Climate And Ocean Circulation Response To The Atmospheric CO₂ And Paleogeography Changes During The Paleogene

Global climate had undergone the transition from a warm-house state to a colder one during the Paleogene.The early Eocene was the warmest climate period of the Cenozoic associated with high atmospheric CO2([CO2]atm),which can be served as a potential analogue for future climates.Therefore the model-data comparison for the early Eocene climate is useful for us to assess the model skill of reproducing warm-house climate,and enhance understanding of paleo-and future climate.In this study,simulations of the latest Paleocene-early Eocene period have been carried out in the framework of DeepMIP,and the model outputs are compared comprehensively with proxies.The results show that on a global scale,the best agreement with proxy-based surface temperatures of the latest Paleocene(LP),the Paleocene-Eocene Thermal Maximum(PETM)and the early Eocene climate optimum(EECO)occurs at[CO2]atm of around 1120 ppm,around 4480 ppm,and between 1120 and 2240 ppm,respectively.However,the simulated low-(high-)latitude temperatures are too high(low)as compared to the proxy data.At[CO2]atm of 1120 ppm,the EECO land surface temperatures in the midlatitudes obtain the most reasonable match between modelling and proxy data.Considering this[CO2]atm is within the reconstructed range during this period,it is suggested that the EECO global climate is probably best constrained by land surface temperatures in the midlatitudes.Although previous studies have shown that the Paleogene ocean circulation could be influenced by changes in[CO2]atm and paleogeography,the potential effect of the varying Eurasia epicontinental sea during the early Eocene is less investigated.In this modelling,results show that when there is open connections between the peri-Tethys and the Nordic Sea,and between the West Siberian Sea and the Arctic,the North Atlantic deep water forms in Labrador Sea and drives the global ocean circulation.There occurs no switch of the deep-water source region and circulation with elevated[CO2]atm.However,when the connection between the peri-Tethys and the Nordic Sea is closed,the deep convection pattern is largely altered,as there occurs a bimodal structure of sinking at both the northern and southern end of the Pacific.From the view of this study,it is possible that the sea level rise during the PETM could have enlarged the seaway between the peri-Tethys and the Nordic Sea and triggered the short-term circulation reversal.The results also imply that the isolation of the West Siberian Sea is from the Arctic at the end of the early Eocene is a possible mechanism accounting for the freshing of the Arctic and the Nordic Sea at 49-48 Ma.Recently,the paleolatitudinal and paleoelevational evolution models of the proto-Tibetan Plateau(proto-TP)during the Eocene-early Miocene have been updated,which should be considered when modelling the relationships between the tectonic evolution of Tibet Plateau and regional paleoclimate.In this study,influence of the latitudinal movement of the proto-TP on East-Central Asian climate is explored using numerical sensitivity experiments.The results show that under modern boundary conditions,proto-TP migrating northward within the subtropics can intensify East Asian monsoonal climate and dry Central Asia.In comparison with previous modeling and proxy-based climatic records,we suggest that the northward migration of the proto-TP in the Paleogene could have intensified the aridity in Central Asia,but its influence on East Asian precipitation and monsoonal circulation could be dependent on the paleogeography and other boundary conditions.