East Asian Summer Monsoon Interdecadal Change Research

Statistical analysis and numerical model are employed to investigate the East Asian summer monsoon weakening in the late 1970s. The dissertation includes following three parts:Based on observation, a comparison between ERA-40 and NCEP/NCAR reanalysis data sets on interdecadal changes of East Asian summer monsoon is carried out. It is found that both reanalysis data display pronounced features of monsoon variation in the late 1970s which is characterized by decreasing surface air temperature, rising sea level pressure,and northern wind anomalies over Jianghuai valley. Besides the elements over monsoon domain, systems related also demonstrate clear changes including southwestern shift of subtropical high over northwestern Pacific, strengthened westerlies over the middle latitudes, positive Antarctic Oscillation trend, strengthened Mascarene high, and Australian high. Excessive amplitude of monsoon weakening in the NCEP/NCAR reanalysis data diminishes its utility in climate research on decadal timescale.To further investigate the extent to which the global sea surface temperature (SST) and sea ice contribute to monsoon weakening, an experiment with IAP-9L atmospheric general circulation model (AGCM) driven by monthly global SST and sea ice in 1958~1999 is conducted. It is shown that only one of five integrations qualitatively reproduces the features of monsoon change. The discrepancy among simulated results suggests that the influence of global SST and sea ice on monsoon change is weak.The relationship between the Arctic Oscillation (AO)/the North Atlantic Oscillation (NAO) and the East Asian summer monsoon weakening is proposed. Associated with the strong AO/NAO, a negative geopotential height tends to be formed over Lake Baikal region. With the anomalous southern flow to the east and strengthen westerlies to the south of negative anomalies, a significant convergence (divergence) at upper (lower) level favors an anomalous sink motion over the northern part of Korea Peninsular. Downward movement enhances the local Hadley cell. Anomalous northern flow of enhanced Hadley cell at low troposphere over the East China and adjacent area contribute to southern flow weakening.