Projected Intensification of Hydroclimatic Extremes in Rio Grande <scp>do</scp> Norte, Brazil, Under <scp>CMIP6</scp> Scenarios

ABSTRACT Understanding changes in precipitation extremes is essential for assessing climate variability in vulnerable regions. This study evaluates historical trends (1963–2023), historical simulations (1850–2014), and future projections (2015–2100) of extreme precipitation indices in Rio Grande do Norte (RN), a semi‐arid state in northeast Brazil characterised by high climatic variability. Daily rainfall data from 115 meteorological stations and multi‐model simulations from the Coupled Model Intercomparison Project Phase 6 (CMIP6) were analysed under three Shared Socioeconomic Pathway (SSP) scenarios: SSP1‐2.6, SSP2‐4.5, and SSP5‐8.5. Eleven standardised precipitation indices defined by the Expert Team on Climate Change Detection and Indices (ETCCDI) were computed. Model performance was assessed using Taylor diagrams, and temporal trends were analysed using the Mann–Kendall test. As an innovation, a Hydroclimatic Extremes Index (HCEI) was developed to integrate the effects of both prolonged dry periods and intense rainfall events by combining Consecutive Dry Days (CDD) and R95pTOT through Z ‐score normalisation. This metric revealed a significant upward trend in HCEI values across all mesoregions of RN, particularly in the West and East, indicating an increased occurrence of compound hydroclimatic extremes. Peaks in HCEI coincided with major El Niño episodes, suggesting strong links between large‐scale climate drivers and local hydroclimatic dynamics. The results point to a consistent intensification of dry spells and extreme precipitation, especially under the high‐emission SSP5‐8.5 scenario. Despite limitations in spatial resolution, CMIP6 models captured large‐scale spatial patterns and project escalating hydroclimatic extremes throughout the 21st century. These findings offer critical insights for regional adaptation planning and water resource management under a changing climate.