In terms of the characteristic topology parameters of climate complex networks,the spatial connection structural complexity of the circulation system and the influence of four teleconnection patterns are quantitatively described.Results of node degrees for the Northern Hemisphere (NH) mid-high latitude (30°N-90°N) circulation system (NHS) networks with and without the Arctic Oscillations (AO),the North Atlantic Oscillations (NAO) and the Pacific-North American pattern (PNA) demonstrate that the teleconnections greatly shorten the mean shortest path length of the networks,thus being advantageous to the rapid transfer of local fluctuation information over the network and to the stability of the NHS.The impact of the AO on the NHS connection structure is most important and the impact of the NAO is the next important.The PNA is a relatively independent teleconnection,and its role in the NHS is mainly manifested in the connection between the NHS and the tropical circulation system (TRS).As to the Southern Hemisphere mid-high latitude (30°S-90°S) circulation system (SHS),the impact of the Antarctic Arctic Oscillations (AAO) on the structural stability of the system is most important.In addition,there might be a stable correlation dipole (AACD) in the SHS,which also has important influence on the structure of the SHS networks.
In this paper,a variation series of snow cover and seasonal freeze-thaw layer from 1965 to 2004 on the Tibetan Plateau has been established by using the observation data from meteorological stations.The sliding T-test,M-K test and B-G algorithm are used to verify abrupt changes of snow cover and seasonal freeze-thaw layer in the Tibetan plateau.The results show that the snow cover has not undergone an abrupt change,but the seasonal freeze-thaw layer obviously witnessed a rapid degradation in 1987,with the frozen soil depth being reduced by about 15 cm.It is also found that when there is less snow in the plateau region,precipitation in South China and Southwest China increases.But when the frozen soil is deep,precipitation in most of China apparently decreases.Both snow cover and seasonal freeze-thaw layer on the plateau can be used to predict the summer precipitation in China.However,if the impacts of snow cover and seasonal freeze-thaw layer are used at the same time,the predictability of summer precipitation can be significantly improved.The significant correlation zone of snow is located in middle reaches of the Yangtze River covering the Hexi Corridor and northeastern Inner Mongolia,and the seasonal freeze-thaw layer exists in Mt.Nanling,northern Shannxi and northwestern part of North China.The significant correlation zone of simultaneous impacts of snow cover and seasonal freeze-thaw layer is larger than that of either snow cover or seasonal freeze-thaw layer.There are three significant correlation zones extending from north to south:the north zone spreads from Mt.Daxinganling to the Hexi Corridor,crossing northern Mt.Taihang and northern Shannxi;the central zone covers middle and lower reaches of the Yangtze River;and the south zone extends from Mt.Wuyi to Yunnan and Guizhou Plateau through Mt.Nanling.
This paper presents an analysis of the mechanisms and impacts of snow cover and frozen soil in the Tibetan Plateau on the summer precipitation in China, using RegCM3 version 3.1 model simulations. Comparisons of simulations vs. observations show that RegCM3 well captures these impacts. Results indicate that in a more-snow year with deep frozen soil there will be more precipitation in the Yangtze River Basin and central Northwest China, western Inner Mongolia, and Xinjiang, but less precipitation in Northeast China, North China, South China, and most of Southwest China. In a less-snow year with deep frozen soil, however, there will be more precipitation in Northeast China, North China, and southern South China, but less precipitation in the Yangtze River Basin and in northern South China. Such differences may be attributed to different combination patterns of melting snow and thawing frozen soil on the Plateau, which may change soil moisture as well as cause differences in energy absorption in the phase change processes of snow cover and frozen soil. These factors may produce more surface sensible heat in more-snow years when the frozen soil is deep than when the frozen soil is shallow. The higher surface sensible heat may lead to a stronger updraft over the Plateau, eventually contributing to a stronger South Asia High and West Pacific Subtropical High. Due to different values of the wind fields at 850 hPa, a convergence zone will form over the Yangtze River Basin, which may produce more summer precipitation in the basin area but less precipitation in North China and South China. However, because soil moisture depends on ice content, in less-snow years with deep frozen soil, the soil moisture will be higher. The combination of higher frozen soil moisture with latent heat absorption in the phase change process may generate less surface sensible heat and consequently a weaker updraft motion over the Plateau. As a result, both the South Asia High and the West Pacific Subtropical High will be weaker, hence causing mo
