地球轨道对北大西洋淡水注入影响印度夏季风的调节作用:以8.2 ka和4.2 ka事件为例

ORBITAL MODULATION OF INDIAN SUMMER MONSOON RESPONSE TO NORTH ATLANTIC FRESHWATER FORCING:CASE STUDY OF 8.2 KA AND 4.2 KA EVENTS

北大西洋淡水注入触发的千年尺度气候突变事件发生在不同地球轨道背景下,理解地球轨道参数对印度夏季风千年尺度变率特征的调节作用,对理解未来印度夏季风对北大西洋淡水注入的响应具有重要的科学意义。本研究利用通用地球系统模式CESM,探讨印度夏季风在8.2 ka B.P.和4.2 ka B.P.对相同北大西洋淡水注入的响应差异。模拟结果显示,北大西洋淡水注入使得印度夏季风强度显著减弱,其中夏季风降水变化在这两次事件中没有显著的空间差异,但变化幅度在4.2 ka B.P.要显著大于其在8.2 ka B.P.,表明地球轨道参数对印度夏季风千年尺度变率特征具有重要的调节作用。进一步分析显示,地球轨道并非通过影响温盐环流强度进行调节,而与夏季太阳辐射的高低有关。在4.2 ka B.P.时,在相同的淡水注入下,由于夏季太阳辐射较低,加剧了北大西洋的降温,同时也增强了其对下游大气的冷却作用,使得欧亚大陆南部对流层中上层大气具有更大的降温幅度,这进一步削弱了欧亚大陆南部与赤道印度洋对流层中上层大气的经向温度梯度,从而导致印度夏季风相较于8.2 ka B.P.具有更大的衰退幅度。因此,在较低夏季太阳辐射背景下,印度夏季风对北大西洋淡水注入的响应更为敏感。

The Indian summer monsoon(ISM),which is the dominant factor controlling precipitation changes in South Asia,Southeast Asia,and Southwest China(5°~25°N,60°~110°E),is sensitive to the variability of the Atlantic meridional overturning circulation(AMOC)resulting from the input of the freshwater into the North Atlantic.Understanding millennial⁃scale ISM variability is critical for climate projection in ISM⁃controlled regions in the context of slowing AMOC caused by global warming.This study intends to explore the modulation of the Earth's orbital parameters in the ISM response to North Atlantic freshwater forcing based on climate simulations using the Community Earth System Model(CESM).We performed two groups of sensitivity experiments at 8.2 ka B.P.and 4.2 ka B.P.,respectively,during which the orbital parameters are significant different.In the control runs(Exp_8.2ka and Exp_4.2ka),the orbital parameters were set at the conditions of 8.2 ka B.P.and 4.2 ka B.P.,respectively.The concentrations of greenhouse gases(CO_(2),CH4,N2O)were set to the values at 8.2 ka B.P.for both the Exp_8.2ka and Exp_4.2ka,to exclude the possible effect of the greenhouse gases.In the sensitivity experiments(Exp_8.2ka_FW and Exp_4.2ka_FW),we added additional 0.2 Sv freshwater into the North Atlantic.The orbital parameters and greenhouse gases concentrations of the Exp_8.2ka_FW and Exp_4.2ka_FW were kept the same with the Exp_8.2ka and Exp_4.2ka,respectively.We compared their differences(Exp_8.2ka_FW minus Exp_8.2ka and Exp_4.2ka_FW minus Exp_4.2ka)to evaluate the effect of the Earth's orbital parameters on the ISM response to North Atlantic freshwater forcing.Simulation results reveal significantly weakened AMOC and ISM precipitation both at 8.2 ka B.P.and 4.2 ka B.P.with the freshwater forcing.There was no significant difference in the spatial pattern of precipitation changes between 8.2 ka B.P.and 4.2 ka B.P.However,ISM precipitation experienced a much greater reduction at 4.2 ka B.P.than at 8.2 ka B.P.,suggesting an important role of the Earth's orbital effect on the ISM response to North Atlantic freshwater forcing.Orbital modulating effect was not via the strength of the AMOC,since the reduction of the AMOC was a bit greater at 8.2 ka B.P.than at 4.2 ka B.P.Further analyses suggest that the difference in the imprint of the freshwater forcing on ISM precipitation was probably triggered by summer insolation.North Hemisphere received lower summer insolation at 4.2 ka B.P.than at 8.2 ka B.P.due to the difference in Earth's axial precession.This leaded to a much colder North Atlantic and the resultant larger drop in the tropospheric temperature downstream at 4.2 ka B.P.relative to that at 8.2 ka B.P.Especially,the tropospheric temperature decrease over the southern Eurasia caused a weakened meridional tropospheric temperature gradient(ΔTT)between the southern Eurasia and tropical Indian Ocean.The reduction inΔTT weakened the ISM via shortening the monsoonal season and inducing anomalous downward motions over South Asia.Accordingly,a much weakerΔTT resulted in a much bigger collapse of the ISM at 4.2 ka B.P.Hence,the ISM was much more sensitivity to North Atlantic freshwater forcing at 4.2 ka B.P.than at 8.2 ka B.P.