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7/11/2019: Séminaire de Michio Watanabe (JAMSTEC, Japan)

Importance of faithful reproduction of El Nino for ocean data assimilation system in terms of historical CO2 flux fluctuations in the equatorial Pacific
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Since an increase in atmospheric CO2 concentration leads to global warming, the need for predicting fluctuations in atmospheric CO2 concentration is increasing, and uptake CO2 at ocean surfaces is one of the key processes in the global carbon cycle that determines atmospheric CO2 concentration. The air-sea CO2 flux is known to fluctuate due to internal climate variability, predominantly by El Nino/Southern Oscillation (ENSO) in the equatorial Pacific.

Therefore, representing the interannual fluctuation of CO2 fluxes in Earth system models (ESMs) is essential in exploring the carbon cycle response to human induced radiative perturbations and predicting future global CO2 concentrations.

In this study, we attempted to reproduce the observed air-sea CO2 flux fluctuations in the equatorial Pacific using two ESMs, NEW used for 6th phase of Coupled Model Intercomparison Project (CMIP6) and OLD for CMIP5, to which observed ocean temperature and salinity data have been assimilated.

Our results show that OLD with data assimilation system failed to capture the observed relationship between the air-sea CO2 flux and the sea surface temperature (SST) in the equatorial Pacific, but NEW with data assimilation system well reproduced the observations.

In OLD without data assimilation system, the inherent ENSO variability is weak due to a diffuse thermocline.

When observations are assimilated to OLD, the above bias lead to non-negligible correction terms to ocean temperature, causing an anomalously false equatorial upwelling during El Ninos and bringing dissolved inorganic carbon rich water from the subsurface layer to the surface layer.

This resulted in, unlike observations, an unusual upward air-sea CO2 flux anomaly that should not occur during El Ninos.

Such unrealistic upwelling anomalies were not seen in NEW that shows improved ENSO and the mean thermocline.

Our results demonstrate that adequate simulation of ENSO in ESM is crucial for reproducing the variability in air-sea CO2 flux and hence carbon cycle.

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