Possible physical mechanism of water vapor transport over Tarim River Basin

Atmospheric water vapor transport (WVT) plays an important role in the Earth's climate system, especially in arid and semi-arid regions, but its physical mechanism has escaped wide attention. Here the distribution of WVT over Tarim River Basin and its relationships with local topography and precipitation are analyzed; the variation and trend of WVT and its correlation with westerly circulation index are also calculated; and the possible physical mechanism is discussed. All statistics and analysis are based on daily precipitation data of 25 weather stations in Tarim River Basin of China and NCEP/NCAR monthly mean reanalysis data (2.5° × 2.5°).It is shown that the western and the northwestern-based horizontal WVT is the main path for Tarim River Basin. But, the greatest contribution of the average annual water vapor net input (WVNI) is 3483.57×10 11kga -1 through eastern boundary. According to Bernoulli's equation, the reason is that the U-shaped terrain around Tarim River Basin causes the shift of WVT from west and northwest to north and northeast. Moreover, in lower troposphere, there is a significant positive correlation between the zonal and meridional WVNI due to the U-shaped terrain. Additionally, zonal WVNI in upper and middle troposphere grow up steadily as well as westerly circulation index; but WVNI in lower troposphere, meridional WVNI in middle troposphere and horizontal WVNI in whole troposphere all decrease significantly. This implies that there are other factors that reduce the WVNI, which counteracts the increase of WVNI caused by enhanced west winds. Last but not least, due to the pumping function of plant on local moisture recycling, precipitation in mountains increased rapidly, but decreased smoothly in plains between the late 1970s and the early 21st century.So we can conclude that climate in interior regions links closely to the local moisture recycling, which depends, to a large extend, on local forest-covered situation; and that the trend of the drying and desertification in Tarim River Basin is not likely to slow down because of the block of the U-shaped terrain, decreasing of WVNI in meridian and sparse vegetation. However, we still do not know how the climate of TRB will develop. There are some other factors such as NAO, runoff and the East Asia Monsoon that need to be further explored. It is also very necessary and urgent to study the mutual feedback relationship between water cycle and climate, and we should pay more attention to the influence of ecosystem. © 2011 Elsevier B.V.