The El Niño Southern Oscillation (ENSO) is the leading mode of interannual climate variability, with large socioeconomical and environmental impacts. ENSO main conceptual model, the Recharge Oscillator (RO), considers two independent modes: the fast zonal tilt mode in phase with central-eastern Pacific Temperature (Te), and the slow recharge mode in phase quadrature. However, usual metrics do not orthogonally isolate the slow recharge mode, being correlated with Te. Furthermore the optimal Oceanic Heat Content (OHC) region is currently debated. Here, through an objective approach to optimize RO equations fit to observations, we develop a new recharge index based on Te-independent OHCind (Te-variability regressed out). The optimal region is the western and southwestern Pacific (5N-15S,120E-155W): OHCind_w+sw. Southwestern Pacific OHC anomalies are caused by ENSO windstress and Ekman pumping meridional asymmetry. This index is more physically and statistically coherent, reconciling RO conceptual model with observations, and more relevant for ENSO operational forecasts.