Hao Zhixin

Meiyu in the middle and lower reaches of the Yangtze River since 1736

Abstract“Yu Xue Fen Cun” records during the Qing Dynasty are used to identify the starting and ending dates of Meiyu at the period of 1736–1911. These results, along with the instrumental meteorological records, are used to reconstruct the series of length and precipitation of Meiyu during 1736–2000 over the middle and lower reaches of the Yangtze River. The characteristics of Meiyu are analyzed since 1736. Moreover, the strength of East Asian Summer Monsoon and locations of rainband are discussed, based on the relationship between the length of Meiyu and the Index of East Asian Summer Monsoon. It is found that the starting and ending dates and the length of Meiyu have significant interannual and interdecadal variations. Apart from 7–8 years, 20–30 years and 40 years cycles for the lengths of Meiyu, the centennial oscillation is also presented. The length of Meiyu, monsoon rainband movement over eastern China, and the strength of East Asian Summer Monsoon (EASM) have a very good correlation, which can be expressed in the following: during the periods of 1736–1770, 1821–1870 and 1921–1970, the EASM was stronger, and the monsoon rainband was located in North China and South China easily, corresponding to the decreased length of Meiyu. Whereas during the periods of 1771–1820, 1871–1920 and 1971–2000, the EASM was weaker and monsoon rainband usually stopped at the middle and lower reaches of the Yangtze River, corresponding to the increased length of Meiyu.

The climate regionalization in China for 1981-2010

The scheme of climate regionalization in China was conducted by the classification criteria of temperature zone, arid/humid region and climatic sub-region, and the variables used in the criteria were calculated as mean values of the 30 years by using the daily meteorological data of 658 stations from 1981 to 2010. In the classification criteria, the temperature zones were classified by the general guideline of the days with daily temperature steady above 10℃, and the secondary guideline of January mean temperature, or by their referenced variables including the accumulated temperature with daily temperature steady above 10℃ and annual minimum temperature, respectively. The arid/humid regions were classified by the annual aridity index and annual precipitation amount in turn. The climatic sub-regions were classified by the July mean temperature. The result shows that China can be divided into 12 temperature zones, 24 arid/humid regions and 56 climatic sub-regions. Compared with the climate regionalization scheme for the period of 1951- 1980, several boundaries of temperature zones in eastern China shifted northward in 1981-2010 due to the climate warming. The east part of the northern boundary of warm temperate zone shifted more than 1.0° at a maximum. On average, the east part of the northern boundary of north subtropical zone shifted 1.0°. The middle part of the northern boundary of mid-subtropical zone shifted 2.0° at a maximum. The west part of the northern boundary of south subtropical zone shifted 0.5°-2.0°. In West China, the shift of temperature zone was not significant in horizontal due to the vertical landform. However, the plateau sub-cold zone was shrunk while the plateau temperate zone was enlarged in the Tibetan Plateau. Because precipitation decreased in North China, southeastern part of Northeast China and eastern part of Northwest China, the boundary of the semi-arid and sub-humid region in Northern China shifted eastward and southward, and in which, the boundary between 36°-41°N shifted 0.5°-2.5°at longitude. Moreover, the climate in the most of arid regions and semi-arid regions in Hexi Corridor, Xinjiang and the Tibetan Plateau changed to be more humid.

Recent advances on reconstruction of climate and extreme events in China for the past 2000 years

China is distinguished by a prominent monsoonal climate in the east of the country, a continental arid climate in the northwest and a highland cold climate on the Qinghai-Tibet Plateau. Because of the long history of Chinese civilization, there are abundant and well-dated documentary records for climate variation over the whole of the country as well as many natural archives (e.g., tree-rings, ice cores, stalagmites, varved lake sediments and corals) that enable high-resolution paleoclimatic reconstruction. In this paper, we review recent advances in the reconstruction of climate and extreme events over the last 2000 years in China. In the last 10 years, many new reconstructions, based on multi-proxies with wide spatial coverage, have been published in China. These reconstructions enable us to understand the characteristics of climate change across the country as well as the uncertainties of regional reconstructions. Synthesized reconstructed temperature results show that warm intervals over the last 2000 years occurred in AD 1–200, AD 551–760, AD 951–1320, and after AD 1921, and also show that cold intervals were in AD 201–350, AD 441–530, AD 781–950, and AD 1321–1920. Extreme cold winters, seen between 1500 and 1900, were more frequent than those after 1950. The intensity of regional heat waves, in the context of recent global warming, may not in fact exceed natural climate variability seen over the last 2000 years. In the eastern monsoonal region of China, decadal, multi-decadal and centennial oscillations are seen in rainfall variability. While the ensemble mean for drought/flood spatial patterns across all cold periods shows a meridional distribution, there is a tri-pole pattern with respect to droughts south of 25°N, floods between 25° and 30°N, and droughts north of 30°N for all warm periods. Data show that extreme drought events were most frequent in the periods AD 301–400, AD 751–800, AD 1051–1150, AD 1501–1550, and AD 1601–1650, while extreme flood events were frequent in the periods AD 101–150, AD 251–300, AD 951–1000, AD 1701–1750, AD 1801–1850, and AD 1901–1950. Between AD 1551–1600, extreme droughts and flood events occurred frequently. In arid northwest China, climate was characterized by dry conditions in AD 1000–1350, wet conditions in AD 1500–1850, and has tended to be wet over recent decades. On the northeastern Qinghai-Tibet Plateau, centennial-scale oscillations in precipitation have occurred over the last 1000 years, interrupted by several multi-decadal-scale severe drought events. Of these, the most severe were in the 1480s and 1710s. In southwest China, extreme droughts as severe as those seen in Sichuan and Chongqing in 2006 are known to have occurred during historical times.

Dryness and wetness variations for the past 1000 years in Guanzhong Plain

In this study, spatial-temporal statistical technologies were utilized to interpolate a dryness and wetness grade dataset, which was reconstructed from previous Chinese historical documents, but with significant data gaps in the past 1000 years, and a complete grade dataset of dryness and wetness was developed for Guanzhong Plain including seven sites at Linfen, Changzhi, Xi’an, Fengxiang, Luoyang, Nanyang and Hanzhong. The probit model was used to interpolate the missing values at the spatial scale depending on the highest variance explanation between neighbouring stations, and the Markov chain model was used to interpolate the temporal series depending on the known features of the dataset from 1470 to 2010. On the basis of this dataset, long-term variation features and extreme drought and flood events in the Guanzhong Plain were identified. By means of wavelet analysis and ensemble empirical mode decomposition methods, the main cycles of interannual-interdecadal-centennial scales of dryness and wetness variations were detected and the relationships between periodicity and driving forces were analyzed. Results indicated that the dry-wet change of the Guanzhong Plain has had three stages in the past 1000 years, that is, the climate changed from wet to dry in 960–1150, then from dry to wet in 1151–1800, and again from wet to dry after 1801. On the half-centennial time scale, five wet periods in 960–1000, 1151–1200, 1251–1300, 1351–1400 and 1651–1900, and five dry periods in 1001–1150, 1201–1250, 1301–1350, 1401–1650 and after 1901 were identified. Extreme drought and flood events occurred with high frequency in the 17th and 19th–20th centuries, about once every two years. Extreme drought events lasting 5 years occurred in 1070–1089, 1212–1216, 1327–1331, 1431–1445, 1481–1491, 1634–1641, 1688–1692, 1714–1722, 1875–1878, 1925–1931 and 1993–1997. Extreme flood events lasting 5 years occurred in 981–985, 1647–1653, 1658–1664, 1676–1680, 1725–1730, 1848–1854 and 1882–1889. The dry-wet changes of the Guanzhong Plain has an inter-annual oscillation with a 3–7-year scale, inter-decadal oscillations with quasi-10-year, quasi-30-year and quasi-70-year scales and a centennial oscillation with a quasi-100-year scale. The climate of Guanzhong Plain tends to be drier than in a normal year in the year of the occurrence of the El Nino or the following year. On the quasi-70-year scale, the correlation of the Pacific Decadal Oscillations and the dry-wet change shows a positive (negative) relationship before (after) 1435, which suggested that the relationship between them is not stable in the long term. In addition, the solar maximum corresponds to the dry climate. It is worth noting that due to the lack of long climate series, both reconstruction and model simulation approaches are used to study climate change during historical times. The reconstructed dryness and wetness variations for the past 1000 years in Guanzhong Plain may still lack certainty, and in the future, further analyses and comparisons between the reconstructions and simulations need to be conducted, in order to diagnose the dynamic mechanisms of the climate features in Guanzhong Plain.