Exploring the relationship between SPI and SPEI in a warming world

Over the last 50 years, global air temperature has increased substantially especially over continents, causing higher rates of evaporative demand that can lead to an increase in drought occurrence and intensity worldwide. The Standardized Precipitation Index (SPI) is a drought index that solely uses precipitation, while the Standardized Precipitation Evapotranspiration Index (SPEI) uses the difference between precipitation and potential evapotranspiration and, therefore, incorporates the impacts of warming more directly. Here, we explore trends in SPI and SPEI, the relationship between SPI and SPEI, and how this relationship varies across different climate types. We find that a majority of the terrestrial surface has positive trends in SPI while most has negative SPEI from 1971 to 2022, a period of consistent global warming. Although the two indices show positive correlation with each other across the globe, we find decreasing correlation across all aridity types from the humid to the hyper-arid regions. The difference of correlation across two recent 26-year periods indicates that humid regions have consistent correlations through time while arid regions experience substantial interdecadal variation. Using a spatially weighted multiple linear regression, we find that SPI and SPEI trends have opposing relationships with temperature trends; specifically, it has a direct relationship with SPI trends and an inverse relationship with SPEI trends. In contrast, both vapor-pressure deficit and incoming solar radiation have an inverse relationship with SPI and a stronger inverse relationship with SPEI. Ongoing increases in temperature and vapor-pressure deficit are likely to cause further decoupling of the two indices worldwide.