East Asian Summer Monsoon Variations In The Last Millennium Simulations:Modeling Evaluation And Mechanism Investigation

The East Asian summer monsoon(EASM)has an important influence on the climate of China,especially of the eastern China region.Against the background of global warming,the change of EASM is becoming more complicated.The study of EASM variations in paleoclimate is of great significance for us to understand the changes of modem EASM.Reconstructions show that there are three typical climate anomalies intervals over the last millennium:Medieval Climate Anomaly(MCA),Little Ice Age(LIA)and modem warm period.Because last millennium period was the most recent typical climate anomalies period from modem times,it has become one of the focuses in paleoclimate research in recent years.During the last millennium,especially during the MCA and LIA,the EASM underwent significant changes.Due to limitation of reconstruction data,the dynamic mechanism behind the EASM changes remain unclear.Paleoclimate models simulation has become an important tool in the research of climate changes during past thousands of years.Paleoclimate Modeling Intercomparison Project Phase III(PMIP3)has joined the last millennium experiments,which will allows us to discuss the possible causes of changes in EASM during the past time with multi-model simulations.Essentially,the EASM variations feature changes in atmospheric circulation,while generally reconstructed by precipitation-based proxies in paleoclimate studies.In this paper,based on the PMIP3 last millennium experiment,we directly define the lower-troposphere meridional winds over East Asia as the EASM strength.Then we explore the possible responses of the EASM intensity to external forcings,as well as its impact on precipitation over East Asia during the last millennium(850-1850 A.D.).We further discuss the influence of the EASM on climate of Asia and surrounding areas.The main conclusions are given as follows:(1)Changes in the EASM strength over the last millennium.The EASM strength in the PMIP3 last millennium simulation is possibly affected by total solar radiation and volcanic eruptions on centennial timescale.In addition,majority of the PMIP3 models and their ensemble average result showed that the EASM was stronger during the MCA and weaker during the LIA than its average status over the last millennium,which was consistent with multi-reconstructions.Although the total radiation during the MCA was stronger than that during the LIA,the spatial distribution and amplitude of temperature change are different among PMIP3 models due to the different sensitivity of eachmodel to the incident radiation.This leads to different changes of land-sea temperature contrast and upper-level westerly jet,ultimately making the spatial and amplitude variation of EASM varies among PMIP3 models.Typically,the stronger(weaker)EASM during the MCA(LIA)resulted in the increase(decrease)of precipitation over East Asia,but they are not corresponding to each other on multidecadal timescale.The decrease of East Asian rainfall is not only caused by a weaker EASM,but possibly also a global phenomenon during a reduction of incoming radiation.(2)The unstable relationship between the EASM strength and East Asian precipitation.The relationship between the EASM strength and precipitation in East Asia varies with timescales in PMIP3 last millennium simulations.On centennial scale,the relationship between the EASM strength and Asia rainfall are quite stable:strong summer monsoon induces enhanced precipitation over the whole East Asia,and vice versa;while on multidecadal scale,the relationship between them varies with time,with a significant-60 years fluctuation.Such unstable relationship between the EASM and East Asia precipitation was also detected in the Community Earth System Model Last Millennium Ensemble,and may be the result of the internal variability of the climate system.This result suggests that the precipitation-related proxy used to reconstruct the EASM intensity possibly reflects only monsoon variability on centennial timescale.(3)The in-phase variation of the EASM and Australian summer monsoon during the last millennium.Humidity reconstructions in East Asia and northern Australia reflect the simultaneous enhancement(weakening)of the EASM and Australian summer monsoon(ASM)during the MCA(LIA).The results of PMIP3 simulations show that the synchronization of two monsoons changes exists on centennial timescale,and the synchronization during the LIA is more-obvious than that during the MCA.During the MCA,relative to the LIA period,the EASM is stronger due to the enhanced land-sea temperature difference and the northward shift of northern hemispheric subtropical westerly jet;the ASM is increased related to the easterly anomalies over the equatorial Pacific and the southward movement of southern hemispheric subtropical westerly jet.Solar radiation is the main reason for the simultaneous changes of the two monsoons in different hemispheres.Moreover,the zonal temperature gradient between the eastern and western Pacific plays an important role.(4)Tripole anomalous humidity pattern over Asia over the last millennium.Reconstructions show that arid central Asia(ACA),North China and South China exhibited a "dry-wet-dry" tripole humidity anomalies distribution during the MCA and an opposite pattern during the LIA.Using MRI-CGCM3,we found that the tripole anomalous humidity distribution occurs on at least multidecadal timescale over the last millennium.The main cause of such tripole distribution is the precipitation anomaly in spring and summer over three regions.In spring of the MCA(LIA),water vapor transportation from the North Atlantic and the Mediterranean to arid areas of central Asia reduced(enhanced)due to the northward(southward)movement of the westerlies.It further results in less(more)precipitation over ACA.While in summer,an anomalous high(low)pressure companied with descending(ascending)airflow over ACA is unconducive(conducive)to precipitation over that region.In East Asian monsoon region,due to the strengthened(weakened)summer monsoon during the MCA(LIA),precipitation over both North China and South China increase(reduce).However,owing to the weaker(stronger)western Pacific subtropical high in spring,precipitation over South China reduces(enhances)and makes the annual precipitation decrease(increase).In addition,the tripole anomalous Asian precipitation during the MCA(LIA)is closely linked with a La Nina(El Nino)-like sea surface temperature anomalies.