sulfate aerosols) The simulation did not include changes in anthropogenic land-use Orbital variations were neglected due to the comparatively short period being investigated Both the solar irradiance and volcanic forcing data used to drive the model were from Crowley et al [24] The solar irradiance data are a combination of observed sunspot numbers and an ice core record of the cosmogenic isotope 10 Be (the 10 Be /Lean splice) The effect of volcanism was applied as a negative deviation from the solar radiation The greenhouse gas concentrations (CO 2,CH 4 and N 2 O) were from Ammann et al [25] The influence of tropospheric sulfate aerosols was taken into account from AD 1850, and the aerosol data were from the CMIP3 project [26] 23 Analysis method The effect of volcanism was applied as a negative deviation from the solar radiation in this study As in Peng et al [27], large volcanic eruptions were defined based on the strongest reduction in solar radiation of greater than 8 W/m 2,whichis approximately equal to a volcanic explosivity intensity (VEI) of 4 or more For the case of consecutive years with a greater than8w/m 2 reduction, the year with the largest reduction was taken as the year of eruption In so doing, 18 cases of volcanic eruptions were selected through the simulation period of AD 10001999 (Table 1) It should be noted that this definition could have led to a discrepancy between observation years and eruption years in the present analysis, since the aerosol layer accumulates leading to a delayed response (in some cases associated with a shift of the eruption year, eg the Tampora eruption in 1816 (1815 in the observation), and the Pinatubo eruption in 1992 (1991 in the observation)) Composite analyses were conducted to investigate the responses of summer temperature and precipitation (JuneJulyAugust) to reductions in solar irradiance induced by large volcanic eruptions The temporal evolutions during the five pre-volcanic eruption years, the volcanic eruption years themselves, and the five

Chin Sci Bull (2014) 59(31):44129 4125 Table 1 List of the selected 18 large volcanic eruptions during 10001999 No Year Name VEI 1 1190 Unknown 2 1196 Unknown 3 1258 Unknown 4 1276 Unknown 5 1286 Unknown 6 1345 Unknown 7 1453 Kuwae 6 8 1601 Huaynaputina 6 9 1621 Iceland 10 1642 Parker 6 11 1674 Gamkonora 5 12 1695 Serua? /Helka? 13 1790 Kilauea 14 1810 St Helen? 6 15 1816 Tambora 7 16 1832 Babuyan Claro 4? 17 1884 Krakatau 6 18 1992 Pinatubo 6 VEI values are included, and question marks indicate uncertainty (source: http://wwwvolcanosiedu/world/largeeruptionscfm) post-volcanic eruption years, as well as the spatial patterns in the first summer after eruption years, were analyzed 3 Results 31 Global-scale temperature and precipitation response to large volcanic eruptions The temporal pattern of global mean surface air temperature (SAT) and precipitation anomalies for the 18 cases of large volcanic eruptions in the five pre-volcanic eruption years, the volcanic eruption years themselves, and the five post-volcanic eruption years are shown in Fig 1 There is a peak global cooling during the volcanic eruption year and the year after, after which the SAT slowly returns to pre-eruption levels The precipitation also significantly decreases during the volcanic eruption year and the year after, and returns to normal conditions after 2 years This temporal pattern of the precipitation response to large volcanic eruptions is similar to that revealed by modeled and observed tropical precipitation [13] and global precipitation [10] The global pattern of SAT and precipitation anomalies in the first summer after the 18 large volcanic eruptions are shown in Fig 2 Cooling anomalies in the first summer after the large volcanic eruptions are seen almost on a global scale (Fig 2a) The global mean SAT anomalies are -018 C with respect to the millennial mean value SAT ( C) Precipitation (mm/d) Fig 1 Superposed epoch analysis of global summer mean SAT ( C) and precipitation (mm/d) for the 18 cases of large volcanic eruptions in the five pre-volcanic eruption years, the volcanic eruption years themselves, and the five post-volcanic eruption years The dashed lines represent confidence intervals of 95 % and 99 % derived from 1,000 Monte Carlo simulations Cooling anomalies are enhanced over the Northern Hemispheric continents and the tropical eastern Pacific The cooling over the continental region (-026 C) is larger than that over the ocean (-016 C) The SAT anomalies also exhibit a polar-amplification phenomenon, which is consistent with previous findings that a global cooling is seen after volcanic eruptions and that the cooling is most evident over the mid-high latitude continental regions [28] The precipitation anomaly patterns in the first summer after the large volcanic eruptions are shown in Fig 2b Negative precipitation anomalies are seen in the tropical and subtropical regions The values for the tropical and subtropical mean precipitation anomalies are -02 mm/d and -01 mm/d, respectively There are negative precipitation anomalies in the mid-high latitude continental regions The mean value for the mid-high latitude continents is -005 mm/d The precipitation responses to large volcanic eruptions are consistent with previous findings that negative precipitation anomalies are seen in many regions after large eruptions, such as in the tropics [13] and central and eastern parts of North America [14] 32 EASM responses to large volcanic eruptions The temporal pattern of SAT and precipitation anomalies over eastern China (25 40 N, 105 1225 E) for the 18 cases of

4126 Chin Sci Bull (2014) 59(31):44129 Fig 2 The composite means of global SAT ( C) and precipitation (colored illustration, mm/d) and 850-hPa winds (vectors, m/s) in the first summer after the 18 large volcanic eruptions The anomalies were calculated relative to the five pre- and post-volcanic eruption years large volcanic eruptions in the five pre-volcanic eruption years, the volcanic eruption years themselves, and the five post-volcanic eruption years is further shown in Fig 3 The largest reduction of the SAT anomalies over eastern China appears in the volcanic eruption year, and the year after Summer precipitation over eastern China also significantly decreases during the volcanic eruption year and the year after, after which the precipitation returns to pre-eruption levels The composite means of SAT and precipitation anomalies over East Asia (0 60 N, 100 140 E) in the first summer after the 18 large volcanic eruptions are shown in Fig 4 There is a coherent cooling with amplitude of -03 C over the East Asian continent and the tropical ocean (Fig 4a) The cooling over the middle-high latitudes of the East Asian continent is stronger than that over the tropical regions, which suggests a reduced landsea thermal contrast The 850-hPa wind anomalies feature a strong cyclone over the East Asian monsoon area, and the East Asian continent is controlled by northeasterly wind anomalies This corresponds well to a weak EASM after the large volcanic eruptions (Fig 4b) The summer rainfall anomalies exhibit a coherent reduction over all of eastern China (Fig 4b), which could be related to the reduced northward water vapor transport Using millennial climate simulations driven by natural and anthropogenic forcing, Peng et al [27] also suggested a reduction in summer precipitation after large volcanic eruption Our results are consistent with proxy data, which also indicate that volcanic forcing is a major factor affecting summer rainfall over eastern China Based on Chinese drought/flood proxy data of the past 500 years, Shen et al [17] showed that the peaks in the frequency of anomalous precipitation events occurred in active periods of volcanic eruption, and there were fewer anomalous precipitation events when less volcanic eruptions occurred Further analysis indicated that there was a significant correlation between volcanic eruption events and droughts over the whole of eastern China A typical case of coherent drought was in 1641 In that year, dry conditions covered most parts of eastern

Chin Sci Bull (2014) 59(31):44129 4127 05 Fig 3 Superposed epoch analysis of summer SAT ( C) and precipitation (mm/d) over eastern China for the 18 cases of large volcanic eruptions in the five pre-volcanic eruption years, the volcanic eruption years themselves, and the five post-volcanic eruption years The dashed lines represent confidence intervals of 95 % and 99 % derived from 1,000 Monte Carlo simulations China, including North China and the middle and lower Yangtze River Valley [16] Shen et al [17] also indicated that during explosive low-latitude volcanic eruption years, the occurrence probability of coherent drought was significantly higher, indicating a statistically significant connection between explosive volcanic eruptions and subsequent coherent drought over eastern China; while there was no significant connection between spatial patterns of precipitation over eastern China and explosive mid-high latitude volcanic eruptions Since the latitudinal dependence of volcanic aerosol was not considered in this study, the above result will be further assessed in future work by using simulations driven by monthly and latitudinal volcanic forcing data 33 Attribution of the EASM response to large volcanic eruptions The surface heating budget was analyzed to understand the mechanisms underlying the response of the EASM to large volcanic eruptions The composite means of surface net shortwave flux, surface net longwave flux, surface latent heat flux, and surface sensible heat flux for the 18 cases are shown in Fig 5 There are significant reductions in both the surface shortwave radiation and the longwave Fig 4 The composite means of SAT ( C) and precipitation (colored illustration, mm/d) and 850-hPa winds (vectors, m/s) over East Asia in the first summer after the 18 large volcanic eruptions during the last millennium The anomalies were calculated relative to the five pre- and post-volcanic eruption years radiation The reduction of the shortwave radiation reaches its largest value of more than -6 W/m 2 in the first summer after the large volcanic eruptions (Fig 5a); the longwave radiation reduction is weaker than in the shortwave radiation, with a reduction of approximately - 2 W/m 2 in the model (Fig 5b) The latent heat flux over the ocean decreases after the large volcanic eruptions, indicating a decline in evaporation in tropical regions, which could lead to the reduction of precipitation over eastern China (Fig 5c) Robock and Liu [13] attributed

4128 Chin Sci Bull (2014) 59(31):44129 (c) (d) Fig 5 The composite means of surface heat fluxes over East Asia in the first summer after the large volcanic eruptions (W/m 2 ) a surface shortwave flux; b surface longwave flux; c surface latent heat flux; and d surface sensible heat flux The anomalies were calculated relative to the five pre- and post-volcanic eruption years the reduction of tropical precipitation to reduced evaporation caused by tropospheric cooling The connection between explosive volcanic eruptions and droughts over eastern China suggested by our results seems to support their hypothesis Further, a reduction in sensible heat flux is observed over land, while it is not evident in tropical regions (Fig 5d) This implies that the decrease of SAT over land is larger than that over tropical oceans, and thus a reduced landsea thermal contrast and a weakening EASM 4 Discussion and conclusions Responses of summer temperature and precipitation to large volcanic eruptions were analyzed using a millennial simulation with the LASG/IAP s climate system model, FGOALS-gl The association between the monsoon response and the surface heat flux change was studied The main results can be summarized as follows: (1) The largest reduction in global mean SAT and precipitation anomalies appears in the volcanic eruption year itself, and the year after, after which the anomalies returns to pre-eruption levels Cool anomalies in the first summer after large volcanic eruptions are seen almost on a global scale The cooling over the continental region is stronger than that over the ocean Precipitation generally decreases in tropical and subtropical regions in the first summer after large volcanic eruptions (2) The largest reduction in SAT and precipitation anomalies over East Asia also appears in the volcanic eruption year itself, and the year after, and returns to normal conditions after 2 years Cooling with amplitudes up to -03 C is seen over eastern China in the first summer after large volcanic eruptions The East Asian continent is dominated by northeasterly wind anomalies and the

Chin Sci Bull (2014) 59(31):44129 4129 corresponding summer rainfall exhibits a coherent reduction over the whole of eastern China (3) The analysis of the surface heat flux suggests the reduction of summer precipitation over eastern China can be attributed to the decrease in moisture vapor over the tropical oceans and the weakening of the EASM may be attributed to the reduced landsea thermal contrast after large volcanic eruptions The present study shows reasonable spatiotemporal patterns of temperature and precipitation response to large volcanic eruptions during the last millennium However, the climatic processes of volcanic eruptions are much more complicated than the effect of volcanic eruptions simply being converted to a direct reduction in global solar irradiance, as in our model Further, the seasonal and latitudinal dependence of volcanic aerosols was not taken into account in the simulation, which is potentially important for a realistic simulation of the last millennial climate The timing and location of volcanic eruptions can affect the transport of aerosols, and thus affect the nature of volcanic forcing on the climate [29] Analysis of proxy data indicates that tropical and high-latitude volcanic eruptions differ distinctly in their climate effects on regional climate [17] Therefore, further simulations driven by monthly and latitudinal volcanic forcing data are needed for a better understanding of the effects of volcanic aerosol on global and regional climate Acknowledgments This work was supported by the National Natural Science Foundation of China (41305069), the Open Project Program of the Key Laboratory of Meteorological Disaster of Ministry of Education, Nanjing University of Information Science and Technology, China R&D Special Fund for Public Welfare Industry (meteorology) (GYHY201406020), and the National Basic Research Program of China (2010CB951904) Conflict of Interest of interest References The authors declare that they have no conflict 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