Influence Of Solar Activity On The Decadal Climate Changes Of The Asian Monsoon

Solar radiation is an important driving force of the Earth system,and its influence on climate change cannot be ignored.The change of the Asian monsoon has a profound impact on agriculture,ecosystem,food security,and drought/flood disasters in Asian countries(including China,India,Japan,and Southeast Asian countries).The Asian monsoon precipitation provides water resources for half of the world's population.In the past century,a series of major decadal climate events took place in the world,such as the decades-long drying over the Sahel in Africa and northern China,and the strong sandstorm in the United States in the 1930s.These decadal climate changes have had a crucial impact on the living environment of human beings.Although many scholars have conducted researches on the decadal variation of the Asian monsoon,the mechanism of the Asian monsoon decadal variation to the 11-year solar cycle is still not clear.Here,based on one control(CTRL)experiment and four solar-only forcing(SSI)experiments of the Last Millennium Ensemble model project carried out by the Community Earth System Model(CESM-LME),the linkages between the 11-year solar cycle and decadal variation of the Asian summer monsoon(ASM)and Asian winter monsoon(AWM)have been explored;meanwhile,the mechanisms that the 11-year solar cycle impacting on the ASM and AWM have been revealed.Then,based on three datasets provided by the European Centre for Medium-Range Weather Forecasts,including ERA-20C from 1901-2010,ERA-40 from 1958-2001,and ERA-Interim from 1979-2018,a dataset with a spatial resolution of 2.5°×2.5°covering surface temperature,precipitation,sea level pressure,wind and other climate elements over 1900-2018were combined.Meanwhile,a sea surface temperature(SST)data during 1871-2018 has been made by simply taking the arithmetic means of the Hadley Centre Sea Ice and Sea Surface Temperature(Had ISST),and the National Oceanic and Atmospheric Administration Extended Reconstructed SST(ERSST).Using these two datasets of observations,in combination with three greenhouse gases(GHGs)-only forcing experiments,SSI experiments and CTRL experiment derived from CESM-LME,the origins of the“Warm Arctic-Cold Siberia”(WACS)during the past two decades have been revealed.In addition,the spatial-temporal structure of equatorial central Pacific(ECP)quasi-decadal oscillation(QDO)and the feedbacks that control its development and decay have been analyzed in this paper.1.Decadal variations of the East Asian summer monsoon forced by 11-year insolation cycle;A previous study suggests that the Asia monsoon circulation may be affected by the solar activity,but the process that the 11-year solar radiation cycle affects the Asian summer monsoon(ASM)remains unexplained.The ASM has a complex structure,depending on the character and location,the ASM can be divided into three sub-monsoon systems,namely,the East Asian summer monsoon(EASM),the Indian summer monsoon,and the Northwest Pacific summer monsoon.Based on the results of the SSI experiment derived from the CESM-LME,a diagnostic analysis of the decadal signal(9-13 years)spatial distribution pattern of summer precipitation over the Asian monsoon region found that only the precipitation over the EASM region has significant decadal signals during the strong 11-year solar cycle period;while the decadal signals over the Indian summer monsoon precipitation and Northwest Pacific summer monsoon precipitation are not obvious.It shows that the precipitation over East Asia derived from the SSI experiment is characterized by a distribution pattern of“northern floods and southern droughts”,then,the area-averaged precipitation over the north of 35°N in the East Asian monsoon region is defined as the EASM index(EASM).During the strong 11-year solar cycle epoch,the EASM index has an obvious 11-year periodicity,while during the weak 11-year solar cycle,there is only a 15-year periodicity in the EASM index;which is consistent with the results derived from the CTRL experiment.During the strong 11-year solar cycle epoch,the East Asian summer monsoon index has a significant positive correlation with total solar radiation(TSI)(r=0.41,p<0.05),but there is no link between the EASM index and TSI during the weak 11-year solar cycle epoch(r=0.002).The strong 11-year solar cycle can excite a Pacific decadal oscillation(PDO)-like SST anomaly with a strong 11-year periodicity.During the strong 11-year solar cycle epoch,a cold PDO mode coincides with an anomalous anti-cyclone dominating the northern North Pacific.To the west of the North Pacific anticyclone prevail anomalous southerlies that strengthen EASM.Associated with EA continental warming,a zonally oriented low-pressure trough occurs around 35~oN over East China,increasing cyclonic vorticity and precipitation over the northern EA.2.Decadal variability of northern Asian winter monsoon shaped by the eleven-year solar cycle;The Asian winter monsoon(AWM)is the most powerful winter circulation system in the northern hemisphere,but the 11-year cycle of solar activity affects the decadal variation of AWM requires further study.The circulation structure of AWM is relatively simple,mainly controlled by the Asian high.Observational data show that the empirical orthogonal function(EOF)of the winter surface air temperature in Asia has two leading modes on the interannual and interdecadal timescale.One is the northern mode with the center of variability in the range of 40°N-70°N,and the other is the southern mode with a dipolar zonal structure over the entire Asia.Based on SSI experiment results,it is shown that the solar radiative forcing does not appreciably change the structures of the two leading modes of the AWM,but the principal component(PC)of the two leading modes have different responses to the 11-year periodicity of solar radiation.The power spectrum analysis results of the PC corresponding to the AWM southern mode during both strong and weak 11-year solar cycle period coinciding with those in the CTRL experiment have obvious interannual signals.Whereas,the PC of the AWM northern mode has a strong 11-year periodicity,but it lags behind the peak year of solar radiation by 3-4 years and has a simultaneous relationship with the cumulative solar radiation.A new mechanism is proposed to explain this delayed response,in which the 11-year solar cycle affects the AWM via modulating the sea ice variation over the Barents-Kara Sea during the preceding summer.At the peak of the accumulative solar irradiance(i.e.,4 years after the maximum solar irradiance),the Arctic sea ice concentration reaches a minimum over the Barents-Kara Sea region accompanied by an Arctic sea surface warming,which then persists into the following winter,causing Arctic high-pressure extend to the Ural mountain region,which enhances Siberian High and East Asian trough,causing a bitter winter over northern Asia.3.The mechanism of the winter surface air temperature“Warm Arctic-Cold Siberia”;Over the latest two decades,adverse cold winters have occurred more frequently than before in Eurasia,which is concurrent with pronounced warming over the Arctic,especially over the Barents-Kara sea region.This“warm Arctic-cold Siberia(or Eurasia)”pattern has attracted prodigious attention in the climate research community because such unexpected cold winters in Eurasia have caused severe damage to livelihoods and socio-ecological systems.Results from an atmosphere-only model simulation suggest that the sea-ice reduction in the Barents-Kara Sea can double the probability of severe winters in central Eurasia.However,the simulation results of the global coupled climate models suggest that the winter“warm Arctic-cold Siberia”(WACS)pattern is unlikely to be the dominant mode of future climate change.Based on the 119-year observations and CTRL experiment,SSI experiments,and GHGs experiment,we find that the WACS pattern is a major EOF mode of SAT variability over Arctic-northern Eurasia,and the frequent occurrence of the WACS in the recent decades is due to a change in the leading internal mode from the uniform warming to the WACS.Results derived from GHG experiments and SSI experiments indicate that both external forcings enhance the uniform warming mode.Observational and simulated results suggest that frequent occurrences of that WACS pattern are instigated by the warm phases of the Atlantic Multidecadal Oscillation(AMO).North Atlantic warming may activate the WACS by generating a background Atlantic-Eurasian wave train characterized by enhanced Ural Mountain ridge and East Asian trough,which is conducive to recurrent WACS pattern.The wave train-induced Barents Sea ice melting can act as an amplifier,reinforcing the WACS.Note that the WACS in the recent positive AMO phase(1998–2013)is more prominent than that in the previous positive AMO phase(1927–1965).Results from this paper suggest that the recent amplification of WACS since 1998 may be additionally reinforced by clustering occurrence of the Central Pacific El Ni(?)o.Although increased greenhouse gases favor a uniform warming pattern,they may contribute to WACS formation by affecting AMO and changing El Ni(?)o properties and its teleconnection.4.Emerging Pacific Quasi-Decadal Oscillation over the past 70 years;The Pacific quasi-decadal oscillation has wildly been studied,but its spatial-temporal characteristics and physical processes remain largely unknown.Based on the merged data,here we show that the SST in the equatorial central Pacific(ECP)exhibits a prominent 11-year periodicity since 1951 but not before.According to the ECP index defined by the annual average SST over the ECP region,the correlation analysis between the ECP index and the SST,circulations shows that an episodic-like quasi-decadal warm event is initiated in a northeast-southwest tilted belt from the United States'west coast to the ECP.The initial development involves positive feedback between atmospheric heating-induced Rossby waves and underlying SST in the tropical North Pacific.The amplification of ECP warming is primarily due to equatorward heat advection and deepening thermocline,while the zonal advective feedback mainly controls its decay.The processes governing ECP warming contrasts the central Pacific El Ni(?)o development in which zonal heat advection dominates.Analysis of the factors that lead to the quasi-11-year oscillation over ECP shows that the equatorial central Pacific QDO tends to follow the 11-year solar cycle with a 1-to-2-year phase delay,meanwhile,the years of clustering El Nino or consecutive La Nina events also correspond roughly to the peak ECP years.