[A-CG35] Projection and detection of global environmental change

[ACG35-03] Air-sea CO₂ flux variations in the tropical Pacific simulated with two ESMs embedded an ocean data assimilation system

*Michio Watanabe¹, Hiroaki Tatebe¹, Hiroshi Koyama¹, Tomohiro Hajima¹, Masahiro Watanabe², Michio Kawamiya¹ (1.Japan Agency for Marine-Earth Science and Technology, 2.Atmosphere and Ocean Research Institute, the University of Tokyo)

Keywords:Earth system model, data assimilation, air-sea CO2 flux, ENSO

Interannual-to-decadal fluctuations of the global-mean atmospheric CO₂ concentration are observed associated with internal climate variations, and these fluctuations sometimes cancel out and at other times support the centennial increasing trend of the global-mean atmospheric CO₂ concentration due to accumulation of anthropogenic CO₂ emissions. One direct cause of the fluctuations is the oceanic absorption or release of CO₂ through the global air-sea CO₂ flux variations which are mainly dominated by the tropical Pacific. Therefore, deeper understanding of the tropical oceanic processes and properly initializing oceanic states including marine ecosystem are of importance in future predictions of the global CO₂ concentration together with quantification of human-influences. In the present study, we examined the simulated air-sea CO₂ flux variations in the tropical Pacific in two ESMs (MIROC-ES2L and MIROC-ESM) with the same ocean data assimilation system. Although the assimilation procedures are the same, observed anti-correlated relationship between interannual variations of the upward air-sea CO₂ flux and the sea surface temperature (SST) in the tropical Pacific are well captured in MIROC-ES2L, but the relationship is reversed in MIROC-ESM. Tropical climatic-mean state of MIROC-ESM shows significant biases of weaker trade winds as well as more diffuse equatorial thermocline than observations, and simulated amplitude of interannual variations of NINO3-SST is about half as large as observations. When observations are assimilated into the model, these biases lead to non-negligible correction terms on the governing equations of ocean temperature and salinity, which induces an anomalous spurious equatorial upwelling during El-Niño events. The spurious upwelling brings dissolved inorganic carbon rich water in the subsurface layer to the surface mixed layer. Consequently, an anomalous upward air-sea CO₂ flux is occurred during El-Niño, as opposed to observations and MIROC-ES2L. Better modeling physical processes in the tropical climate system is suggested to be essential for better marine ecosystem modeling and reanalysis.