By analyzing the two types of El Niño Southern-Oscillation (ENSO) indices, i.e., the Central Pacific (CP) type index and the Eastern Pacific (EP) type index by Ren and Jin (2011), this study finds that the low correlation between the two types of indices by some previous studies should be reconsidered. Then based on previous ideas of the unified Niño index systems, the new ocean surface regions for the CP and EP El Niño indices’ calculation are defined. The features of the new CP and EP El Niño indices are consistent with sea surface temperature anomaly (SSTA) evolution along the Pacific equator. This study suggests that, concerning the El Niño characteristics, the CP and EP El Niño indices are not necessarily independent of each other; but their differences are almost absolutely independent of the unified Niño region SSTA. The results quantitatively confirm the relationship between the Trans-Niño Index (TNI) and Niño 3.4 indices (which are nearly independent of each other and provide different flavors for each El Niño event). Results presented here contribute to a better understanding of the nature of the El Niño events.
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