A Modelling Study Of Temperature And Precipitation In East Asian Monsoon Region Over The Past 425 Ka

The orbital scale paleoclimate evolution in East Asia has always been one of the hotspots of palaeoclimatology.Over the past few decades,some progresses about the climate evolution of the East Asian monsoon have been made due to the development of geological reconstructions and modelling studies.However,up to now,there are still considerable controversies regarding the temporal and spatial evolution of temperature and precipitation in the East Asian monsoon region during the glaciation-interglacial cycles.Related disputes are mainly reflected in the following two aspects.First,the traditional paleoclimatological view suggests that monsoonal China was overall wetter when the East Asian summer monsoon(EASM)was strong.However,this view is at odds with the understanding of modern climate,which demonstrates an analogous“tripole precipitation pattern”in monsoonal China.The background of the traditional paleoclimatological view is that the paleo-precipitation reconstructions are mostly in North China,but scarce in southern China.Therefore,the spatial distribution pattern of precipitation in the East Asian monsoon region on the orbital time scale remains to be further investigated.Second,different geological reconstructions and simulation studies show different views about the driving mechanism of temperature and precipitation in the East Asian monsoon region on orbital time scales.Therefore,to further explore the temporal and spatial evolution characteristics of temperature and precipitation in East Asia,in this study a series of equilibrium simulations were conducted by using the low-resolution version of the Norwegian Earth System Model(Nor ESM-L)over the past 425ka.And to investigate the East Asian climate sensitivities to potential forcings,a set of sensitivity experiments were also designed.By comparing with the geological records,our research mainly gained the following insights:(1)Similar to the present understanding of modern East Asian climate,the tripole precipitation pattern in monsoonal China likely occur during the past glacial-interglacial cycles.Our simulations illustrate that more(less)precipitation in North China and South China,and less(more)precipitation in the Yangtze River Valley during strong(weak)EASM periods associated with high(low)boreal summer insolation.Such tripole precipitation pattern in monsoonal China revealed by the simulations can be supported by paleoclimate records available from China to some extent.This temporal and spatial variability in precipitation in monsoonal China highlight the complex hydroclimatic conditions in monsoonal China.Due to this variability,it remains possible that glacial precipitation was higher than modern,particularly in the Yangtze River Valley.(2)Our simulations confirm that the boreal summer insolation is the dominant forcing for the intensity of EASM and the response of subtropical high pressures is fundamental in modulating the precipitation pattern in monsoonal China on orbital timescales.The changes in boreal summer insolation modify the land-sea thermal contrasts in East Asia,which modifies the intensity of EASM,and this is consistent with the previous studies.Our simulations reveal that when the boreal summer insolation is higher,the EASM is strengthened,more moisture reaches North China,resulting in more precipitation in North China;at this time,it also corresponds to the westward extension and strengthening of the western Pacific subtropical high.These make the Yangtze River Valley is controlled by the western Pacific subtropical high,which limits the summer precipitation in this region;at the same time,the upward movement in South China has increased,which is conducive to the precipitation.Conversely,the situation is different when the boreal summer insolation decreases.It should be noticed that this mechanism on the orbital time scales must be distinguished from the mechanism for the tripole precipitation pattern on the decadal time scales.On the decadal time scales,the westward migration of western Pacific subtropical high is caused by a regional anormal heating forcing,for example in the Indian Ocean,not accompanied with the intensification of whole subtropical high pressures.(3)The key forcing factor of the annual mean temperature in the East Asian monsoon region on the orbital time scale is the atmospheric CO₂ concentration.In addition,the ice sheet may also have an important effect on the annual mean temperature to some extent.Our simulations show that the annual mean temperature in East Asia is dominated by the?100-kyr cycle.Further correlation and sensitivity analyses also illustrate that the annual mean temperature in East Asia has a high correlation with the atmospheric CO₂concentration/ice volume,and the sensitivity of the annual mean temperature to the atmospheric CO₂ concentration/ice volume is relatively high.These characteristics all indicate that the atmospheric CO₂ concentration or the inclusion of ice volume plays a major role in the modulation of the annual mean temperature in East Asia.(4)The paleo-temperature reconstructions may indicate the annual mean signal.Unlike the dominated?100-kyr cycles for the simulated annual mean temperature in East Asia,the?20-kyr cycle is dominant in the seasonal mean temperature in East Asia.However,the reconstructed paleo-temperature in East Asia shows the dominant?100-kyr cycle,rather than the?20-kyr cycle.Therefore,the comparison between simulation and reconstruction shows that the reconstructed paleotemperature may reflect the annual mean temperature,rather than seasonal temperature or growing season temperature.(5)Our study demonstrates that the cycles of the simulated annual mean precipitation are highly model-dependent.The dominant cycles of the annual mean precipitation in East Asia revealed by our simulations are quite different from the cycles shown in previous studies.Further sensitivity analyses also show that the sensitivities of the annual mean precipitation calculated by different models to potential forcings are quite different;when the same boundary conditions are changed,the spatial distributions of annual mean precipitation in East Asia based on different models are also different.The comparison of simulated and observed precipitation shows that the main source of uncertainty in simulated annual precipitation in East Asia may be the winter and spring precipitation.However,for the summer precipitation,our simulations are consistent with existing simulation results,and most of them show that summer precipitation is dominated by the?20-kyr cycle.These indicate that the uncertainty of the simulated summer precipitation cycle may be small.Our modelling study about the East Asian monsoonal climate further reveal the advantages and limitations of the current model in paleoclimate research.Especially for precipitation,the advantage of simulation research still focuses on the exploration of variability;its characterization of absolute quantity has greater uncertainty.Because of this,it is difficult to obtain consistent results when comparing the cycles of simulated annual mean precipitation by different models;while exploring the spatial or temporal variability of precipitation,it can match the geological reconstruction better.In this sense,in future,the uncertainty of simulated precipitation needs to be paid more attention.