Recent changes in the dry-to-wet transition season in the Andean Altiplano and related atmospheric circulation patterns (1981–2022)

Drought-related impacts in the southern Tropical Andes are crucial for economic activities. Here, precipitation decreases (p < 0.1) during the dry-to-wet transition season (SON) based on CHIRPS (1981–2023) and meteorological stations (1973–2016). In addition, a decline in specific humidity and moisture flux is detected (1979–2022). The precipitation trend is stronger (p < 0.05) in the southern Titicaca, Desaguadero, Poopó and Coipasa Salt Pan hydrological system (TDPS) and extends southwards (20°S-30°S). A delayed onset and a reduced duration of the wet season are observed in the TDPS (p < 0.05), although precipitation intensifies during DJF in the southwestern TDPS (p < 0.05).To analyze the atmospheric features related to these changes, we derive atmospherics circulation patterns (CPs) from standardized anomalies of daily 200-hPa winds (1979–2022) using a weather classification technique. We identify 9 CPs, characterized by four ‘‘dry’’ (D1, D2, D3, and D4), three ‘‘wet’’ (W1, W2 and W3), and two ‘‘transitional’’ patterns (T1 and T2). During SON, the frequency of the W1 (D3) is negatively (positively) correlated with the onset date of the wet season (p < 0.05). Through the period 1979–2022, the frequencies of the D3 and the predominantly dry T1 increase (p < 0.1 and p < 0.05, respectively). Conversely, W1 frequency diminishes substantially since 2010. The physical mechanisms associated are discussed. In conclusion, increased D3 and T1 frequencies, and decreased W1 are related to the diminution of precipitation during SON and to the shortening of the wet season length. We also show that the extreme drought of 2022 was characterized by an unprecedented frequency of dry CPs, particularly during November.