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Remarkable interannual variability in the thermocline depth in the southern tropical Indian Ocean (STIO) is analyzed using reanalysis data during 1980-2017. Previous studies have shown that the El Nino-Southern Oscillation (ENSO) has a significant relationship with thermocline depth anomalies in this region. We find that both the eastern-Pacific (EP) and central-Pacific (CP) ENSO have important impacts on the STIO thermocline variation. The positive and negative phases of thermocline anomalies in the STIO are induced by asymmetric forcings from the two phases of ENSO. EP-El Nino and CP-La Nina events tend to induce larger thermocline depth anomalies in the STIO. Equatorial westerly and STIO anticyclonic winds during EP-El Nino events can induce downwelling Rossby waves that extend far toward the western Indian Ocean, which dominates the westward propagation of thermocline anomalies, while upwelling Rossby waves during CP-La Nina events cannot extend that far west.

Plain Language Summary Despite more researches mentioned about the global warming slowdown, the extreme events like the El Nino-South Oscillation (ENSO) happen more frequently in the real world. Previous studies proved that the ocean thermocline depth in the southern tropical Indian Ocean (STIO) is influenced by different ENSO events (El Nino and La Nina), and the ENSO events have been divided as the eastern-Pacific (EP) and central-Pacific (CP) El Nino/La Nina based on different locations of the maximum sea surface temperature anomalies centers during the past decades. In this study, we present the relationships between the STIO thermocline depth and EP/CP El Nino/La Nina in this complex circumstance. We find that forcings from El Nino and La Nina are asymmetric, and the EP-El Nino and CP-La Nina events tend to induce larger thermocline variations. Considering these two different ENSO events, as mentioned above, happen more frequently, the variation of the STIO thermocline might be much greater in the future.