Abstract
Abstract. Several recent observation-based studies suggest that ocean anthropogenic carbon uptake has slowed down due to the impact of anthropogenic forced climate change. However, it remains unclear whether detected changes over the recent time period can be attributed to anthropogenic climate change or rather to natural climate variability (internal plus naturally forced variability) alone. One large uncertainty arises from the lack of knowledge on ocean carbon flux natural variability at the decadal time scales. To gain more insights into decadal time scales, we have examined the internal variability of ocean carbon fluxes in a 1000 yr long preindustrial simulation performed with the Earth System Model IPSL-CM5A-LR. Our analysis shows that ocean carbon fluxes exhibit low-frequency oscillations that emerge from their year-to-year variability in the North Atlantic, the North Pacific, and the Southern Ocean. In our model, a 20 yr mode of variability in the North Atlantic air-sea carbon flux is driven by sea surface temperature variability and accounts for ~40% of the interannual regional variance. The North Pacific and the Southern Ocean carbon fluxes are also characterised by decadal to multi-decadal modes of variability (10 to 50 yr) that account for 20–40% of the interannual regional variance. These modes are driven by the vertical supply of dissolved inorganic carbon through the variability of Ekman-induced upwelling and deep-mixing events. Differences in drivers of regional modes of variability stem from the coupling between ocean dynamics variability and the ocean carbon distribution, which is set by large-scale secular ocean circulation.
Highlights
In recent years, several observation-based and model-based studies have suggested that ocean anthropogenic carbon uptake has slowed down due Sto othleidimEpaactrtohf anthropogenic forced climate change in various ocean regions (e.g. Corbiere et al, 2007; Le Quereet al., 2007)
Possible mechanisms driving this decline are associated to climate-induced modifications of both oceanic and atmospheric dynamics, e.g., rising sea-surface temperature (Corbiere et al, 2007), increasing ocean stratification (Schuster et al, 2009) or changes in horizontal oceanic currents owing to a shift in the North Atlantic Oscillation (Thomas et al, 2007; Schuster et al, 2009)
The core of this study is an extended preindustrial simulation of 1000 yr, in which the initial state of marine carbon cycle comes from a 3000 yr offline spinup plus 300 yr of online adjustment, while the dynamical components of IPSLCM5A-LR have been spun-up for 600 yr (Dufresne et al, 2013)
Summary
Several observation-based and model-based studies have suggested that ocean anthropogenic carbon uptake has slowed down due Sto othleidimEpaactrtohf anthropogenic forced climate change in various ocean regions (e.g. Corbiere et al, 2007; Le Quereet al., 2007). Authors demonstrated that the southward shift of westerlies associated with the positive trend in the Southern Annular Mode (SAM) could drive a weakening of the Southern Ocean carbon sink because of changes in water mass overturning It seems that such a response of both water mass overturning and ocean carbon sink may be overstated at the decadal time scale since no changes in Southern Ocean circulation have been detected from observations, while these data detect coherent warming and freshening trends in subsurface waters (Boening et al, 2008). Gruber et al, 2002; Bates, 2007; Bates et al, 2012) that may be poorly representative of the basinscale dynamics (McKinley and Follows, 2004) In this context, long model simulations (> 500 yr) are the only way to circumvent spatiotemporal sampling issues and have been demonstrated to include the minimum years of data to assess significantly variability at decadal time-scales Using output of this model simulation, we performed several statistical time-series analyses to (i) track the low-frequency modes variability, (ii) locate the oceanic regions that contribute the most to these modes and (iii) identify the main drivers of the ocean carbon fluxes variability at decadal time scales
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