Recent hydroclimate changes in a tropical-subtropical Andean-lowland transitional region (1981–2024): Implications to Pilcomayo River

Study region The Pilcomayo River Basin, located within the tropical–subtropical transition zone of South America, spans from the Bolivian Andes to the Chaco lowlands. The basin's marked topographic gradient and socio-environmental vulnerability make it a representative case of Andean-lowland hydroclimatic interactions. Study focus This study characterizes seasonal precipitation regimes of the Pilcomayo River Basin (1981–2024) and their links to atmospheric circulation patterns (CPs). The objectives were to (i) characterize the annual precipitation cycle across homogeneous regions and (ii) assess CPs influence on seasonal cycles and trends. CPs were identified using weather classification techniques of daily winds at 850 and 200 hPa. New hydrological insights for the region The analysis reveals three hydroclimatic regions (Andean, Transitional, and Lowlands) with distinct seasonal cycles. Nine CPs were identified, reflecting vertically coupled atmospheric modes that influence the basin's precipitation variability. They include three wet (W1-W3, predominant in austral summer), four dry (D1-D4, austral winter), and two transitional patterns (Tdw and Twd). A significant decline in austral spring (SON) precipitation was detected in the Transitional region (p < 0.05), representing a 28 % reduction over the study period. It is associated with an increased frequency of a dry CP (D1, p < 0.05) and a reduced occurrence of a wet CP (W1, p < 0.1). These changes are linked to shifts in large-scale ocean-atmosphere interactions, including the strength of Hadley and Walker circulations and tropical Atlantic warming.