Water Vapor Factors Affecting Drought Events in Northwest China

Abstract

We calculated the monthly and yearly water vapor fluxes and water vapor flux divergence based on 1961 to 2017 data from NCEP. We studied the water vapor transport routes, water vapor convergence, the water vapor budgets as well as the changes of water vapor fluxes and budgets over time at four borders encompassing the eastern part of Northwest China. The Mediterranean Sea, Black Sea, Caspian Sea, Indian Ocean, Bay of Bengal, and the South China Sea were the main sources of water vapor in the eastern part of the Northwest China, supplemented by the mid to high latitude continental sources. In the winter half-year, water vapor transport was mainly from the Mediterranean, Black, and Caspian Seas and the Indian Ocean supplemented by water vapor transport from the mid to high latitudes. The Bay of Bengal and the South China Sea were the main sources for water vapor in the autumn. In the summer the four sources of water vapor all contributed. The monthly water vapor flux change in the eastern part of Northwest China was parabolic. The transport of water vapor in Northwest China was mainly towards the east and north. The eastern part of Northwest China was in the water vapor convergence zone, and the convergence center was at the junction of Gansu, Sichuan and Shaanxi provinces. Anomalous water vapor transport was a major cause of extreme drought events, and weak divergence was also an important factor. During extreme drought events, water vapor expenditures on the eastern boundary increased; the percentage of water vapor in the western boundary decreased, and the percentage change in water vapor in the southern boundary was the largest. During droughts the proportion of water vapor in the northern boundary increased. In the 1970s and 1980s, the yearly and seasonal water vapor fluxes in the eastern parts of Northwest China rapidly changed and the water vapor flux decreased. The spring abrupt change occurred in 1975, and during the summer of 1978. The autumn and the annual abrupt changes were similar and first appeared in 1980, while the winter abrupt change occurred later (2005). The water vapor input into the eastern part of Northwest China mainly came from the west and south borders. The north boundary also contributed a small water vapor input. The percentage of water vapor in the western boundary had only slight monthly changes, while the southern boundary had obvious seasonal changes. The output of water vapor in the eastern part of Northwest China had clear seasonal differences. The water vapor output in winter was greater than the input, which led to the drought in the eastern part of Northwest China during winter. Since 1981, the input of water vapor in the western and southern borders has decreased, while the output of water vapor in the eastern boundary has increased. The input of water vapor in the northern boundary has increased slightly. The interannual variation of the four boundaries was largest in summer.