Nonlinear response of soil moisture drought persistence to climatic aridity at the global scale

Soil moisture deficit persistence affects drought development, ecosystem resilience, and land–atmosphere feedbacks, but its dependence on climatic aridity is still not well understood at the global scale. We examine whether persistence changes gradually along the aridity gradient or has a clear break between hydroclimatic regimes. Using monthly near-surface soil moisture data for 2001–2023, we estimate the characteristic persistence timescale (t L ) and the upper tail of deficit duration (t 99 ). Persistence increases from arid climates toward semi-arid and dry-subhumid conditions, then declines under humid climates. The estimated transition occurs near AI ≈ 0.68 for t L and AI ≈ 0.67 for t 99 , close to the dry-subhumid to humid boundary. Soil moisture memory, represented by lag-1 autocorrelation, also peaks within a similar intermediate-aridity range, although not exactly at the persistence breakpoints. Similar behaviour across ERA5, GLDAS, and ESA CCI indicates that the aridity persistence pattern is not tied to one soil moisture product. Overall, the analysis identifies a clear non-linear relationship between climatic aridity and soil moisture deficit persistence, although the mechanism still needs to be tested with process-based studies.