Past-to-future drought across hydrographic microregions in Northeast Brazil: SPI-based diagnostics and bivariate (duration–severity) risk under CMIP6 scenarios

Abstract Drought is a recurrent challenge for water management in Northeast Brazil. Understanding future changes is important for planning. In this study, we used observed precipitation and QDM corrected CMIP6 simulations to examine future meteorological drought across 82 hydrographic microregions under SSP2-4.5 and SSP5-8.5. We calculated SPI-12 relative to a fixed 1961–2014 reference period and evaluated dry year frequency, the area affected by drought, trends, drought events, and the recurrence of events with similar duration and severity. Our results show lower SPI-12 values in most microregions under both scenarios, with a stronger shift under SSP5-8.5. The median frequency of moderately dry years increased from 16.1% in the observed record to 26.5% under SSP2-4.5 and 32.0% under SSP5-8.5. Dry conditions also affected larger areas of the region in more years. Most simulations projected negative trends, mainly under SSP5-8.5, but statistically supported local trends were less widespread. This shows that future changes are not expected to follow a simple and uniform pattern. Fewer new dry sequences occurred, but they lasted longer and accumulated larger deficits. Events that reached an annual SPI-12 value of −2 or lower became more frequent in 70 to 80 of the 82 microregions. Events similar to the local 2012–2018 reference drought also had shorter return periods in many microregions, although results varied by location and event definition. These results can help identify areas where drought planning deserves priority.