Climate Extremes over the Brazilian Caatinga Based on Performance-Based Projections from Selected NEX-GDDP-CMIP6 Models

The Caatinga biome, located in northeastern Brazil, is a semi-arid region highly exposed to hydroclimatic variability, recurrent droughts, and increasing thermal stress. As the driest and socioeconomically most vulnerable region of the country, robust assessments of climate extremes are essential to support adaptation and resilience planning. This study investigates historical climate extremes and future projections over the Caatinga using a performance-based subset of three bias-corrected global climate models from the NEX-GDDP-CMIP6 dataset: CESM2, TaiESM1, and MRI-ESM2-0.

The historical evaluation covers the period 1981-2014 and is based on gridded observations and reanalysis data. ERA5 exhibits good agreement with observations for percentile-based temperature indices (TN10p, TN90p, TX10p, TX90p) and the Warm Spell Duration Index (WSDI). However, large Percent Bias values (>80%) are identified over the São Francisco River Basin, indicating regional discrepancies. For precipitation extremes, the R20mm frequency index reveals dominant drying trends in the same basin, highlighting a regional hotspot of hydroclimatic stress.

Observed extremes show a clear intensification of hot events, while increasing consecutive dry days (CDD) exacerbate drought impacts across the Northeast. The northern Caatinga and the central-southeastern sector associated with the São Francisco Basin exhibit consistent drying signals, despite an increase in the frequency of extreme precipitation events since the 1980s. In contrast, coastal areas show a reduction in the frequency of hot days, alongside a general decline in annual precipitation totals and extreme rainfall frequency across most of the Caatinga.

Future projections are analyzed for near-term (2021-2040), mid-term (2041-2060), and long-term (2081-2100) periods, indicating a substantial reduction in total precipitation and an intensification of compound heat-dryness extremes. These changes pose severe risks to water availability, ecosystem stability, and human livelihoods, threatening millions of people and reinforcing the urgency of climate adaptation policies in semi-arid regions.