From Rare to Recurrent: Intensifying Hot–Dry Extremes Across Romania

ABSTRACT Using a novel high‐resolution monthly gridded observational dataset for Romania (i.e., 1901–2024; derived from 165 homogenised stations), we quantify the evolution of compound hot–dry months, defined as concurrent exceedances of a Standardised Temperature Index (STI ≥ +1) and deficits in the 1‐month Standardised Precipitation–Evapotranspiration Index (SPEI1 ≤ −1), in terms of frequency, spatial extent, and co‐occurrence probabilities. Late‐century projections are assessed using bias‐corrected EURO‐CORDEX regional climate simulations (RoCLIB dataset) under RCP4.5 and RCP8.5. Our results show a pronounced hydroclimatic transition toward more frequent and spatially extensive compound events, accelerating since the 1990s amid rising country‐mean temperatures and declining short‐term water‐balance anomalies. Persistent hotspots of joint probability emerge in the eastern Moldavian Plateau and southeastern lowlands (i.e., lower Danube–Bărăgan Plain–Black Sea coast), while elevated terrain exhibits relative buffering. Period‐wise comparisons (i.e., 1901–1931, 1932–1962, 1963–1993 and 1994–2024) reveal widespread increases in joint probability (typically 12%–20% across most of the country, with conditional heat–drought coupling (P(dry|hot)) strengthening across the eastern Carpathian foreland and Transylvanian Plateau but attenuating along the Lower Danube and coastal margins, indicating that amplified compound risk stems predominantly from the rising prevalence of hot months. The complementary conditional probability P(hot|dry) also rises across the southern and eastern lowlands, indicating that recent dry months are increasingly likely to be hot. Projections for 2071–2100 anticipate further intensification to 4–5 months yr. −1 (RCP4.5) up to 6–7 months yr. −1 (RCP8.5), with joint probabilities reaching 20%–35% and 35%–60%, respectively, rendering such conditions recurrent to near‐normative across key agricultural lowlands.