SPEI-Based Drought Frequency, Intensity, and Duration from CMIP6 Models Under SSP1-2.6 and SSP5-8.5 Across Tropical–Subtropical Monsoon Asia

Tropical–subtropical monsoon Asia faces pronounced hydroclimatic change under future warming, yet the regional patterns and emission scenario dependence of drought risk remain unclear. Using a 14-member CMIP6 multi-model ensemble from the NEX-GDDP-CMIP6 dataset, we assessed drought evolution under SSP1-2.6 and SSP5-8.5 (2015–2100). Combining the 3-month Standardized Precipitation Evapotranspiration Index (SPEI-3) with event-based run theory and Mann–Kendall trend analysis, we evaluated four complementary drought dimensions: annual mean SPEI, frequency, intensity, and duration. Regional drought area was used as a spatial diagnostic of extent. A persistent west-to-east moisture gradient characterizes present-day conditions: chronic deficits prevail in southwestern China, southern Myanmar, and Thailand, whereas southeastern China and the equatorial belt maintain relative surpluses. This contrast intensifies through the century, with negative SPEI anomalies deepening by 0.2–0.4 units and chronic drought zones expanding into previously marginal areas. Under SSP1-2.6, no metric shows statistically significant trends. By contrast, SSP5-8.5 produces robust drying, with 8.0% of the area showing significant SPEI declines and up to 30.9% exhibiting significant intensification. Scenario-driven increments affect 89–99.7% of the area across all indicators, confirming that higher emissions systematically elevate drought frequency, persistence, and severity. Scenario uncertainty surpasses model structural uncertainty first for intensity (2067), then area (2078), duration (2079), and frequency (2090), reflecting earlier model convergence on the forced thermodynamic shift than on threshold-dependent frequency counts. Stringent mitigation consistent with the Paris Agreement can largely stabilize regional moisture regimes, whereas continued high emissions will drive pervasive drought intensification across one of the world’s most populous and biodiverse forest regions.