Asian Monsoon Climate Change And Effects Of Different External Forcings During Historical Period

Over the past century, the effects of global warming on Asian monsoon have been paid much attention to by the scientific community and the role of human activities in climate change is controversial. To know the influence of human activities on climate change, it is necessary to put the modern climate change in the context of the historical climate change and the past two thousand years is one of the important periods for historical climate change research. Based on the climatic simulation results from the Community Earth Systems model (CESM), the responses of the Asian summer monsoon(ASM) precipitation and the Asian winter monsoon (AWM) temperature to the major external forcings during the past 1500 years are studied in this paper, respectively. The characteristics and influencing factors of the ASM precipitation and the AWM temperature on centennial time scale are investigated too.To evaluate the model performance for the Asian monsoon, the results from the all forcings experiment are compared with observation/reanalysis datasets, the simulated data from the CMIP5 models and the reconstructed datasets, respectively. The results show that the spatial structure of the modern climate state and the temporal characteristics of historical temperature over the Asian monsoon region are well simulated by the CESM. So,the results of the CESM simulations can be used in this study.The responses of the ASM precipitation and the AWM temperature to the major external forcings during the past 1500 years are discussed through the comparison between the results of control experiment and several sensitivity experiments driven by single forcing. On the centennial time scale, the total solar irradiation and the volcanic eruptions are the main factors to modulate the ASM precipitation during the past 1500 years. The precipitation increases (decreases) according to the strengthening (weakening) of the total solar radiation. During the periods of volcanic activity,the precipitation correspondingly reduces. The greenhouse gases and the land use/land cover change have some effects on the precipitation,too. Increased concentrations of greenhouse gases make the precipitation have a certain degree of increase. While the land use/land cover change has some extent of influence on the centennial cycles of the ASM precipitation. The modulation effects of the total solar irradiation, the volcanic eruptions and the greenhouse gases on the AWM temperature are significant. While the effect of the land use/land cover change on the AWM temperature is not so significant as the other three external forcings. Under the action of the total solar radiation, the change of the AWM temperature is very consistent with the solar radiation. The strengthening of solar radiation makes the whole Asian monsoon region consistently warm up, while volcanic eruptions make the whole Asian monsoon region cool in winter. The AWM temperature change is closely related to the concentration of greenhouse gases. The concentration of greenhouse gases increases and the temperature rises. The modulation effect of the land use/cover change is relatively weak.The precipitation and temperature present an in-phase variability on the centennial time scale over both the Asian monsoon region and its sub-regions. The results of spectrum analysis indicate that there are three periodicities (100-yr, 150-yr and 200-yr) in the ASM precipitation. It can be found that the 100-yr cycle is affected by four external forcings through the comparison with the results of other experiments. The 150-yr cycle is mainly related to the internal variability of climate system. While the volcanic eruptions are the main factor affecting the 200-yr cycle. To find out the spatial characteristic of precipitation,the empirical orthogonal function (EOF) is used. The spatial structure of the first EOF mode of the ASM precipitation (EOF1) shows negative value over the eastern Arabian Sea,the southern Indian peninsula, the North China Plain and the eastern region of the plain of the same latitude and shows positive value over other regions. The main factors to affect the EOF 1 are the internal variability of climate system and the volcanic eruptions.Meanwhile, the spatial structure of the second EOF mode of the ASM precipitation (EOF2)shows the "negetive-positive-negetive-positive" banded structure over the east part of 100°E while it shows the "negetive-positive - negetive" structure from southwest to northeast over the India monsoon region. The EOF2 is related to the total solar irradiation and the internal variability of climate system.The winter temperature variabilities over the whole Asian monsoon region and its sub-regions are consistent on the centennial time scale over the past 1500 years. There are three periodicities (100-yr, 150-yr and 200-yr) both in the AWM temperature and the temperature of its sub-systems. The 100-yr cycle and the 200-yr cycle of the AWM temperature are both mainly influenced by the internal variability of climate system. While the 150-yr cycle is related to the total solar irradiation and the land use/land cover change.The EOF1 mode of the AWM temperature on the centennial time scale is an in-phase pattern in the whole region and the EOF2 is a cold-north and warm-south (or vise versa)pattern. The main causes of the EOF 1 are the total solar irradiation and the greenhouse gases. The EOF 1 modes of the temperature over the sub-regions are also in-phase patterns and the corresponding PC series are similar to the sequences of regional average temperature over the sub-regions.