Changes In The Meiyu Belt And Its Association With Water Vapor Transport In Terms Of Global Warming

Recently it is still hard to predict the rainfall during flood season over the Yangtze-Huaihe River Valley （YHRV） of China. Especially, as the climate rapidly warms, the rainbelt pattern tends to be more complex and diverse and the rain intensity. Thus, further research is necessary to gain an in-depth understanding of the trends of the Meiyu and to provide a scientific basis for developing climate change policies.Changes in the Meiyu belt over China, the possible causes of these changes, and the link to intensity and continuity of rainfall during1958-2009are examined by using daily rainfall datasets from756stations in China and ERA-40and ERA-Interim daily reanalyses. The onset and termination of the regional Meiyu over the Yangtze-Huaihe River Valley （YHRV） are defined by a700hPa relative humidity index. Variations of the water vapour transport and their potential influence on the Meiyu belt are also explored, based on the HYSPLIT₄model and Euler’s method of calculating water vapour budget.The results indicate that the Meiyu belt experienced a northward shift in the late1990s in response to global warming, which corresponds with the increasing frequency and contribution of1-to2-day and more than6-day continuous rainfall in the central and northern YHRV. Moreover, the spatial variability of rainfall in various intensity categories is a crucial to the changes in the rain band. This variability is primarily characterised by a positive heaviest rainfall trend over the north-central YHRV, a negative trend elsewhere and a statistically significant increase in the rain-free days over the southern YHRV. A composite of the atmospheric circulation indicates that the intense northward horizontal transport and convergence of water vapour flux are the immediate causes of the rain band shift. Furthermore, the change in the wind field mainly contributes to the change in the water vapour flux and Meiyu belt, whereas a strengthened land-sea thermal contrast intensifies the southwest monsoon.The South China Sea （SCS） and Bay of Bengal （BOB） are the principal sources of water vapour leading to the Meiyu over the YHRV, and the transports from the two account for more than50percent of total water vapour transports. Other transports come successively from local evaporation in lower atmosphere, the mid-level cool air over the North Atlantic Ocean and the Caspian and the Aral seas. During the Meiyu after2000, water vapour from the SCS decrease, of which the mid-level moisture transport is replaced by the transport from the northern Indochina; low-level water vapour from the BOB increases double and mid-level water vapour from the passage declines. The enhanced low-level moisture transport and mid-level moisture transport from the BOB and the northern Indochina, respectively, are primary causes of an obvious increase in the inflow through the southern boundary over the YHRV. Meanwhile, more water vapour that leaves the southern region heads northward into the northern region, which due to the enhanced low-level flow from the BOB.The effect of the meridional transport of water vapour from the SCS focuses on the southern mid-lower reaches of the Yangtze River, whereas the transport of water vapour from the BOB can cause an opposite variation on the Meiyu over the northern and southern region separated by the Yangtze River. Thus, weakening transport of water vapour from the SCS and strengthening water vapour transport from the BOB jointly drive the northward shift of the Meiyu belt.