Quantifying the climatic drivers of drought using a standardized aridity index

Drought is one of the costly natural disasters that affect water resources, agriculture and ecosystems. This study developed a standardized aridity index (SAI) to quantify the short- and long-term drought, and then decipher the climate drivers of the drought on local, regional and continental scales.  The ratio of total precipitation (P) to total potential evapotranspiration (PET) for a given month or multiple months was firstly calculated, and then normalized to calculate the SAI. The contribution of P, PET as well as temperature, solar radiation, wind speed and relative humidity on SAI can be decomposed by apply partial derivation of SAI and PET algorithm (i.e., Penman-Monteith model). The SAI is highly correlated to several frequently used drought indexes.  We also examined the temporal variations and spatial extent of different droughts across the global. The contributions of different climate variables on these droughts were also examined. The spatial distribution of individual droughts and their intensity revealed by SAI are comparable to those calculated using existing drought indexes and drought monitors. For example, the 12-month SAI and other drought indexes all suggested a several drought condition in the central Europe during 2015-2020, which is unprecedented in the past 2,000 years. We found that this drought was firstly initiated by precipitation deficit, but the PET became important in the late years of this drought. On average, the precipitation contributed to 70%, while the PET contributed to another 30% to this multi-year drought. The temperature warming alone contributed to about 20% of the drought intensity.