East Asian summer monsoon has experienced significant decadal shift around the late 1970s. This shift is characterized by a weakening summer monsoon circulation and a shift in rainfall patterns over East China that is often referred to as the “south flooding and north drought”. Using model simulations with NCAR CAM3 and GFDL models with different forcings, such as global SSTs (GOGA), tropical SSTs (TOGA), global SSTs plus greenhouse gases and aerosol forcings (GOGAI), and climatological SSTs with time-varying greenhouse gas and aerosol forcings (RADATM), we examined the relative roles of these individual forcing in causing the observed monsoon and rainfall changes over East Asia.

The CAM3 and GFDL model capture the inter-decadal weakening of East Asian summer monsoon and corresponding circulation, such as stronger subtropical high and southward shift of high level jet over East Asia. However, the models failed to simulate the inter-decadal change of sea level pressure over the northwestern Pacific and the “south flooding and north drought” precipitation change pattern in summer over East China, which is a relatively small regional feature in a global model.

Both East Asian summer monsoon indices based on 500hPa geopotential height and wind shear between 850hPa and 200hPa show marked inter-decadal shift around the late 1970s. Although the correlation coefficient between Pacific Decadal Oscillation (PDO) index and East Asian summer monsoon index of the GOGA simulations is a little bigger than TOGA simulation, both are significant. The consistent change in GOGA and TOGA indicates that the East Asian climate variations are mostly forced by tropical SSTs. And the high correlation of TOGA simulation and PDO also reveals that the tropical SSTs play a key role in North Pacific and East Asian Summer monsoon inter-decadal variability.

With greenhouse gases and aerosol forcings added, the precipitation change pattern simulated by GFDL model is similar to observations, but the location is shifted to the north of the observations. With climatological SSTs and time-varying greenhouse gases and aerosol forcings, the CAM3 produces the “south flooding and north drought” precipitation change pattern over East China, which indicates that the greenhouse gases and aerosol forcings play an important role in simulating rainfall changes over East China.