Effect Of Increased Anthropogenic Sulfate Aerosols Concentration On The Decadal Shift Of Summer Precipitation In The Middle And Lower Reaches Of The Yangtze River

To explore the direct effect of the increased anthropogenic sulfate aerosols concentration on the decadal shift of summer precipitation in the middle and lower reaches of the Yangtze River valley since the late-1970s, a global climate model (GFDL-AM2) is employed to conduct ensemble sensitive experiments. The results are compared with observation and reanalysis data to understand the mechanism for the impact. Then, this effect is compared with that of black carbon. Particularly, the synergy effect of the combined two kinds of aerosols is examined.Two sets of ensemble experiments are performed. Each ensemble constitutes5members with a31-year integration beginning from1970. In the first ensemble, the AGCM is prescribed with historically evolving aerosol forcing, while the anthropogenic sulfate aerosol is fixed at the level of1970over East Asia in the second ensemble. The difference of the first ensemble minus the second ensemble is used to analytze the impact of anthropogenic sulfate. As far as the30-year average is concerned, anthropogenic sulfate aerosols causes obvious negative radiative forcing at the top of atmosphere and the surface in East Asia from1971-2000, with the maximum close to-3W·m-2. It results in significant changes in temperature within (-0.3,0.1)℃and precipitation change within (-0.1,0.2) mm-day-1in most areas in China. As far as the interdecadal variation is concerned, the spatial-temporal structure shift of modeled summer rainfall bears a big similarity to the observed, exhibiting a rainfall increase in the middle and lower reaches of the Yangtze River. In particular, the shift characteristics in several East Asian Summer Monsoon subsystems, including the southward and westward stretch of the western Pacific subtropical high and the anomalous northerly along the east coast of East China as well as those in the vertical distribution of air temperature and vertical velocity, can be captured. Mechanisticly, the increased sulfate aerosols causes intensified negative radiative forcing, results in cooling in much of east China from the surface to the middle troposphere with the maximum cooling rate of more than-1℃on the surface. Subsequently, this weakens the land-sea thermal contrast and East Asian Summer Monsoon, and favors the rainband staying in the Yangtze River basin rather than moving northward to North China and Northeast China in middle summer (July-August). Thus, it is concluded that the increased anthropogenic sulfate aerosols have contributed substantially to the decadal shift of summer precipitation in the middle and lower reaches of the Yangtze River since the late-1970s.When it is compared with Black carbon(BC), BC causes a surface temperature cooling close to-1℃and weakened summer monsoon and increased precipitation in the Yangtze River basin, which are similar to those of anthropogenic sulfate. Synergy impact of the combined two kinds of aerosols shows completely different characteristics in that decreased precipitation close to-1mm-day-1and warmed surface air temperature as large as0.6-18℃are seen over the south of the Yangtze River. A further analysis displays that synergy impact of the two kinds of aerosols tends to block moisture transport toward east Asia, causing decreased precipitation in eastern China. This suggests that the synergy impact of the combined two kinds of aerosols is different from the sum of their individual impacts. Thus, a nonlinear interaction between the impacts of the two kinds of aerosols may exist...