Decadal Variations Of Air Water Resources In North China And Its Linkage To The Large-scale Land-sea Surface Temperature

China is serious short of water resources. The precipitation in North China has appears a decreasing trend with the global warming over the past decades, and further enhance the drop of local water resources. The inadequate water supply in North China has seriously hampered local economic development and caused great concern by central government. Summer air water is the main source of available water resources in North China, the well-understanding of the changes of water resources and its possible causes in North China is an important prerequisite in making the national water resources strategy. In this regard, based on East Asian sounding datasets, 722 station-observed precipitation and temperature, NCEP/NCAR and ECMWF reanalysis datasets, the annual and decadal variations of air water resources in North China during 1951-2010 are extensively discussed. In present study, the addressed variables include the vertical components of precipitation, evaporation and available precipitation, and the horizontal components of water vapor, vapor flux and vapor budget. In addition, the accuracy of NCEP/NCAR and ECMWF reanalyzed data sets are examined on the basis of air sounding datasets, and the large-scale circulation anomalies associated with variations of air water resources in North China and its possible linkages to the changes of land and ocean temperature are explored. The main results are as follows:(1) The seasonal variation of climate temperature, precipitation, evaporation and available precipitation in North China appears a single peak in summer. The local evaporation accounts for 69% losses of annual precipitation in North China, and the most losses are observed in spring and winter. The summer available precipitation is the main water resource in North China, and its variation on decadal time scale is consistent to that of the local summer precipitation. The summer available precipitation exhibits a decreaseing trend from1970s to 1980s, and from 1990s to the early 21st century, with the abrupt drop observed in 1979 and 1996, respectively.(2) It is found that the seasonal variations of water resource in North China can be well described by NCEP/NCAR and ECMWF reanalysis data; however, the annual variations based on the NCEP /NCAR reanalyzed data are too large until it starts to be consistent with ECMWF reanalysis data and air sounding data after 1965. The summer column water content in North China significantly reduces in the late 1970s and the early 1990s, respectively, and it appears continually negative anomalies in recent decade. The zonal vapor flux starts to reduce from the late 1980s and further decreases in the early 21st century. Meridional vapor flux decreases in the late 1970s and the late 1980s, respectively, but it increases in the early 21st century compared with the case in 1990s.(3) The water supply in North China are mainly from the southwest vapor flux of East Asian Summer Monsoon and Westerly vapor flux of water vapor transport in North China, which transport water from southern and western boundaries, respectively. The climate vapor budget of summer in North China is positive, with the sinks in low levels and sources in middle and high levels. Comparison suggests the vapor flux from the southern and western boundaries are comparative. The water vapor comes from the southern boundary is mainly concentrated in the level below 700hPa, while the counterpart from the west boundary is distributed uniformly in the high, middle and low levels in the tropospheric atmosphere. Interdecadal variations of vapor budget show an increase in 1960s, and decrease in 1970s and 1980s, and then increases again in 1990s and 21st century. The results based on the NCEP data suggest that the East Asian summer monsoon vapor flux displays downward trend in 1980s, the weakening of East Asian summer monsoon and westerly vapor flux jointly reduced the water vapor supply in 1990s, but the weakened westerly vapor flux possibly reduced the local water in the early 21st century.(4) The cyclone over the Mongolian region and East Asian summer monsoon are the main circulation systems affecting the summer available precipitation in North China. Base on the NCEP reanalysis data and air sounding data, it is found 500hPa temperature, geopotential height enhance when the surface air temperature becomes warming around Baikal region. The warming can generate an anomalous warm anticyclone and result in the weakening of vapor in west wind and southwest wind of East Asian summer monsoon, which causes the recent reduction of air water resources in North China.(5) The long-trem changes in the large-scale sea and land surface temperature play important roles in affecting the water resources in North China. Analysis suggests the available precipitation in North China is negatively correlated with the land surface temperature in north of Asia and the SST in East Pacific equatorial and western Indian Oceans. The 11-years moving correlation between SST and available precipitation in North China shows a stable negatively relationship, but an abrupt reversal correlation coefficient is observed after 1996. In contrast, the significantly negative correlation with land surface temperature only appears in late 1970s and 1990s when the linear trend was removed. This result indicates that the large-scale land surface temperature possibly paly the more important role in modifying the available precipitation in North China on decadal time scale.