Distinct and Synergistic <scp>ENSO</scp> Modulation of Winter Extreme Rainfall Over Central Vietnam

ABSTRACT Previous studies have primarily focused on tropical cyclone–related extreme rainfall in Central Vietnam, whereas comparatively little attention has been given to extremes during the boreal winter (November–January; NDJ). Recent winter flooding events highlight the need to better understand the large‐scale climate drivers of non‐cyclonic extreme precipitation. This study investigates the individual and synergistic roles of the El Niño–Southern Oscillation (ENSO) and the East Asian Winter Monsoon (EAWM). To disentangle their coupled influences, a linear regression‐based framework is applied to isolate the ENSO‐independent monsoonal component (EAWMres). Results show that winter extreme precipitation exhibits robust, statistically significant negative correlations with both ENSO and the total EAWM indices. However, once the ENSO signal is removed, EAWMres shows no significant relationship with rainfall extremes, while the ENSO‐related monsoonal component displays stronger and more spatially coherent associations. Dynamical diagnostics indicate that ENSO establishes a favourable large‐scale environment by coupling low‐level convergence with enhanced upper‐level divergence, forming an efficient ‘vertical pump’ that sustains deep convection. In contrast, EAWMres lacks this vertical coherence. Moisture budget analysis further shows that convergence dominates over horizontal advection, particularly in the lower troposphere. This effect is amplified by the Annamite (Truong Son) Range, where orographic lifting concentrates rainfall within a narrow coastal band. These findings provide a physically consistent framework linking large‐scale variability with regional topographic effects, offering implications for seasonal predictability and climate risk management.