Interannual variability of regional daily precipitation extremes in subtropical Argentina: circulation features during early austral summer

Abstract Subtropical Argentina concentrates most of the country's population and economic activity, including rainfed agriculture in the La Plata Basin, where daily precipitation extremes drive flash flooding and crop losses. This study characterizes their interannual variability in early austral summer (November–January) through joint statistical and dynamical analysis. The R95p index was computed at rain gauges over 1961–2020, and a stepwise correlation-based regionalization grouped them into seven coherent regions spanning the full climatic gradient. Regional centroids were analysed for trends, breakpoints and periodicities, and used to composite ERA5 fields. Extreme precipitation increased over the north of the domain, as a significant linear trend in the northeast and as an abrupt 1984 breakpoint in the northwest. In every region, variability is organized by anomalous low-level moisture transport and convective instability, but moisture sources follow a longitudinal rule opposite to ocean proximity: eastern regions are fed by northerly transport along the eastern Andes, inland regions by easterly Atlantic inflow, except the northwest, which switched to the northerly pathway after 1984. Large-scale forcing follows the same partition: ENSO imprints a robust, linear signal over the Littoral and northeast, weakens along the semi-arid diagonal, and gives way to the Southern Annular Mode in the northwest, where the relationship reverses sign after 1984, shifting from a low-level mass-field pathway to jet-entrance ageostrophic ascent. Over coastal Buenos Aires, a significant Niño3.4 correlation is not supported by the circulation, showing that statistical association alone can mislead attribution.