Decoupling Between South American Low‐Level Jet Intensity and Precipitation Extremes in Southeastern South America

ABSTRACT The South American Low‐Level Jet (SALLJ) is the primary structure responsible for transporting moisture from the Amazon to Southeastern South America (SESA). While its influence on the mean hydroclimate is well documented, its efficiency in triggering extreme precipitation events on daily timescales remains less understood. This study analyzes the SALLJ–precipitation relationship using ERA5 reanalysis and IMERG satellite estimates for the austral summer (2002–2024). By applying a rigorous detection algorithm based on joint wind speed and vertical shear thresholds, we isolate dynamically coherent jet days from general northerly flows. Results indicate a marked non‐linear response: the presence of a SALLJ increases the probability of extreme rainfall by a ~7‐fold factor compared to non‐jet days. However, the majority of SALLJ days do not result in heavy precipitation, indicating that the jet acts as an essential moisture supplier that requires additional dynamical support to trigger deep convection. Composite analyses reveal that the outcome is strongly modulated by a stationary Rossby wave train imposing mid‐tropospheric subsidence over SESA. Such dry days are characterized by a dynamically forced vertical decoupling, where strong low‐level moisture transport is suppressed by mid‐tropospheric subsidence associated with the rear flank of an upper‐level trough. Conversely, extreme rainfall occurs only when the jet couples with favourable large‐scale dynamics that support deep upward motion. These findings suggest that operational forecasting should move beyond a sole dependence on low‐level wind intensity and integrate upper‐level dynamic diagnostics to minimise false alarms.