Abstract
Abstract. Based on a set of climate simulations utilizing two kinds of Earth system models (ESMs) in which observed ocean hydrographic data are assimilated using exactly the same data assimilation procedure, we have clarified that the successful simulation of the observed air–sea CO2 flux variations in the equatorial Pacific is tightly linked to the reproducibility of coupled physical air–sea processes. When an ESM with a weaker ENSO (El Niño–Southern Oscillations) amplitude than that of the observations was used for historical simulations with ocean data assimilation, the observed equatorial anticorrelated relationship between the sea surface temperature (SST) and the air–sea CO2 flux on interannual to decadal timescales could not be represented. The simulated CO2 flux anomalies were upward (downward) during El Niño (La Niña) periods in the equatorial Pacific. The reason for this was that the non-negligible correction term in the governing equation of ocean temperature, which was added via the ocean data assimilation procedure, caused an anomalous, spurious equatorial upwelling (downwelling) during El Niño (La Niña) periods, which brought more (less) subsurface layer water rich in dissolved inorganic carbon (DIC) to the surface layer. On the other hand, in the historical simulations where the observational data were assimilated into the other ESM with a more realistic ENSO representation, the correction term associated with the assimilation procedure remained small enough so as not to disturb an anomalous advection–diffusion balance for the equatorial ocean temperature. Consequently, spurious vertical transport of DIC and the resultant positively correlated SST and air–sea CO2 flux variations did not occur. Thus, the reproducibility of the tropical air–sea CO2 flux variability with data assimilation can be significantly attributed to the reproducibility of ENSO in an ESM. Our results suggest that, when using data assimilation to initialize ESMs for carbon cycle predictions, the reproducibility of the internal climate variations in the model itself is of great importance.
Highlights
Since the industrial revolution, vast quantities of greenhouse gases (e.g., CO2) have been released into the atmosphere through human activities such as fossil fuel use and land use change
In the present study, comparing the results of two Earth system models (ESMs) in which observed ocean hydrographic data are assimilated, we have clarified that the representation of the processes in the equatorial climate system is important to reproduce the observed anticorrelated relationship between the sea surface temperature (SST) and CO2F in the equatorial Pacific
When the ocean temperature and salinity observations were assimilated into the other ESM with a rather realistic El Niño–Southern Oscillation (ENSO) representation, the anticorrelated relationship between the SST and CO2F was reproduced
Summary
Vast quantities of greenhouse gases (e.g., CO2) have been released into the atmosphere through human activities such as fossil fuel use and land use change. Observation-based studies have reached the consensus that significant interannual variability in the air–sea CO2 flux (hereafter CO2F) exists in some specific regions, such as the equatorial Pacific and the high latitudes of both hemispheres (e.g., Park et al, 2010; Valsala and Maksyutov, 2010; Landschützer et al, 2014; Rödenbeck et al, 2014), and the variation in CO2F associated with the El Niño–Southern Oscillation (ENSO) in the equatorial Pacific has been highlighted in many previous observationbased and simulation-based studies (Keeling and Revelle, 1985; Feely et al, 1997, 1999; Jones et al, 2001; Obata and Kitamura, 2003; McKinley et al, 2004; Patra et al, 2005).
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