OBSERVED AND GCM SIMULATED DECADAL VARIABILITY OF MONSOON RAINFALL INEAST CHINA

Variability and associated mechanisms of summer rainfall over east Chi na are identified and described using both observations and a general circulation model (GCM) simulation. The observations include two data sets: the 90-station, 1470-1988 annual drought/flood index and the 60- station, 1889-1988 monthly mean precipitation measurements. The GCM da ta set is a 100-year equilibrium simulation of the present climate. Sp ectra of the drought/flood index indicate decadal cycles which decreas e from north (approximately 47 y) to south (approximately 21 y). Corre lation coefficients show decadal variability in the relationship betwe en index values along the Yangtse River valley and those over northeas t and southeast China. Analysis of the measured data confirms this res ult; for example, the correlation was small during 1889-1918, but sign ificantly negative during 1930-1959. When compared with precipitation measurements, the GCM better simulates monthly means and variances alo ng the Yangste River valley. Three distinct 30-year periods of interan nual variability in summer rainfall are found over this area. During e ach period, rainfall is negatively correlated with spring surface temp erature over a remote region and is identified with variations in a sp ecific component of the east Asian monsoon circulation: (1) when Euras ian temperatures decrease, the thermal contrast across the Mei-Yu fron t increases and frontal rainfall intensities; (2) lower temperatures o ver the Sea of Japan/northwest Pacific Ocean are identified with enhan ced easterly flow, moisture transport and rainfall; (3) when tropical east Pacific Ocean temperatures decrease, rainfall associated with the low latitude monsoon trough increases. Given that the GCM generates d ecadal changes in the relationship between the physical mechanisms, th e east Asian monsoon and planetary general circulations and east China rainfall, future studies should focus on the predictability of these changes with the use of improved and much longer GCM simulations.