Study on change of weather and climate extremes has become an important aspect in modern climate change research. Based on the daily surface air temperature data from 200 stations and daily precipitation data from 739 stations during the second half of the 20th century, schemes for analyzing climate extremes were designed mainly according to percentiles of a non-parametric distribution and the gross errors in the daily data were removed based on a newly designed quality control procedure. The spatial and temporal characteristics of change of climate extremes over northern China were studied. The main conclusions are summarized as follows: 1) The number of days with maximum temperatures over 35oC decreased slightly. The decreasing trends are obvious in the North China Plain and the Hexi Corridor. However, since the 1990s, the extreme hot days increased greatly. Meanwhile, the frost days decreased significantly in northern China, especially in the eastern part of northern China and Xinjiang Uygur Autonomous Region. Increase trends were found for the 95th percentiles of daily maximum temperatures except in the southern part of North China, while obvious decrease trends were found for the 5th percentiles of daily minimum temperatures. 2) The extreme intense precipitation events obviously increased in much of northwestern China but decreased in the eastern part of northeastern China and most parts of North China. The number of heavy rain days increased in eastern Inner Mongolia and eastern Northeast China, but obviously decreased in the Northeast China Plain and North China.
Based on daily mean surface temperature of 110 stations over China from 1951 to 2000, trends of days and intensity of extreme temperature over arid and semi-arid area of China have been analyzed, and the trend of annual extreme temperature and differences among regions were shown; also temporal and spatial characteristics of extreme temperature and its relationship to regional warming have been discussed finally. The results show that there is a remarkable decreasing trend of days of minimum temperature over arid and semi-arid areas during the period, and the beginning date of trend is different in different regions. Compared with the trend of minimum temperature, there is no obvious trend of maximum temperature days before the 1990s, but there is a remarkable increasing trend in maximum temperature days since 1990. Except days of extreme temperature, there is decrease trend of annual minimum temperature. The results have also shown the trend of zero temperature(under 0oC), i.e., there is reduction trend of zero temperature days, and its beginning date delays, and ending date is earlier. In addition, we found that the regional warming in northern China is closely related to reduction of minimum temperature days, and increase of maximum temperature days has intensified regional warming since 1990.
Trends on the daily mean air temperature and its anomalous strength in China for the warm season (May-September) in the last 40 years (1961-2000) were analyzed using observational 453-station datasets including daily temperature and daily precipitation. Calculation methods are standard deviation and Skewness. Results show that there were different spatial-temporal characteristics in the daily air temperature variability. In the last 40 years, a warming trend was commonly noted in China but the stronger warming was located in the area north of the Yellow River. For the time evolution, a transition time point was found in the late 1970s from a cooling trend to a warming trend but the stronger warming appeared since the late 1990s. The amplifying daily temperature anomaly indicated a trend of climate variability in many places of China. The relationships between daily air temperature and rainfall events were also discussed particularly in the coastal regions.
Based on daily mean temperature data from 62 stations of northern China since 1951, the trend of frost days and temperature has been analyzed, and the variation trend of the starting date in autumn and the ending date in spring have been shown. The results indicate an obvious reduction trend of frost days, an increase trend of frost-day temperature in northern China from 1951 to 2000, and the increasingly longer growth season due to early start date of spring and late ending date of autumn. We can also find a good relationship between frost-day temperature and mean temperature in the period, this may foreshow the variation of mean temperature is produced by frost-day temperature.
Annual mean monthly snowfall series are analyzed by using daily snowfall (≥5 mm) data of 111 meteorological observations in Inner Mongolia from each year during October 1961 to March 1998. October to the next March is defined as a more objective winter for describing the variation of heavy snowfall or snowstorm in Inner Mongolia. According to such a definition, distributions of heavy snowfall or snowstorm and its varibility in Inner Mongolia are analyzed. Finally spatial patterns of the variation of the snowfall anomaly and their corresponding time variable scales are examined with REOF decomposition. It was found that: (1) the anomalies of the occurrence of heavy snowfall or snowstorm in Inner Mongolia correlate positively to the snowfall itself; (2) the key regions with large variability are located mainly within 40-45oN and 105-123oE; (3) the anomalies of snowfall in winter are mostly attributed to the variation of early winter, secondly to that of late winter and least to that of midwinter; and (4) the heavy snowfall and snowstorm is characterized by a decadal-scale variation.
This paper first deals with the empirical orthogonal function (EOF) expansion using four seasonal rainfall data for 1884-1995 in eastern China from 33 stations. The sea level pressure anomalies (SLPA), then, are used to explain the leading modes of rainfall in northern China from the composite of typical years. The first mode of summer rainfall shows the opposite trends in the central area of the Yellow and the Yangtze river basins related to the coastal areas of South China. More rainfall in the area appeared when the positive SLPA occurred in northeast Asia while a pressure trough in eastern China. For the autumn rainfall, the first three modes show a center in the middle and lower reaches of the Yangtze River, reversed trend between southern and northern China, and between eastern coastal and western areas of China, respectively. These modes are closely linked with circulation patterns, particularly for the autumn third mode that was linked to the circulation in the typhoon season. The central area of the first mode for the winter rainfall is located in the middle reaches of the Yellow River and the Yangtze River. For the first mode its amplitude of interdecadal variability gradually became large in the 20th century. The second mode of winter rainfall shows an opposite relation in signs in northern and southern China. The tendency indicates that the decadal rainfall was above normal for the 1920s-1970s but below normal for recent years in northern China. For the spring rainfall the first mode shows a reversed relation in northern China related to South China. The second mode shows that the rainfall in North China and Northeast China was opposite to other regions and a quasi-30-year oscillation is noted in the series of this mode. The dry trend in northern China has become severe since the 1980s.
Based on China's monthly precipitation data of 629 stations during 1950-2000, we calculated Z indices and separated them into seven Dryness and Wetness grades. Furthermore, a drought area index was proposed to study changes of drought extent. The results revealed that the different grades of droughts all show expanding trends in northern China's main agricultural area. Moreover, the area coverage of droughts in different seasons and different regions displayed different trends.
Using monthly precipitation and monthly mean temperature, a surface humid index was suggested. According to the index, the distribution characteristics of extreme dryness has been deeply analyzed. The results indicated that there is an obvious increasing trend of extreme dryness in the central part of North China and Northeast China in the last 10 years, which is a high frequent period of extreme dryness; and a low frequent period in the regions during the last 100 years. To compare with variation trend of the temperature in these regions, the high frequent extreme dryness region is consistent with the warming trend in the same region.
The station data, including precipitation, air temperature and soil humidity are employed to investigate the arid climate trend for the past 50 years over northeastern China. "Atmospheric drought index" is proposed based on monthly mean temperature and precipitation anomalies as well as mean variance. The variations in the index and soil humidity show that the arid situation gets deteriorated throughout the northeastern China, this arid trend is more significant since the mid-1990s, especially in semi-arid area located in northeastern Inner Mongolia. Air temperature increase plays a more important role than precipitation decrease does in terms of the formation of drought. Although rainfall has slightly increased over the southern part of northeastern China in recent 50 years, arid trend remains to be developing, which is obviously associated with global warming. The responses of aridity developing over northeastern China to the global warming are explored using linear regression analyses. The results show that under circumstance of global temperature increase by 1oC the aridity extent will rise 5-20%, its maximum will reach 22%.
Regional characteristics of soil moisture evolution in northern China are analyzed based on the monthly retrieved soil moisture data of 139 stations from 1951 to 1999. The results show that evolution of soil moisture at different levels coincides with each other very well. North China experienced a wet period during the late 1960s to late 1970s. An abrupt change occurred at the end of the 1970s and the soil moisture began to decrease. The descending trend of soil moisture in North China is especially evident in the late 1990s. For Northeast China, the 1970s is a distinct dry period. In terms of seasonal trend, however, no apparent decrease of soil moisture has been found since 1980 in North China. In addition, a notable phenomenon is the general decrease of soil moisture in autumn over wide regions of northern China, especially in Northeast China.
Drought during the period 1999-2000 in North China is studied by using monthly mean temperature and precipitation data from 160 stations covering January 1951 to October 2000. The revised Palmer drought severity index (PDSI) of the 160 stations covering the same period was also calculated. It is found that the protracted drought was a manifestation of an abnormal form of general circulation. The PDSI can characterize the temporal and spatial distributions of remarkable droughts and floods in China. During 1999-2000 there was a persistent negative PDSI in North China, the PDSI also shows that there is a marked interannual and interdecadal variation of drought in North China. From 1950-1970 there is a wet summer climate, while since 1970 there have been more drought years in North China.
By applying rotated empirical orthogonal function (REOF) analysis on dryness/wetness (DW) series from 1470 to 1999 at 100 selected stations over the eastern part of China, defined by the large contours of the seven REOF modes, 7 dryness/wetness active centers are identified: Northeast China, northern North China, southern North China, Northwest China, the middle and lower Yangtze River valley, South China and Southwest China. The temporal variations of regional dryness/wetness for monsoon boundary zone in northern China are investigated on the basis of the regional division of dryness/wetness. Dryness occurred fairly frequently in Northeast China, southern North China and Northwest China in the 16th century and the first half of the 17th century. In the 18th and 19th centuries, the occurrence of dryness is more frequent in northern North China, but the occurrence of wetness is more frequent in Northeast China, dryness/wetness occurred alternately in southern North China and Northwest China. The changes of dryness/wetness jumped from wetness to dryness in northern North China and southern North China during the late 19th century and the early 20th century respectively. Dryness had kept still in two divisions.The study shows extreme dryness has similar trend, which increased at both ends and less in the middle in Northeast China, northern North China and Northwest China during the last 530 years. But it is a frequently occurring area in southern North China.
In this research, a new type of desertification index is built by using the anomalies of differences between the air temperature (Ta) and the 0cm soil temperature (Ts) over northern part (35oN north, 100oE east) of China. Depending on the data from 1956 to 2000, the most serious desertification happens over the eastern part of Northwest China (35o~45oN, 100o~110oE) in both spring and summer, which began from the 1970s, and became much remarkable from the 1980s. A tendency of desertification is also found over Northeast China (45o~52oN, 120o~135oE) from the 1990s. Although the precipitation over central North China (35o~45oN, 110o~120oE) decreased notably from 1956 to 2000, we have not found the tendency of desertification over this area. The desertification indices (Ta-Ts) over the above three areas are correlated to the precipitation very well, which indicates the key roles of precipitation to the desertification index. The high correlations between Ta-Ts and NDVI in summer of eastern part of Northwest China and both spring and summer of central North China show the feasibility of the desertification index used in this research.
The monthly PDSI, surface moisture index and percentage of precipitation anomaly are calculated for the past 49 years with data from 160 stations in China. The result shows that PDSI and surface moisture index can grasp rainfall, a decisive factor of drought. Moreover, they reflect the process of drought on a greater space and time scale. In the region with more actual evaporation, PDSI describes the severity of drought more precisely than percentage of precipitation anomaly. In North China, before the 1980s, the rainfall was more sufficient, the temperature was higher and the potential evaporation was lower. It was a relatively moist period. Since the 1980s, the rainfall has been less sufficient together with higher temperature, the drought in North China has become more and more severe.
Based on duststorm observations during 1954-2000 from around 700 meteorological stations, this paper analyzed seasonal and inter-annual variation features. The impacts of strong wind and precipitation on the duststorms were especially emphasized. The results show that the frequency of China's duststorms is influenced by many climate factors. In NW China and northern Inner Mongolia, the main duststorm regions, the influences of climate conditions are most obvious. Wind speed and humidity are the most important controls for the seasonal cycle of duststorm frequency, and precipitation is one of the most dominant factors impacting inter-annual variability. The good correlation between the summer rainfall and duststorm frequency of next spring suggests some climate predictability.
Methodology and theoretical basis of the spatial-temporal series analysis is discussed in this paper, based on the phase space reconstruction theory and Taken's embedding theory, according to the prediction method of the phase space dynamics reconstruction theory for single variable time series, the dynamics prediction idea and method of the spatial-temporal series analysis is conducted then, which is also used in the long-term prediction of 5-, 10- and 20-year scales of drought and flood in the region of northern China. The preliminary results are as follows: to a certain degree, the method of the spatial-temporal series analysis has the predictable ability, and the dryness and wetness grades for 5- and 20- year scale on northern China are as normal, the dryness and wetness grades for 10-year scale in this region is a little wet above the normal.
This paper summarizes the researches on the marginal active belt of East Asian summer monsoon. A range of sections are presented, dealing with the meaning of this research, the influencing factors in marginal monsoon activity, and studies on advance and retreat of monsoonal margins as well as interannual and interdecadal variations of northern boundary of East Asian summer monsoon.
