Interannual and Interdecadal Variations of Winter Water Budgets in the East Asian Monsoon Humid Region

Using monthly means of NCAR/NCEP reanalysis datasets and rainfall data from 160 surface stations in China, a water budgets sequence over the East Asian Monsoon Humid Region is calculated by a water vapor transport method for the period from 1958 to 2007, referring to changes in water budgets associated with the anomalous atmospheric circulations and precipitation in the East Asia Monsoon Humid Region. Relationships between the water budget index and the variability of the atmospheric circulation and rainfall in China are examined. Results show that the established water budgets sequence displays the variability on the interannual and interdecadal scales. The higher-index values mainly occur in the 1960s and the 1980s and the low-index values mainly occur in the 1970s and the 1990s. The index reflects the variations of the meridional winds anomalies and indicates an intensity of winter monsoon and an anomaly of rainfall along the valleys in the East Asian Monsoon Humid Region. Corresponding to the higher-index values, the high pressure centering in Mongolia and the low pressure centering in the Aleutian Islands are weaker. Meanwhile, the southerly wind anomalies are prevailing in the lower troposphere in the East Asia and cyclonic circulation anomaly appears in the vertical troposphere over the mainland of China. This anomaly intensifies the convergence and upward motion to the south area of 30°N, driving the warm water vapor transport coming from the Bay of Bengal and the South China Sea, and increases the water budgets and precipitation, with the difference of rainfall above 40 mm. The anomalous water vapor transport mainly appears between 600 hPa and 900 hPa, where the western and southern boundaries are the major input regions, the eastern and northern boundaries are the major output regions. The anomalous water budget is larger in meridional than in zonal direction, accounting for 91.3% for the changes of net incoming and expenditure while the anomalous zonal water budget is smaller. So the main cause for the difference and significant anomaly for rainfall is the changes of the incoming and expenditure water budgets in meridional. The differences for water budgets and rainfall in north and south parts are significantly different, which contribute greatly to the changes of zonal income and expenditure. The inter-decadal variability of water budgets reflects the strength of the water vapor transport and the amount of water vapor from the ocean, and also indicates an anomaly of the interdecadal rainfall variation in the East Asian Monsoon Humid Region with a rainfall anomaly above 30 mm.