The Possible Effects Of External Forcings On The Global Monsoon Rainfall Change

Using GPCP and GPCC precipitation observations, the simulations of NCAR CCSM3.0 climate model driven by natural and anthropogenic forcings (greenhouse gases, sulfate aerosol, ozone, black carbon aerosol, volcanoes and solar activity) in the project "20th Century Climate in Coupled Models" (20C3M) and the simulations of the A1B scenario to discuss the possible effects of external forcing on the global monsoon rainfall. Firstly discuss the possible impact on the global monsoon rainfall from the PDO and AMO. Further evaluate CCSM3.0 model’s performances in the global monsoon precipitation to discuss the possible effects of external forcing in the 20th century on the global monsoon rainfall trend and to explore the volcanic eruption of the 20th century impact on the global land monsoon precipitation. Finally, we discuss the changes in the 21th century global monsoon precipitation from the closest to the real situation of the A1B scenario. We get the following conclusions:(1) PDO (AMO) has obvious negative (positive) relationship to the global land monsoon precipitation through the observation and CCSM3.0 simulation data. Under the different phase composition of the PDO and AMO index play a key role for the global monsoon rainfall. PDO negative (positive) corresponding to the AMO positive (negative) phase, the global monsoon rainfall anomaly more (less). Comparison control and simulation results show that global land monsoon rainfall with AMO significantly positive correlation after 1990s, significantly positive correlation with PDO at north American monsoon region, monsoon region in northeast China monsoon rainfall significantly positive correlation with AMO and under the different phase composition of the 20th century the PDO and AMO is mainly caused by external forcing effect.(2) The global monsoon rainfall in full forcing simulation exhibits a distinct increasing trending the 20th century, which is mainly due to the change of the anthropogenic forcing, especially the greenhouse gases. The simulation driven by the greenhouse gases could produce "Eastern Pacific (EP) cooling and the west pacific (WP) warming" pattern and the increasing land-ocean and inter-hemispheric thermal gradients will increase the global monsoon rainfall with a significant rise trend in the 20th Century. The global land monsoon precipitation anomaly large when the strong volcanic eruption in the 20th century. The global land monsoon precipitation decreased within two years after the eruption, two years later start rise. The influence of strong volcanic eruption on global land monsoon precipitation in twentieth century is mainly caused by the volcanic action in the middle and low latitudes.(3) The global monsoon rainfall at the beginning of the 21st century (2000— 2020) as the downward trend, and after 2020 as the growth trend. The global monsoon rainfall is a growing trend in the 21st century. In the twenty-first century the PDO and AMO have the same change trend, Until the 2030s as the rising trend, the 30s to 60s as the downward trend, after 60s to rise. In the 21th century the PDO is more important than AMO to the global monsoon rainfall change, play a leading role.