Atmospheric Water Vapor Flux and Its Association with Rainfall overChina in Summer

Abstract

The summer moisture transport by the large-scale circulation over China has been investigated using ECMWF?WMO twice-daily analyses for the period 1980?96. The horizontal flux of atmospheric water vapor and its divergence has been calculated over two target regions, namely, southeast China (25°?35°N, 110°?120°E) and northeast China (40°?50°N, 120°?130°E). The time-averaged fluxes show the southeastern Asian and Indian monsoon circulations bringing abundant moisture from the South China Sea and the Bay of Bengal, respectively, to southeast China while the midlatitude westerlies dominate the moisture transport over northeast China. The association between the interannual variations of moisture flux and rainfall over China has been examined. The comparison of the fluxes for wet and dry years over the southeast showed there to be, for the former, much stronger moisture transport by the southeastern Asian monsoon through the southern boundary but little change associated with transport by the Indian monsoon. Furthermore, comparison between mean and transient eddy transport budgets in wet and dry years and in the climatological mean shows that the mean component is dominant in the total transport. The moisture convergence associated with the transient eddies assumes its largest values in the eastern part of the country and, for the most part, assumes a sign opposite to that of the moisture flux convergence associated with the time-mean circulation and moisture fields. The results of this study suggest that the transient eddies do not play a significant role in the initiation and maintenance of the abnormal climate events over the two domains used in the study. Over both of these domains the two largest terms in the climatological moisture budget are seen to be the evaporation and the precipitation, while for interannual variations the largest terms are the atmospheric moisture convergence and the precipitation. China has the largest irrigated area in the world and much of this is in the form of flooded paddy fields. It is suggested that this large area of saturated surface may result in evapotranspiration rates that differ significantly from those implied by atlases that have used estimates obtained at ?typical? locations. The extent of irrigation over China may also make the task of interpreting its moisture budget more difficult.