Extreme Climate Change And Its Trend Prediction In The Yangtze River Basin

Under the background of climate warming, the extreme climate events are increasingly frequent, which has become the hot topic for government, public and the climatic research community in particular. The scientific assessment of extreme climate events and its trend prediction in the Yangtze River Basin(YRB) will be of guiding significance to disaster prevention and reduction, eco-civilization construction, economic and social sustainable development. With a drainage area of 1.80 million km2, the Yangtze River Basin was selected as the study area in this paper. Based on the meteorological and future climate scenario data, the Mann-Kendall test, Sen slope estimator, Moving average, Double mass curve and other statistical methods were employed to detect the variation trends of annual, seasonal mean air temperature and precipitation, to explore the spatiotemporal changes of extreme temperature and precipitation events with a suite of climate indices recommended by the Expert Team on Climate Change Detection and Indices. Finally, the trends and variability of annual mean temperature, precipitation and extreme climate events under RCP4.5 and RCP8.5 emission scenarios during 2020-2099 were evaluated on the basis of a regional climate model Reg CM4.0 nested by a global climate model BCC-CSM1.1. The main conclusions were as follows:(1) The annual and seasonal mean air temperature all showed significant increasing trends during 1960-2012, and the stronger warming occurred in winter and spring at a rate of 0.23 and 0.21 °C/10 a, respectively. Geographically, warming in the eastern Tibetan Plateau and the northern part of middle and lower reaches of YRB were more significant. Besides, the mean temperature in summer displayed cooling trends in the northestern part of YRB. No significant trends could be detected in the annual mean precipitation and precipitation in the flood season(from April to September). The precipitation in summer and winter exhibited an increasing trend of 9.91 and 3.83 mm/10 a, respectively, but the trends in spring and autumn were decreasing with a trend of-5.64 and-8.21 mm/10 a, respectively. The variation trends in precipitation showed obvious regional difference. For the annual mean precipitation and precipitation in the flood season, stations with increasing trends occurred mostly in the eastern Tibetan Plateau and the middle and lower reaches of YRB. Precipitation in spring were dominated by decreasing trends in the middle and lower reaches of YRB and increasing trends in most parts of the eastern Tibetan Plateau. In summer and winter, the increasing magnitudes were more pronounced in the middle and lower reaches of YRB. Precipitation in autumn showed spatially consistent decreasing trends in most YRB.(2) All temperature indices showed widespread significant warming in the YRB during 1960-2012, with indices derived from daily minimum temperature(TN) warming faster than indices from daily maximum temperature(TX), and the trends in 1986-2012 were more significant. On the whole, cold-related indices, i.e., cold nights(TN10P), cold days(TX10P), frost days(FD), icing days(ID) and cold spell duration index(CSDI) significantly decreased by-3.45,-1.03,-3.04,-0.42 and-1.6 days/decade, respectively. In contrast, warm-related indices such as warm nights(TN90P), warm days(TX90P), summer days(SU), tropical nights(TR) and warm spell duration index(WSDI) significantly increased by 2.95, 1.71, 2.16, 1.05 and 0.73 days/decade. Minimum TN, maximum TN, minimum TX and maximum TX increased significantly by 0.42, 0.18, 0.19 and 0.14 oC/decade. Because of a faster increase in TN than TX, the diurnal temperature range(DTR) exhibited a significant decreasing trend of-0.09 oC/decade for the whole YRB during 1960-2012. However, the decreasing trends all occurred in 1960-1985, while increasing trends though insignificant were found in all sub-regions and the whole YRB during 1986-2012. Geographically, stations in the eastern Tibet Plateau and northeastern YRB showed stronger trends in almost all temperature indices. Time series analysis indicated that the YRB was dominated by a general cooling trend before the mid-1980 s, but a warming trend afterwards. Strong relationships between extreme temperature trends and elevation, longitude as well as latitude were detected in this study. The warming rates increased with elevation when elevation is above 350 m or longitude is less than 112°, but decreased with elevation when elevation is below 350 m or longitude is greater than 112°. The warming trends in cold-related indices increased with latitude.(3) The total precipitation amount(PRCPTOT) did not show any significant trend for the whole YRB against the large interannual and interdecadal variability of precipitation during 1960-2012. However, simple daily intensity index(SDII), heavy precipitation(R95p TOT), extremely heavy precipitation(R99p TOT), maximum 1-day precipitation(RX1day), maximum 5-day precipitation(RX5day) and maximum consecutive dry days(CDD) all increased significantly with a trend of 40.15, 7.78, 6.59, 1.43, 1.47 mm/decade and 0.22 days/decade, respectively. In contrast, weak precipitation days(R10mm) and maximum consecutive wet days(CWD) decreased significantly, implying that the precipitation processes in YRB were dominated by precipitation events with shorter durations. Geographically, a wetting tendency was observed in the eastern Tibet Plateau and the middle and lower YRB, while the other regions experienced precipitation deficits. The increasing precipitation was mainly due to the intensification of heavy precipitation events and the decreasing precipitation may be attributed to the decrease of weak or moderate precipitation events(R20mm). In addition, the regional trends were of greater magnitudes in the middle and lower YRB, indicating more frequent extreme precipitation events in the region. Time series analysis revealed that most precipitation indices exhibited neither a stable nor a gradual pattern during 1960-2012. In most cases, precipitation variation trends had an insignificant relationship with latitude and elevation when elevation is above 500 m as well as longitude when longitude is less than 105°. But when elevation is below 500 m or longitude is greater than 105°, trends in precipitation indices increased significantly with decreasing elevation or increasing longitude in YRB.(4) Based on the observed daily mean temperature and precipitation during 1960-2005 collected from 143 stations in YRB, the capability of a regional climate model Reg CM4.0 one-way nested by the global climate model BCC_CSM1.1 were first examined. Results showed that the simulated mean temperature and precipitation were broadly comparable with the observations in both time and space, but the errors in the eastern Tibetan Plateau were relatively large. During 2020-2099, the mean air temperature increased by 1.53 and 2.21°C under RCP4.5 and RCP8.5 emission scenarios. Geographically, the warming magnitudes were much stronger in the north parts of YRB than the south part of YRB. The annual mean precipitation displayed obviously fluctuated characteristics during 2020-2099 under two emission scenarios. A clear increasing trend during 2020-2034 was observed under RCP4.5 emission scenario. In spatial distribution, the upper reaches of YRB were dominated by increasing trends, while the middle and lower reaches of YRB were mainly characterized by decreasing trends under RCP4.5 emission scenario. But under RCP8.5 emission scenario, the increasing trends were mostly located in the middle and lower reaches of YRB, whereas the decreasing trends were primarily distributed in upper reaches of YRB.(5) Except for the eastern Tibetan Plateau, the climate model has good performance in simulating extreme climate events in YRB during 1960-2005. During 2020-2099, FD, SU, TNn and TXx all increased significantly under two emission scenarios, but the changing magnitudes under RCP8.5 were-1.44, 4.48 days/10 a and 0.44, 0.47 °C/10 a respectively, which were more significant when compared with those under RCP4.5. DTR showed an increasing trend during 2020-2099, but the trends were more evident during 2059-2099 during which the increasing trends were 0.02 and 0.04 °C/10 a under RCP4.5 and RCP8.5 emission scenarios, respectively. Geographically, the extreme temperature indices showed consistently significant warming trends in YRB and six sub-regions under two emission scenarios during 2020-2099, but the changing magnitudes showd large region difference. The magnitudes of FD, SU, TNn and TXx were especially strong in the eastern Tibetan Plateau, central-south YRB, the eastern Tibetan Plateau and mid-northeatern YRB, respectively. DTR showed larger magnitudes in the mid-northeatern YRB and the south parts of middle and lower YRB. The extreme precipitation indices under RCP4.5 emission scenario exhibited increasing trends during 2020-2059, especially in 2020-2034, meaning higher risk of flood and drought in YRB. The extreme precipitation events showed obviously regional features in the future. R95 p TOT and SDII displayed significant decreasing trends in the north parts of middle and lower YRB, but significant increasing trends in the south parts of middle and lower YRB, with a trend of 4.40 and 13.63 mm/10 a, respectively. RX1 day and RX5 day exhibited decreasing trends in central-northwestern YRB and the south parts of middle and lower YRB. Under RCP8.5emission scenario, all the extreme precipitation indices showed a weak increasing trend during 2020-2099, especially in the middle and lower YRB.