Research On The Extreme Climate Events Over Northeast China Under Global Climate Change

Extreme climate events(ECEs) have inflicted severe and adverse effects on human life, social economy, and natural ecosystems. One of the current main and important hotspots of meteorology and climatology is to analyze the ECEs. The study area is the Northeast China(NEC). This study analyzes the uncertainty of ECEs, and proposes a more objective method for determining ECEs. On the basis, this study investigates the spatio-temporal characteristics of extreme temperature and extreme precipitation over NEC during 1961~2013. Considering the differences between precipitation and temperature, this study proposes an idea to research extreme persistent precipitation events. This study also discusses the mechanism of extreme precipitation over NEC, and predicts the changes in ECEs over NEC using the CMIP5 model outputs of different RCPs scenarios. The main conclusions are as the follows:(1) Over the NEC, the changes in annual precipitation are mainly fluctuant decrease type; the changes in annual mean temperature(Tmean) and maximum temperature(Tmax) are mostly fluctuant increase type; in contrast to Tmean and Tmax, there are more steady increase type for the changes in annual minimum temperature(Tmin), which mainly occur in the west of Liaoning Province and the Zhangguangcai Mountain. For the entire NEC, the change in the annual precipitation is fluctuant decrease, whereas the changes of annual Tmean, Tmax, and Tmin are all fluctuant increase during 1961~2013.(2) The percentile-based method, which is the most popular method for defining the ECEs, has four aspects of uncertainty. The remove-detrended fluctuation analysis(R-DFA) method proposed in this study can distinguish the extreme events from the normal ones by calculating and analyzing the long-range correlation index. The threshold based on the R-DFA method is relevant to the local climate and is site specific. Compared with the percentile methods, a test of plus or minus extremes for a daily series existing extreme values show that the R-DFA method can definitely determine the critical value between the extreme and non-extreme events. Although the definition of ECEs has some uncertainties, the R-DFA method is a robust method for defining the threshold of ECEs.(3) Significant occurrence frequencies of ELMT are dominated in the central region of NEC. Comparatively, the notable occurrence frequencies of EHMT are mainly distributed in Liaoning Province and the northwestern region of NEC. The strong ELMT intensity mostly occurred in Liaoning Province and the Songliao Plain. The zones of strong intensity EHMT are mainly located in the north-central and southwestern regions of NEC. The trends in ELMT frequency of the majority of stations(approximately 58 %) are decreasing. The increasing trends in EHMT frequency of the overwhelming majority of the stations(about 94 %) are found. About 58 % stations have decreasing trend in ELMT intensity, whereas 69 % stations are increasing trend of EHMT intensity. The serious ELMT mainly occur in the Songliao Plain and the Sanjiang Plain, especially in the Songliao Plain. The serious EHMT distinctly occur in the Sanjing Plain, and the southwestern and northwestern regions of NEC.(4) Both high-frequency and high-degree intensity extreme daily precipitation events(EPs) are mainly distributed in Liaoning Province and the piedmont regions of the Changbai Mountains and Xiao Hinggan Mountains. Generally, EPE frequency shows a negative trend, whereas EP intensity has a weak positive trend for the entire NEC in the period of 1961~2009. However, the trends are not significant at the 0.05 confidence level. The study reveals obvious NE-SW and NW-SE spatial gradients in EP severity index(EPSI). The EPSI in northwestern and southeastern NEC is small, whereas the EPSI in southwestern and northeastern NEC is large. The area with the most serious EPSI is in Liaoning Province, and the severity decreasingly spreads along the SW-NE direction to central NEC.(5) The regional difference of extreme persistent precipitation event(EPP) frequency is notable over NEC. The occurrence frequency of EPP in mountains is more than that in plain regions. High frequencies of EPP are mainly located in the Changbai Mountains and the Xiao Xinggan Mountains, whereas low EPP frequencies are mainly observed in southwest of NEC, west of Hulun Buir, and parts of the Sanjiang Plain. The EPPs with high intensity are observed in the south of NEC, whereas low-intensity EPPs are mainly occurred in the north of NEC. The persistent duration of EPPs in mountains is longer than that in plain regions. The EPPs in Liaoning Province are more severity than those in the northwest(Hulun Buir and Mohe) and east(the Sanjiang Plain) of NEC. For the entire NEC, the severity of EPPs show non-significant increase. Moreover, the EPPs in the last ten years are more and more serious.(6) The intensity of north cold air is one of the causes of regional EP in NEC. But, the intensity of north cold air and EP frequency in July and August have significant negative correlation, that is, the stronger the cold air intensity, the less frequency of EP. Cyclone is also one of the causes of regional EP in NEC. About 15~20% of regional EP is related to the cyclone over NEC. The East Asia and southeast Asia monsoon intensity are not obviously associated with the EP frequency in NEC. The ENSO phenomenon in a summer and fall tend to produce more summer EP events in NEC in the next year. Therefore, the occurrence of ENSO could be a sign that the summer EPs will be increased in the next year over NEC. According to the correlation analysis between summer EPPEs and the cut-off lows(COLs), more than a third of EPPs are associated with COLs, and more than half of the COLs are one of the factors resulting in the EPPs over NEC.(7) The precipitation, Tmax, and Tmin are all increase trends under the RCP2.6, RCP4.5, and RCP8.5 scenarios in the first half of the 21 st century. For the second half of the 21 st century, precipitation, Tmax, and Tmin are all decrease under the RCP2.6 scenario and increase under the RCP8.5 scenario, whereas the precipitation and temperature are decrease and increase under the RCP4.5 scenario, respectively. However, for the entire 21 st century, both precipitation and temperature show significant increase trends under the three RCPs scenarios. On this basis, extreme Tmax and Tmin are increase and decrease under the three RCPs scenarios in 2021~2040, respectively. Both EP and EPP show increase trends under the three RCPs scenarios in 2021~2040. In this period, high increase of both EP and EPP frequencies will occur in Changbai Mountains and the north of Da Xinggan Mountains, whereas low increase of EP and EPP frequencies will distribute in the Sanjiang Plain and Xiao Xinggan Mountains. For the return period, in general, the longer the return period is, the more intensity extreme precipitation will be. What’s more, the return levels of persistent precipitation are greater than that of daily precipitation. The largest return value is located in the south of NEC(Liaoning Province) for both EP and EPP under three levels of return period(20 a, 50 a, and 100 a), which gradually decreasing towards north.According to a series of site observation data and simulated data, this study discusses the extreme events uncertainty, and comprehensively analyzes the spatio-temporal characteristics of frequency, intensity, and severity of extreme temperature and extreme precipitation in the past, present and future over Northeast China. These findings can not only provide theoretical basis to the study of regional response to global change, but also can provide a scientific basis for policy making under the background that the extreme climate disasters become more and more sevrious.