Decadal-Centennial Variations And Mechanisms Of East Asian Monsoon Over The Past 1500 Years

Studying the characteristics and mechanisms of East Asian monsoon changes on decadal-centennial timescales over the past 1500 years has an important scientific significance in prediction of the future changes of the East Asian summer monsoon(EASM) and a better understanding of the climate variability during Present Warm Periods (PWP). Based on the simulation over the past 1500 years using the Community Earth System Model (CESM), the spatial-temporal variabilities of the EASM precipitation and the East Asian winter monsoon (EAWM) temperature on decadal-centennial timescales during the past 1500 years are investigated, as well as the different characteristics of the EASM interdecadal variability during typical warm periods. The causes and mechanisms of the EASM and the EAWM changes on the decadal-centennial timescales are also examined in this paper.Firstly, to assess the performance of CESM, the simulated surface temperature,precipitation rate,850 hPa winds, sea level pressure and sea surface temperature are compared with observation/reanalysis and reconstruction data. Overall, the agreement between the simulated and observed annual mean and annual cycle, as well as the decadal-centennial variation of temperature in China over the past 1500 years adds confidence to our subsequent analysis of the decadal-centennial timescales variability based on the outputs from the CESM runs.Secondly, the spatial-temporal variabilities of the EASM precipitation and the East Asian winter monsoon (EAWM) temperature on decadal-centennial timescales during the past 1500 years simulated by all forcings (AF) experiment are investigated. The results show that there are decadal droughts over East Asia around 1250 AD, 1450 AD,1600 AD, 1750 AD and 1815 AD. The spatial distribution of the EASM precipitation is characterized by positive rainfall anomalies over northern China and the south of the Yangtze River valley and negative rainfall anomalies over the Yellow River basin to the south of Japan, which is consistent with the characteristics of the EASM rainfall changes on interdecadal timescale during Medieval Warm Period (MWP).Furthermore, the interdecadal leading mode of the EASM during PWP is characterized by a north-south dipole rainfall pattern,and the significant interdecadal periods of the EASM during MWP and PWP are about 10 and 20 years. On centennial timescale, there exist five dry periods within the simulated EASM precipitation variation over the past 1500 years, including 530-710 AD, 850-950 AD, 1250-1350 AD, 1440-1530 AD, 1630-1860 AD, and four wet periods involving 730-840 AD,1060-1240 AD, 1330-1420 AD and 1520-1620 AD. The spatial pattern of the EASM precipitation is overwhelmed by higher summer precipitation over the whole eastern China.There are decadal coolings over East Asia around 1250 AD, 1450 AD and 1815 AD. The EAWM temperature increases simultaneously in the whole East Asia with warmer amplitude over land than the sea due to the weak EAWM. On centennial timescale, there are two warm periods within simulated EAWM temperature variation over the past 1500 years, i.e., 800-1250 AD and 1900-2000 AD, and a cold period in 1400-1900 AD. The spatial pattern of EAWM temperature shows that warmer conditions dominate over the whole East Asia. Under the influence of warm air from mid-low latitude, the temperature over the west of 110°E is warmer than that over the east of 110°E.Thirdly, Comparing the EASM precipitation and the EAWM temperature changes on decadal-centennial timescale from AF runs with the outputs from individual forcing experiments under each external forcing, it is found that total solar irradiation (TSI), volcanic eruptions (Vol) and internal variability play primary roles in driving model's EASM precipitation changes on decadal timescale. Anthropogenic forcings make no difference to the decadal EASM precipitation changes. The spatial pattern of floods and droughts over the East Asia is mainly controlled by the internal variability. TSI forcing and Vol forcing have modulating effects on this spatial pattern.Under the influence of internal variability, the tropical central-eastern Pacific cooling plays an essential role in the development of the western Pacific warming by strong equatorial easterly wind and Walker circulation. The enhanced convection over the southern tropical northwest Pacific drives cyclonic anomalies centered over the Philippine Sea according to the Gill model. The atmospheric perturbation propagates poleward as Rossby waves and forms an anticyclonic anomaly over northwest Pacific,giving rise to suppressed convection over the Yellow River basin to the south of Japan with decreased precipitation. However, on the modulation of natural forcings, the SST cooling anomalies over the west of northwest Pacific are less obvious than the outputs of control run, thus the summer precipitation over the Yellow River basin to the south of Japan are higher than that of control run. A further study indicates that the Pacific Decadal Oscillation (PDO) has a significant negative correlation with the interdecadal change of the EASM during MWP and PWP. On centennial timescale, the fluctuations of the EASM precipitation over the past 1500 years are mainly affected by TSI forcing, Vol forcing and greenhouse gases (GHGs) forcing, but land use and land cover (LUCC) have little influence on it. The spatial pattern of EASM precipitation is attributed to TSI forcing and Vol forcing. Under the influence of natural focings, the wetting pattern over eastern China is caused by the increase of latent flux over Indian Ocean and tropical Pacific Ocean with favorable water vapor transport from Indian Ocean and tropical Pacific Ocean.Vol forcing and internal variability of climate system are main factors responsible for the change of EAWM temperature on decadal timescale, except TSI and anthropogenic focings. The spatial structure of EAWM temperature on decadal timescale is caused by internal variability. Furthermore, Vol forcing regulates the effect of the EAWM temperature change. Under the Vol forcing, the winter temperature over East Asia mainland of AF run is lower than that of control run,which is induced by reduced effective solar radiation. On centennial timescale, TSI and GHGs are the dominant factors for the EAWM temperature changes during the past 1500 years. Vol and LUCC have no significant effect. The warm condition of the EAWM temperature after industrial revolution is mainly caused by the increase of CO2 concentration. For the spatial pattern, the winter temperature over the west of East Asia mainland is higher than that over the east of East Asia mainland due to the distribution of TSI. The warmer condition over the west of East Asia is related to the higher surface net radiation flux for more shortwave radiation absorbed.