Variability, Trends and Dynamic Control of Precipitation in Southeastern China

中國東南部降水的變化、長期趨勢以及其動力響應

Student thesis: Doctoral Thesis

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Award date18 Sep 2018

Abstract

In this study, variations in the moisture budget in regions of China and the long-term trends and behaviors of precipitation of different intensities, as well as the modulations of vertical motion and moisture over southeastern China, with a focus on the Yangtze River region, are investigated. 

The precipitation data used in this study are 54 years (1960−2013) of continuous observation data obtained from the Climate Data Center, China Meteorological Administration (CMA). The atmospheric data are mainly from the National Centers for Environmental Prediction (NCEP) reanalysis.

Long-term variations in the moisture budget and drought/flood events in six regions of China are analyzed. Water vapor is transported mainly zonally during winter, while meridional transport is much stronger during summer. This study found that during winter, both the input moisture from the western boundary and the output moisture from the eastern boundary were slightly weakened; thus, the total amount of regional moisture in these regions did not change much in eastern China. During summer, the decline of input moisture from the south was slightly weaker than the decrease of output moisture via the northern boundary; therefore, the net moisture budget was slightly increased in eastern China, resulting in more flood events during recent decades in this region.

Trends in rainfall amount and frequency in China in terms of light, moderate, heavy, and torrential categories are studied. The results indicate a rising trend in precipitation amount over western China, due to a higher yearly frequency in light and moderate precipitation. A comparable trend was also extracted in southeastern China, but with increasing heavy and torrential precipitation. In southwestern and northern China, decreasing trends were found together with a lower frequency in light and moderate precipitation. These results suggest that the time and spatial distribution of precipitation amount and frequency in China can vary considerably and may play an important role in regional climate change. Further, variation in atmospheric circulation related to the four categories of rainfall anomalies over southeastern China was discovered. Results show that the anomalous patterns of atmospheric circulation are similar for the different categories of rainfall, with an anomalous cyclone and anomalous anticyclone located over the subtropical eastern Pacific, which leads to water vapor convergence over the Yangtze River region. Stronger anomalous circulation can result in heavier rainfall.  

The relative forcings of vertical motion and moisture on the variation in precipitation over southeastern China, with a focus on the Yangtze River region, are investigated. It is found that the linear trend in summer precipitation over the Yangtze River region is contributed solely by the trend in vertical motion. This trend is caused by a decreasing trend in static stability in the lower troposphere. On the other hand, the variation is controlled by both vertical motion and moisture. The change in vertical motion is forced by anomalous convergence in the midtroposphere and static stability in the lower troposphere. The change in moisture is contributed by the anomalous convergence of moisture transport. These changes are related to the southwestward extension of the western North Pacific subtropical high.

Variation in summer precipitation over southeastern China during a global warming period (1976−1997) and a hiatus period (1998−2013) are further studied. The results show that during the warming period, precipitation over both the Yangtze River region and South China shows an increasing trend, attributable to increasing ascending motion, a decreasing East Asian westerly jet (EAWJ), and a decreasing temperature gradient. During the hiatus period, the rainfall belt moves from the Yangtze River region to the Huang-Huai River region in association with the northward displacement of the EAWJ, attributable to the increasing trend in the temperature gradient at high latitudes. The second EOF mode of summer precipitation can represent the variation in rainfall over the Yangtze River region. During the warming period, rainfall over the Yangtze River region is controlled by a dipole mode of water vapor transport, induced by SST cooling in the Maritime Continent, which can lead to a strong Pacific–Japan teleconnection; warming SST in the east-central tropical Pacific can also strengthen this pattern. During the hiatus period, the Pacific–Japan pattern is weak, and the water vapor transport pattern over the western subtropical Pacific changes from a dipole structure to a weak monopole structure.