Inter-decadal Variations of Summer Water Budgets in the Monsoon Wetness Region of Eastern China

Using monthly means of NCAR/ NCEP reanalysis datasets and rainfall data from 660 surface stations in China, a water budgets time series over the monsoon wetness region in Eastern China is calculated by a water vapor transport method for the period from 1983 to 2002, referring to changes in inter-decadal variability of water budgets associated with the anomalous atmospheric circulations and precipitation in the monsoon wetness region of Eastern China. Relationships between the water budget index and the variability of the atmospheric circulation and rainfall in China are examined. The results show that the summer water budget time series displays the variability on the inter-decadal scale from the 1980s to the 1990s in the region. The correlation coefficient is 0.71 between the water budgets and regional average precipitation and decreases to 0.55 after removing the trends. The index reflects the variations of the meridional winds anomalies and indicates an intensity of summer monsoon and an anomaly of rainfall along the valleys in the monsoon wetness region of Eastern China. The anomalous water vapor transport is affected by four anomalous circulations: The anomalous cyclonic circulation between 20°N and 30°N over the Eastern China, and the anomalous anti-cyclonic circulations between 40°N and 60°N over East Asian continent, between 18°N and 30°N over the west Pacific, and between 5°N and 20°N over the Bay of Bengal. Anomalous moisture flux convergence appears in the south of the Yangtze. Corresponding to the higher-index value decades, the low pressure centering in Mongolia is weaker and the surface temperature, sea surface temperature, the convergence in lower troposphere and divergence in upper troposphere are generally stronger in East Asia and the western north Pacific. Meanwhile, the southerly wind anomalies are prevailing in the eastern coast of China. This anomaly intensifies upward motion over the south mainland of China and drives the water vapor transport from South China Sea, and increases the water budgets and precipitation, with the difference of rainfall above 100 mm in the southern China. The water budgets are net surplus though uneven from 1983 to 2002. Vapor income in the low value year is about 24.3% less than in the high value year, and the precipitation is less by 18%. The anomalous meridional water budget is larger than zonal one, accounting for 71.3% of net budget. The anomalous water budget of low-index value decades accounts for 7% in latitude and 24% in longitude relative to the high-index value decades. The variation of the meridional water budget is larger than zonal one and is greater in magnitude. Therefore, the net water budget is quite different between high and low value decades of water budget and the anomalous precipitation is also significantly different.