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Abstract

Although the thermocline feedback is considered as the dominant mechanism for generating positive sea surface temperature (SST) anomalies associated with the canonical El Niño, the role of vertical mixing has not been fully quantified in modelling studies even though active vertical turbulent mixing in the upper ocean is observed in the eastern equatorial Pacific due to intense current shear. In this study, a completely closed mixed layer heat budget analysis using outputs from a hindcast simulation of a regional ocean model is performed to elucidate the mechanisms of positive SST anomalies associated with the canonical El Niño. It is shown that the vertical mixing process plays a more important role in the development of SST anomalies than the thermocline feedback. The anomalous warming by the vertical mixing process may be explained by anomalously thick mixed layer that reduces sensitivity to cooling by the mean vertical mixing and an anomalous decrease in vertical temperature gradient associated with positive thermocline depth anomalies. On the other hand, effects of surface heat fluxes damp positive SST anomalies, not only due to negative latent heat flux and shortwave radiation anomalies, but also due to less effective warming by the mean surface heating due to positive mixed layer depth anomalies.