Simulation Of Volcanic Impacts On Tropical Climate Variability: Past And Future

Volcanic eruption is one of the important natural external forcings.Studing the impacts of volcanic eruption on tropical climate variability（ENSO and global monsoon）is of great scientific significance for coping with the current global warming and improving the theory of global change.At the same time,it also helps to prevent and reduce the loss of social economy and people’s life and property,and has important practical significance for the sustainable development of social economy.Using the Community Earth System Model（CESM）,as well as the window analysis,spatial interpolation,multiple data composite analysis,diagnostic analysis and other methods,the temporal and spatial characteristics of ENSO influenced by tropical and northern high-latitude volcanic eruptions are studied and the mechanisms are also explored.Meanwhile,the changes of global land monsoon precipitation under the future stratospheric aerosol injection（SAI,one type of geoengineering）scenario are predicted,and the different effects of greenhouses gases,tropical SAI and Arctic SAI on global monsoon changes are discussed.1 Impacts of large tropical volcanic eruptions on ENSO changeUsing an ensemble of nine El Ni?o/Southern Oscillation（ENSO）reconstructed proxies and volcano eruption proxies for the past 1500 years,this study shows that a significant La Ni?a state emerges in the second year（year（2）hereafter）after large tropical volcanic eruptions.The reasons for the development of La Ni?a are investigated using the Community Earth System Model（CESM）.In the volcanic eruption experiment（Vol）,a robust La Ni?a signal occurs in year（2）,resembling the proxy records.The eastward positioning of the western North Pacific anomalous anticyclone（WNPAC）in Vol plays a critical role in the advanced decay of year（2）warming and the strong intensification of cooling in the equatorial eastern Pacific.The enhanced easterlies located on the southern edge of the WNPAC can stimulate consecutive oceanic upwelling Kelvin waves,shallowing the thermocline in the eastern Pacific,thereby resulting in a greater cooling rate by the enhanced thermocline feedback and cold zonal advection.Over the equatorial eastern Pacific,the reduced shortwave radiation contributes to the advanced decay of warming,while the upward latent heat flux augments the strong intensification of the cooling.Essentially,the eastward positioning of the WNPAC is a result of the volcanic forcing.The volcanic effect cools the maritime continent more than its adjacent oceans,thus pushing convective anomalies eastward during year（1）.This induces vertical thermal advection and upward surface latent heat flux,thereby suppressing the development of warm Sea Surface Temperature over the central-western Pacific and causing the eastward positioning of the WNPAC in Vol.2 Impacts of northern high-latitude volcanic eruptions in different seasons on ENSO changeThe impact of northern high-latitude volcanic（NHV）eruptions on El Ni?o/Southern Oscillation（ENSO）are investigated based on ensemble simulations with the Community Earth System Model.The seasonality of atmospheric circulation influences the NHV aerosol dispersion,causing the stronger（weaker）Northern Hemisphere cooling after the January and April（July and October）eruptions.ENSO’s response is found to be more dependent on NHV eruption seasons than that on tropical eruption seasons.The January eruption causes an El Ni?o in eruption year（year（0）hereafter）while an El Ni?o occurs in year（1）after the October eruption.No significant El Ni?o occurs after the April（July）eruption.A diagnostic analysis reveals that these El Ni?os’developments are attributed to the positive zonal,meridional advective and thermocline feedbacks,triggered by the western Pacific westerly anomalies.Anomalous North Pacific Cyclone（NPC）and Asian monsoon are key systems to excite anomalous westerlies,which is caused by the NHV-induced mid-latitude cooling and Eurasian continent-North Pacific thermal contrast.After the January eruption,the anomalous NPC develops in early summer and connects with weakened Asian summer monsoon,which excites anomalous westerlies over the Indo-western Pacific,activating the Bjerknes feedback.For the October eruption,the anomalous NPC and enhanced East Asian winter monsoon bring cold air to the Maritime Continent and warm the subtropical central-North Pacific through surface heat flux exchange,exciting the westerly anomalies.These results suggest that the strong dependence on the seasonal timing of NHV should be a critical element of data-model comparisons.3 Global land monsoon precipitation responses to tropical and Arctic stratospheric aerosol injection.Stratospheric aerosol injection（SAI）is considered as a backup approach to mitigate global warming and understanding its climate impact is of great societal concern.It remains unclear how differently global land monsoon（GLM）precipitation changes in response to tropical and Arctic SAI.We address this issue using the tropical and Arctic SAI experiment with different intensities by the Community Earth System Model.The result shows that Arctic SAI has a relatively weak influence on suppressing GLM than tropical SAI does;however,it reduces Northern Hemisphere land monsoon,（NHLM）precipitation more than tropical SAI does,and it tends to increase Southern Hemisphere land monsoon（SHLM）precipitation.There exists a threshold of tropical SAI above 20 Tg yr^-1 to lead the significant decrease in GLM precipitation.When injecting more than 20 Tg yr^-1 aerosols in Arctic region,the NHLM precipitation will be notably suppressed.From the moisture budget analysis,the thermodynamic effect plays a critical role in suppressing GLM precipitation under the effect of tropical SAI,which is caused by the tropical tropospheric cooling substantially reducing the anomalous moisture by climatological vertical motion.Under the effect of Arctic SAI,the anomalous negative advection（positive convergence）process in dynamic effect decreases（increases）the NHLM（SHLM）monsoon precipitation,which is associated with changes in the meridional temperature gradient and NH land-sea thermal contrast.The knowledge gained from this study is important not only for understanding the potential impacts of SAI on the monsoon changes,but also for providing reference for geoengineering.