The South Atlantic--SACZ Pathway Shapes ENSO Phase Selection under Neutral Initial Conditions and Is Gated by Pacific Subsurface State

Pacific recharge--discharge dynamics remain central to El~Ni\~no--Southern Oscillation (ENSO) diversity and duration, but remote tropical variability may also provide predictive information. We test when the subtropical South Atlantic--South Atlantic Convergence Zone (STA--SACZ) pathway predicts subsequent-winter ENSO diversity and La~Ni\~na duration relative to other tropical precursors. Combining regime-conditional causal network analyses with Bayesian model comparison over 1979--2025, and conditioning on ocean heat content and thermocline slope, we find that remote predictors are state dependent. In active ENSO years, the STA--SACZ family contributes most clearly in selected subsurface regimes, consistent with a Pacific gatekeeper that limits remote contributions when the internal oscillator is already strong. Under neutral initial conditions, its role is clearest: it is most informative for the sign of the emerging event, while the broader established-precursor set adds no predictive gain. By contrast, La~Ni\~na duration remains more consistent with subsurface-dominated predictability than with stable remote contributions, although regime-split tests are limited by sample size. Thus, the STA--SACZ pathway's predictive value is gated by Pacific subsurface state and is most closely tied to phase emergence from neutral conditions.