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

[ACG33-08] Interactions between phytoplankton and ENSO simulated in a high-resolution coupled physical-biological ocean model

*Kaito Koike¹, Yoshikazu Sasai², Tomoki Tozuka³ (1.Department of Earth and Planetary Physics, School of Science, The University of Tokyo, 2.Japan Agency for Marine-Earth Science and Technology, 3.Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo)

Keywords:El Niño-Southern Oscillation, Phytoplankton, Shortwave radiation

The El Niño-Southern Oscillation (ENSO) is the major interannual climate mode in the tropical Pacific and has strong impacts on the global climate through teleconnections. It is also known to influence biological activity including phytoplankton in the eastern equatorial Pacific by modulating nutrient supply by equatorial upwelling. Since absorption of penetrative shortwave radiation by chlorophyll of phytoplankton warms the upper ocean, this may feedback onto sea surface temperature (SST) anomalies associated with the ENSO. However, the relative importance of this feedback has not been quantified using a high-resolution coupled physical-biological ocean model. Using outputs from an ecosystem model embedded in an eddy-resolving ocean general circulation model, this study attempts to quantify the interactions between phytoplankton and ENSO. In agreement with observations, phytoplankton concentration decreases anomalously near the surface associated with a decrease in nutrient concentration during El Niño and vice versa during La Niña. As a result, less amount of shortwave radiation is absorbed by the surface mixed layer during El Niño and the absorption increases anomalously during La Niña. When integrated over the development phase from March to December, the contribution from phytoplankton concentration anomalies damps positive SST anomalies associated with El Niño by 1.0°C, which is about 50% of the damping by the SST-cloud-shortwave radiation feedback. On the other hand, during La Niña, the negative feedback associated with phytoplankton concentration anomalies is almost comparable with the SST-cloud-shortwave radiation feedback and contributes to 1.0°C anomalous warming when integrated over the development phase. Thus, this study suggests the possible importance of considering the physical-biological feedback for a more realistic simulation and a more accurate prediction of the ENSO.