Séminaire : Michio Watanabe (JAMSTEC, Japan): Importance of faithful reproduction of El Nino for ocean data assimilation\nsystem in terms of historical CO2 flux fluctuations in the equatorial Pacific
Thursday 07 November 2019, 11:00am - 12:00pm
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Abstract : 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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Location Salle du LMD, 3ème étage, couloir 45-55, pièce 313



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