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.