The East Asian monsoon system, which is an important part of the climate system and the circulation system in the Northern Hemisphere, results in warm, humid summer monsoon and cold, dry winter monsoon over the East Asia continent and exerts a profound influence on living environments in the most Asian regions. East Asian summer monsoon and East Asian winter monsoon are two major branches of the East Asian monsoon system. Significant variabilities of East Asian monsoon, especially the monsoon′s advancing, retreating and conversion are closely related to the global climate system, resulting flood, drought, freezing, rain and snow disasters and may cause great damage to agriculture and industrial production, transportation, and daily life of people in many East Asian countries. Research on long-term monsoon dynamics is directed toward understanding the monsoon climate change and it is of great significance for the prediction and assessment of future global monsoon changes. Scientists have studied East Asian monsoon variability and its mechanism and various levels of factors have been hypothesized as the dominant Asian monsoon forcing, such as the uplift of the Qinghai-Tibet Plateau, orbital forcing, solar activities, high-latitude ice volume, the Inter Tropical Convergence Zone (ITCZ), sea surface temperature (SST), El Niño-Southern Oscillation (ENSO) and global cold events. However, the dominant monsoon forcing can change from interannual to millennial time scales and it is still unclear the links of monsoon forcing between different time scales.
Varieties of data and calculating methods were used to discuss the variability and mechanism of the Asian monsoon between different time scales. Varieties of data included NCEP/NCAR reanalysis data, SST, sea ice, the monthly air temperature and precipitation, monthly mean total sunspot number, Niño-3.4 SST index, as well as the paleo-climatic proxies. Calculating methods included the definition of monsoon index, field analysis, spatial correlation, sensitivity analysis, space check and monsoon simulation methods. Firstly, we discussed the annual variation of East Asian summer and winter monsoon during 1964—2013 and the correlation between East Asian summer monsoon and summer precipitation, East Asian winter monsoon and winter temperature. Further, we researched variability and mechanism of the Asian monsoon on interannual to interdecadal time scales. Secondly, with regards to a comparison between the modern and millennial-scale monsoon change, the monsoon sensitive zone, which is both sensitive to summer and winter monsoon, was selected. The paleo-climatic proxies from the monsoon sensitive zone were used to reconstruct millennial-scale summer and winter monsoon variations. Monsoon simulation data were used to analyze Holocene Asian summer and winter monsoon variations, as well as to explore the paleo-monsoon mechanism. Lastly, by analyzing the relationship between the monsoon and the solar radiation, the Arctic sea ice, atmospheric circulation, we discussed the variability and mechanism of the Asian monsoon between different time scales. We get the following conclusions:
(1) According to the modern observation data, the time series of the East Asian summer monsoon and winter monsoon index from the 1964 to 2013 were calculated. The East Asian summer monsoon and East Asian winter monsoon from the 1964 to 2013 show a strong interannual and interdecadal variation. While there are several extreme values in monsoon indices, the East Asian summer monsoon and winter monsoon show a slight weakening trend.
(2) The correlation area between East Asian summer monsoon and summer precipitation, East Asian winter monsoon and winter temperature has a certain spatial pattern. East Asian summer monsoon and the summer precipitation in the north of the Yangtze river and the Huaihe river basin have a positive correlation, while in the south of the Yangtze river and the Huaihe river basin have a negative correlation. On the whole, East Asian winter monsoon is negatively related with winter temperature over East Asia, and the highly correlated area of more than 95% significance covers Northwest China, East China, the Korean peninsula and the area near the Japan sea.
(3) Based on the modern climatology, the monsoon sensitive region which is both sensitive to summer and winter monsoon is located in the northeast of the Qinghai-Tibet Plateau region. In the monsoon sensitive area, East Asian summer monsoon and summer precipitation have an obviously positive correlation, while East Asian winter monsoon and winter temperature have an obviously negative correlation. This suggests when the East Asian summer monsoon is strong, the summer precipitation in the northeast of the Qinghai-Tibet Plateau region tends to be more than normal, and when the East Asian winter monsoon is strong, the winter surface temperature in the northeast of the Qinghai-Tibet Plateau region tends to be low than normal.
(4) The reconstructed and simulated Holocene summer monsoon indicator reveals a significant increase since the Last Deglaciation, and maintains at a relatively high level during 10000-6000 cal yr BP. In the early Holocene, the East Asian summer monsoon is stronger, and the monsoon precipitation is more, and then the East Asian summer monsoon shows a weakening trend and the monsoon precipitation is least. As a whole, the East Asian summer monsoon in East Asia is gradually weakening from the early Holocene to the late Holocene. The reconstructed and simulated Holocene winter monsoon indicator generally shows that the East Asian winter monsoon is relatively strong during the Last Deglaciation and early Holocene and weakens since the mid-Holocene.
(5) By studying the variability and mechanism of East Asian summer and winter monsoon on different time scales, it can be found that the main driving factor of the East Asian monsoon has significant differences on various time scales. The Holocene East Asian summer and winter monsoon intensity showed a weakening trend that was consistent with the low-latitude summer insolation change for summer monsoon and mid-latitude winter insolation for winter monsoon. On interannual to interdecadal time scales, the Arctic sea ice anomalies and SST anomalies in North Pacific can cause atmospheric circulation anomalies, meanwhile, the atmospheric circulation anomalies can also cause the Arctic sea ice anomalies and SST anomalies.