Secular trend of global drought since 1950

Abstract

Drought is a recurring extreme climate event over most parts of the world featured by long duration and low predictability. The secular trend of drought is of particular interest for investigators in agriculture, climate change and sustainability domains. In this study, we applied the ensemble empirical mode decomposition (EEMD) method and analyzed the spatio-temporal characteristics of the secular trends of meteorological drought over global land surface during the period 1950-2015 using a self-calibrating Palmer Drought Severity Index (PDSI) product. We found that there were 25.98% PDSI samples had turning point, namely the shift of trend, in the corresponding secular trend series; the probability distribution of the turning points position (period) extracted by EEMD closely follows a normal distribution with mean value at Nov. 1981. We showed that there is large discrepancy in the secular trend types extracted by EEMD and Mann-Kendall test, and exemplified the risk of using a monotonic trend to capture the changes of the intrinsic secular trend of PDSI series. We suggested that there was an accelerated drying trend over global land surface as a whole, but large areas with wetting trend existed in the meantime, especially at the high latitudes in the northern hemisphere. Additionally, we found that the PDSI secular trend change rate exhibits a multidecadal variability of about 50 years or so and it implies a potential relationship with periodic variations of the oceanic and atmospheric current. We showed that the secular trend of PDSI series extracted by EEMD could provide more detailed spatio-temporal characteristics, featured by the shifts of trend and nonlinear property of the secular trend, of global drought than that of the non-parametric or linear regression methods. The secular trend of PDSI could present more insights about the transition and progress of wetting/drying trend over global land surface at multidecadal scale.