ENSO-PDO Combined Impacts on Global and Regional Droughts

Abstract

Drought is a climate extreme phenomenon that causes major social and economic consequences on human life. Using a multi-scalar drought index of Standardized Precipitation Evapotranspiration Index (SPEI) (Vicente-Serrano et al., 2010) which is based on monthly differences between precipitation (P) and potential evapotranspiration (PET), this study examines the combined effects of the El Niño – Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) on global and regional droughts in terms of magnitude, timing and duration. Seven drought hotspots are empirically identified based on drought probability and magnitude during ENSO events: five hotspots for El Niño (India, north-eastern China, Indonesia, eastern Australia, Amazon) and two hotspots for La Niña (south-eastern U.S and southern South America). When ENSO and PDO are in phase, all drought hotspots are found to experience an intensification and expansion of droughts, consistent with previous studies. Additionally, these hotspots share the same feature of longer drought duration during ENSO-PDO in phase winters as a consequence of earlier onset (from late summer or early autumn of previous years) and later withdraw (to summer or winter of the next year), which is clearer at longer time scales of droughts (6-12 months). Regional analyses of relative contribution of P and PET suggest the dominant role of reduced P in determining drier condition over India, eastern Australia, Amazon and south-eastern U.S. While over Indonesia and north-eastern China, this pattern is not clear, southern South America witnesses the larger contribution of enhanced PET than reduced P to the SPEI decrease due to ENSO-PDO combination effects. The enhanced PET is found to be dominated by the radiative component which is related to reduction of cloud cover (∆cld<0) or by the aerodynamic term that is associated with warmer condition (∆T>0) and/or drier condition (∆RH<0) depending on regions. Further, physical mechanism by which PDO can modulate the ENSO influence on individual regional droughts is reviewed and examined by analysing atmospheric circulation changes. Results show that when ENSO is in phase with PDO, enhanced descending motions are favored causing P reduction over all drought hotspots. Mechanisms of regional drought intensification are discussed by linking atmospheric circulation changes to changes in PET and its components.