Joint analysis of changes in temperature and precipitation on the Loess Plateau during the period 1961-2011

The Loess Plateau is particularly sensitive to climate change owing to its fragile ecological environment and geographic features. Here, we present a comprehensive analysis of the joint probabilistic characteristics and tendencies for bivariate and trivariate precipitation and temperature indices across the plateau, based on copula theory. The results show that the southeast region of the plateau had a higher potential for flooding: the 10-year return levels for the number of days with heavy and very heavy precipitation (R10mm, R20mm) and for the maximum 5-day precipitation value (RX5day) were higher in this region. The northwest region of the plateau, however, had a higher potential for drought, as reflected in the high and increasing 10-year return levels for the number of consecutive dry days (CDD) and the number of days with low precipitation (R1mm). In a joint analysis of precipitation indices, large areas of the Loess Plateau showed a relatively high risk of concurrent extreme precipitation events. However, the risk of concurrent extreme wet and dry events did not increase over the past half century, as demonstrated by nonsignificant changes in the probability of concurrently long CDD and long consecutive wet days (CWD). A trivariate copula analysis showed that some grid locations in the southeast of the plateau had an increasing risk of extreme precipitation events occurring at a high frequency and a high intensity, and forming a large percentage of the annual precipitation. Joint analysis of precipitation and temperature indices showed that the risk of higher temperatures and longer spells of consecutive dry days had increased over the past 50 years in grid locations scattered in the northern and southern regions: there were negative trends in the bivariate return periods for warm days (TX90p) and CDD. In addition, there was a decreased probability of concurrent long spells of consecutive wet days and colder temperatures, as demonstrated by the positive trends in the bivariate return periods for cold days (TX10p) and CWD across large areas of the plateau. Overall, the risk of severe floods and droughts over the Loess Plateau increased over the period 1961-2011.