[A-CG33] Multi-scale ocean-atmosphere interaction in the tropics

[ACG33-P02] A Study on Generation Mechanisms of Sea Surface Temperature Anomalies Associated with the Canonical El Niño Focusing on Vertical Mixing

Kouya Nakamura¹, Shoichiro Kido², Takashi Ijichi¹, *Tomoki Tozuka¹ (1.Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 2.Application Laboratory, Research Institute for Value-Added-Information Generation, JAMSTEC)

Keywords:El Niño/Southern Oscillation, Vertical turbulent mixing, Mixed layer heat budget analysis, Tropical Pacific

The El Niño/Southern Oscillation (ENSO) plays an important role in global climate variability. Past studies have suggested that the thermocline feedback (i.e. mean vertical advection of anomalous vertical temperature gradient) is the dominant generation mechanism of positive sea surface temperature (SST) anomalies associated with the canonical El Niño. However, most studies are not based on a closed heat budget analysis and the role of vertical mixing has not been quantified even though active vertical turbulent mixing in the upper ocean is observed in the eastern equatorial Pacific due to the intense current shear between the South Equatorial Current and the Equatorial Undercurrent. In this study, a completely closed mixed layer heat budget analysis using a hindcast simulation by the Regional Ocean Modeling System (ROMS) with a sophisticated vertical turbulent mixing parameterization is performed. It is shown for the first time that the vertical mixing process plays a more important role in the development of SST anomalies than the thermocline feedback. Further analyses of vertical diffusion coefficients and turbulent kinetic energy budget reveal that 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 a decrease in vertical temperature gradient associated with anomalous deepening of the thermocline. On the other hand, effects of surface heat fluxes operate as the dominant negative feedback mechanism. In addition to an increase in latent heat loss and a decrease in shortwave radiation reaching the sea surface, less effective warming by the mean surface heating due to the anomalously thick mixed layer contributes to the damping of positive SST anomalies. Thus, this study emphasizes the importance of vertical mixing processes and mixed layer depth variability in the evolution of the canonical El Niño.