Observed and GCM simulated decadal variability of monsoon rainfall in east China

Abstract

Variability and associated mechanisms of summer rainfall over east China 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 data set is a 100-year equilibrium simulation of the present climate. Spectra of the drought/flood index indicate decadal cycles which decrease from north (≈47 y) to south (≈21 y). Correlation coefficients show decadal variability in the relationship between index values along the Yangtse River valley and those over northeast and southeast China. Analysis of the measured data confirms this result; for example, the correlation was small during 1889–1918, but significantly negative during 1930–1959. When compared with precipitation measurements, the GCM better simulates monthly means and variances along the Yangtse River valley. Three distinct 30-year periods of interannual variability in summer rainfall are found over this area. During each period, rainfall is negatively correlated with spring surface temperature over a remote region and is identified with variations in a specific component of the east Asian monsoon circulation: (1) when Eurasian temperatures decrease, the thermal contrast across the Mei-Yu front increases and frontal rainfall intensities; (2) lower temperatures over the Sea of Japan/northwest Pacific Ocean are identified with enhanced 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 decadal changes in the relationship between the physical mechanisms, the 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.