The Influence Of Solar Activity On The Decadal Fluctuations Of The Atlantic Meridional Overturning Circulation During The Last 400 Years

Solar radiation is the most important energy source of the Earth’s climate system,and solar activities can drive the multi-scale climate change of the earth through various ways,among which the total solar irradiance(TSI),solar ultraviolet radiation and space weather(the physical and chemical effects of space energy particles on the atmosphere)are considered to be the three key mechanisms.However,some studies based on recent observations and numerical simulations have pointed out that the solar radiation changes in the last thousand years are not enough to cause climate fluctuations.Therefore,the Sixth Assessment Report of the IPCC argues that the contribution of solar activity to modern climate change is still highly uncertain.However,solar activity is likely to influence the global climate by influencing the sensitive regions in the earth’s surface system,such as the North Atlantic region,and then modulating the North Atlantic Oscillation(NAO)and the Atlantic meridional overturning circulation(AMOC).In particular,through the coupling of surface and bottom currents,AMOC can produce trans-hemispheric and trans-oceanic transport of matter and energy,which has an important impact on regional and even global climate.The intensity of the AMOC is driven directly by the temperature/salinity of the North Atlantic Ocean and shows strong interdecadal variability.It is important to evaluate the contribution of solar activity to the AMOC variability at this time scale to understand the multidecadal variation of the AMOC and predict the evolution trend of the AMOC.In the past 400 years,the earth’s climate has experienced the "Little Ice Age" and global warming since the Industrial Revolution,and is now in the most special period since the Holocene.The gradual weakening of the AMOC since 1800 has attracted wide attention.The significance,driving factors and driving mechanisms of the weakening of the AMOC are still highly controversial.The contributions of natural factors(including solar activity)and human factors are hot issues in this field.Since the Little Ice Age,the TSI caused by solar activity has fluctuated significantly from interdecadal(11 years)to centennial.The study of its influence on the weakening of AMOC is of great significance for understanding the weakening trend of AMOC in modern times.Based on two TSI sequences reconstructed by Coddington et al.(2016)and Lean(2000)(defined as low-value and high-value sequences respectively),this paper uses PLASIM-GENIE,an Earth system model of moderate complexity,to carry out multi-scale wave sensitivity tests of TSI driven by solar activity.To explore the influence of solar activity on AMOC through TSI pathway.Through this study,it is found that:(1)In the past 400 years,there was a significant negative correlation between TSI and AMOC index(AMOCI),but in the background of low solar radiation,there was also a short period of significant positive correlation between them during the Maunder Minimum.The reconstructed TSI amplitude can reach a maximum of 4.9‰,and the increase of TSI has a significant warming effect in the world,resulting in the decrease of salinity and density in the main convective area,which results in the max decrease of 0.58 Sv AMOC in the period.The multi-periodic fluctuations of TSI caused the temperature and salinity changes in the polar region and affected the density of the downstream Nordic Sea and even the Labrador and Irminger Sea region(the L-E Sea)through the ocean circulation such as the East Greenland Current,which further weakened the AMOC.Under the background of lower TSI,the annual average AMOC decreased by about 0.09 Sv at most.It is about 0.13 Sv in the background of higher TSI.(2)The two types of TSI sequences had significant influence on the trend change of AMOCI time series.During the Maunder minimum period(1610-1699,when TSI mainly showed a decline),the thermohaline in the Labrador and Irminger Sea region significantly weakened,leading to an obvious downward trend of the AMOCI,which reached 2.99Sv/100 a in the control test.However,this phenomenon does not occur under higher TSI forcing.After Maunder minimum period(1700-2000),the AMOCI showed a significant decreasing trend under the two kinds of TSI forcing.The reduction rate of AMOCI in LAS of low-value fluctuation TSI test was 0.19Sv/100 a,while that in HAS of high-value fluctuation TSI test was 0.32Sv/100 a.They accounted for 11.1% and 18.7% of the decline of AMOCI in the last century.Under higher TSI forcing,the polar warming process was significant,and the sea ice area decreased more,and the AMOCI decreased more.(3)The changes of TSI modulated the multidecadal periodic fluctuations of the AMOC,which were significantly related to the meridional gradients of temperature and salinity in the North Atlantic Ocean,the changes of sea ice cover and the changes of the AMOC itself.However,the main period of AMOCI was 130 years for low solar radiative forcing control experiment,and 65 years for high solar radiative forcing control experiment,which showed obvious shortening,mainly due to the advance of negative correlation response of polar sea ice under high solar radiation condition.Under the background of high TSI,the 11-year periodic fluctuation components of TSI directly caused the negative thermohaline anomaly in the L-E Sea,and caused the faster sea ice response process,which resulted in the maximum weakening of AMOC.The centennial cycle fluctuation group weakens the effect of 11-year cycle fluctuation by weakening the decrease degree of sub-polar salinity and causing subtropical salinization.The multi-period fluctuation of TSI extends the period of AMOCI relative to the control test,but the centennial period fluctuation component has more obvious effect on the period extension.