Combined impacts of the El Niño‐Southern Oscillation and Pacific Decadal Oscillation on global droughts assessed using the standardized precipitation evapotranspiration index

Using the standardized precipitation evapotranspiration index, this study examines the combined effects of El Niño‐Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) on global droughts in terms of magnitude, timing, and duration. The ENSO‐affected drought hotspots are identified based on drought magnitude and probability of occurrence: five hotspots for El Niño (Amazon, India, central China, Indonesia, and eastern Australia) and four hotspots for La Niña (southeastern United States, southern South America, East Africa, and Southwest Asia). When ENSO and PDO are in phase, most of the hotspots exhibit an intensification and expansion of drought, more clearly at longer time scales (6–12 months), supporting previous studies. Interestingly, the in‐phase PDO advances El Niño‐induced drought onset by early summer of the previous year, whereas it delays the withdrawal of La Niña‐induced drought until the end of the event year. This asymmetric response is found to be in part associated with the earlier start and later end of El Niño itself during warm PDO, which does not hold for the La Niña/cold PDO composites. Further analyses of the responses of precipitation (P) and potential evapotranspiration (PET) to different ENSO‐PDO combinations suggest the important role of P reduction in determining drought magnitude and timing over most of the hotspots, with some contribution of enhanced PET to drier conditions over a few La Niña hotspots. It is also found that the PDO modulation of El Niño‐induced drought occurs primarily through the eastern Pacific El Niño with a limited influence on the central Pacific El Niño. Using a multi‐scalar drought index of standardized precipitation evapotranspiration index (SPEI), this study examines the combined effects of El Niño‐Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) on global droughts in terms of magnitude, timing and duration. When ENSO and PDO are in phase, all nine drought hotspots experience an intensification and area expansion of drought. An asymmetric influence of in‐phase PDO is identified, advancing the El Niño‐induced drought onset while delaying the withdrawal of La Niña‐induced drought. The figure shows the comparisons of ENSO‐induced drought timing and duration between in‐phase and out‐of‐phase PDO conditions based on SPEI12 during (a) El Niño and (b) La Niña.