Spatial-Temporal Variation Of The Precipitation In North China And The Impact Factors Of Precipitation Reduction

This paper was based on 752 stations daily and monthly precipitation datasets supplied by National Meteorological Information Center, NCEP/NCAR reanalysis data, and NOAA_ERSST_V3 data provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA (http://www.cdc.noaa.gov). The aim was to present the spatial-temporal variation of the precipitation in North China and indicate the relationships between variation of key influencing factors, dominating modes of seasonal evolution in North China precipitation, Indian Ocean Dipole (IOD), the East Asian monsoon and the precipitation reduction in North China by using statistical methods, synthesis analysis and other mathematic methods. The main conclusions are as follow:1) The precipitation trend in North China is different significantly. In order to analysis the overall trend of North China, it is more reasonable to select Shanxi, Hebei (including Beijing and Tianjin) as representative region. The reduction of annual precipitation in recent 50 years is due to the decrease of summer rainfall which is manifesting that the reduction of rainy days, especially the reduction of rainstorm days. Both the peak value of summer precipitation decrease and the rainy season becoming longer are the internal characteristics of summer precipitation reduction in North China.2) The summer precipitation in North China had an abrupt change in 1965, followed by a significant reduction. This change is in accordance with the variation of mid-high latitude circulation in Asia. First of all, at 500 hPa height field, the circulation converting into zonal circulation from the previous meridional circulation and Lake Baikal upper trough reducing activities, which leaded to decreasing the upward movement over North China. Secondly, at 500 hPa layer, the temperature in Mongolia obviously dropped. For one thing, the lower temperature would lower potential height at upper troposphere, and lead to the high-altitude jet stream turning toward south, which caused the East Asian summer monsoon to weaken. So water vapor will not reach in North China crossing Yangtze River. For another, the lower temperature caused air pressure of the lower troposphere in this region to increase and the low pressure weather process activities to reduce at the ground.3) There are two dominating modes of seasonal evolution in North China precipitation. The first mode mainly is controlled by both ENSO and IOD evolution, the second mode is mainly affected by IOD evolution. In the past 50 years, the first mode gradually weakened, while the second mode gradually increased, the combined effect of both makes summer precipitation in North China to decrease.4) During the recent 50 years, increased Indian Ocean SST and enhanced IOD index affected summer sea-land thermodynamic contrast by changing the East Asian winter and spring precipitation, resulting in strengthening the West Pacific subtropical high in summer, and its location moving towards north and west. So North China was controlled by the subtropical high. Meanwhile, water vapor carried by southwest monsoon from Bay of Bengal was less. Thus, lack of moisture sources in North China results in summer precipitation in North China appearing a decreased trend.5) With the Indian Ocean and the Pacific Ocean SST increasing and the IOD index enhancing in the past 50 years, significant changes occurred in East Asian monsoon. Southwesterly winds in summer decreased significantly, Mongolia cyclone circulation converted into anticyclone circulation, the circulation convergence converted into divergence near Yellow River Loop, the circulation from "east high pressure with west low pressure" to "east low pressure with west high pressure" at high field over North China. Insufficient moisture sources and lack of favorable dynamic conditions result in a decreasing trend in North China summer rainfall.