[A-CG43] Multi-scale ocean-atmosphere interaction in the tropics

[ACG43-02] Precipitation anomaly enhances the development of El Niño/Southern Oscillation

★Invited Papers

*Takahito Kataoka¹, Fei-Fei Jin², Masahiro Watanabe³, Tatsuo Suzuki¹, Hiroaki Tatebe¹, Hugo Bellenger⁴ (1.JAMSTEC Japan Agency for Marine-Earth Science and Technology, 2.University of Hawaii, 3.Atmosphere and Ocean Research Institute, the University of Tokyo, 4.École normale supérieure)

Keywords:El Niño/Southern Oscillation, freshwater flux, coupled ocean-atmosphere model

During El Niño, positive sea surface temperature (SST) anomalies in the central-eastern Pacific are accompanied by positive precipitation anomalies. While it has been known that precipitation can impact the SST through changing near-surface stratification, observations are spatiotemporally sparse to quantify its role in El Niño -Southern Oscillation (ENSO). On the other hand, modeling studies are mostly based on ocean-only models or a hybrid coupled model that ignores cloud variations, thus shortwave radiation anomalies, despite precipitation varying interannually.
In this study, we investigate the roles of freshwater flux anomaly in SST anomalies associated with ENSO in a coupled system using a state-of-the-art climate model, MIROC6. In addition to the control experiment (CTRL), where the atmosphere and ocean are fully coupled everywhere, two sensitivity experiments are conducted where freshwater flux over the tropical Pacific Ocean is partially replaced with the 3-hourly climatology from CTRL. In the Pclim/Eclim experiment, the precipitation/evaporation flux into the ocean is replaced over the tropical Pacific (the atmosphere can "feel" the ocean variations everywhere). It is found that precipitation anomaly significantly enhances ENSO variability (by up to about 20% for Niño3.4), particularly in the developing season, while evaporation anomaly does not significantly change the ENSO amplitude. A mixed layer heat budget analysis for the CTRL and Pclim reveals that precipitation anomaly enhances the ENSO variability through modifying oceanic vertical mixing. Further analysis shows that precipitation anomalies shoal the mixed layer, which thickens the barrier layer and, thus, reduces the vertical temperature gradient. In addition, the shallower mixed layer enhances the vertical shear, and so the mixing coefficient. These together strengthen the contribution of vertical mixing anomaly.