South Tropical Atlantic and South Atlantic Convergence Zone: Opportunities for Long-Lead ENSO Diversity Prediction

Forecasting skill for the pronounced spatial diversity of the El Niño-Southern Oscillation (ENSO) remains a significant challenge at long lead times. Using Peter and Clark Momentary Conditional Independence Plus with forward-in-time conditioning applied to the fifth-generation European Centre for Medium-Range Weather Forecasts reanalysis (1945--2024), we identify a novel atmospheric bridge linking ENSO evolution to variability of the subtropical South Atlantic (STA) and convection in the Amazon/South Atlantic Convergence Zone (SACZ) during the South American Monsoon System (SAMS) season. An STA sea surface temperature mode in June-July-August of year -1 emerges as the longest lead precursor (six-season lead) of December-January-February of year +1 (DJF(+1)) ENSO diversity, with a stronger influence on Eastern-Pacific (EP) events. Seasonally phased SACZ/Amazon convection from September-October-November of year -1 to March-April-May of year 0 provides complementary 3--5 season pathways. Mechanistic diagnostics reveal a subtropical Gill pattern and extratropical Rossby wave sources over the South Atlantic-South America sector that project over the equatorial band and lead to a reorganization of the Walker Cell, thereby preconditioning the tropical Pacific for ENSO onset. Statistical forecasts restricted to these causal parents improve DJF(+1) E-index and C-index (Eastern-Pacific and Central-Pacific indices, respectively) correlation skill by \((\approx)\) 0.15 over canonical predictors, significantly mitigating the boreal-spring predictability barrier. Collectively, these findings reposition the SAMS as an active agent in pantropical coupling, offering physically interpretable pathways to enhance long-lead predictions of ENSO diversity.