Abstract
Cold sea surface temperature (SST) extremes, known as marine cold spells (MCS), have received less attention than marine heatwaves, particularly at the regional scale. This study examines the origin and drivers of autumn cooling events over the southwestern South Atlantic shelf (SWAS; 32° S–38° S), focusing on two record‐breaking cases in 2016 and 2024. We use ERA5 reanalysis, NOAA OI SST V2.1, and SMAP satellite products for SST and salinity, along with indices for the El Niño–Southern Oscillation (ENSO) and Madden–Julian Oscillation (MJO). These years presented negative SST anomalies in the Río de la Plata estuary and adjacent areas that were the most intense in the 43‐year long satellite record. The events were linked to exceptional river discharge (> 95th percentile) and anomalous surface southerly winds associated with a strong geopotential height dipole over southern South America at upper atmospheric levels. Results suggest these events arose from the interplay of several phenomena acting across different time scales: ENSO, MJO and climate change. El Niño increased rainfall over southeast South America and, by long‐term warming, the Indian Ocean favoured persistent MJO Phases 2 and 3, which reinforced atmospheric conditions sustaining the southerly winds over the SWAS. These winds drove surface heat loss and northward transport of colder, fresher waters from the Río de la Plata. Understanding these compound drivers improves MCS predictability, providing key insights for marine adaptation and management under climate change.