The Record-Breaking Low-Level Jet Event in Subtropical South America During the Winter of 2026

The South American Low-Level Jet (SALLJ) is a major pathway for heat and moisture transport from tropical to subtropical South America, often triggering severe weather. This study characterizes the physical and statistical features of an exceptionally strong SALLJ event that occurred between 17 and 19 July 2026. A 20-year radiosonde dataset from three stations along the jet pathway revealed that the event was unprecedented in persistence, with wind speeds remaining above the local 90th percentile for at least four consecutive soundings taken at 12 h intervals. ERA5 reanalysis further showed that the 850 hPa wind speeds during this SALLJ episode were anomalously high, reaching 3.8 standard deviations above the July 1991–2020 climatological mean and exceeding 4 standard deviations on 17 July. The event developed under the combined influence of an anomalously strong subtropical jet and an upper-level trough extending toward southeastern South America. SALLJ intensification and persistence were favored by two complementary processes: (1) an enhanced pressure gradient between a deepened continental low (associated with upper-level divergence) and the intensified South Atlantic Subtropical Anticyclone and (2) upper-level jet-exit dynamics and orographically induced ageostrophic circulations that maintained the low-level northerly flow for a longer time. El Niño 2026 background conditions for the SALLJ extreme event due to the anomalous upper-level jet configuration were highlighted.