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The role of oceanic feedbacks in determining the asymmetry of El Nino-Southern Oscillation (ENSO) magnitude, spatial structure, and duration is quantified on the basis of a novel temperature variance budget. Results confirm previous studies that in the eastern Pacific, El Nino warm temperature anomalies are larger in magnitude than La Nina cold temperature anomalies mainly due to stronger positive oceanic feedbacks for El Nino. We find that La Nina cold anomalies are typically stronger than El Nino warm anomalies in the central Pacific with a faster growth rate for cold anomalies, due to a stronger positive thermocline feedback and weaker nonlinear damping. The thermocline feedback related to recharge oscillator dynamics plays a dominate role and leads to asymmetry in the duration of El Nino and La Nina events. In particular, the thermocline feedback becomes significantly negative during the late decaying phase of El Nino and speeds up its demise.