Abstract
AbstractThe North Atlantic is a substantial sink for anthropogenic CO2. Understanding the mechanisms driving the sink's variability is key to assessing its current state and predicting its potential response to global climate change. Here we apply a time series decomposition technique to satellite and in situ data to examine separately the factors (both biological and nonbiological) that affect the sea‐air CO2 difference (ΔpCO2) on seasonal and interannual time scales. We demonstrate that on seasonal time scales, the subpolar North Atlantic ΔpCO2 signal is predominantly correlated with biological processes, whereas seawater temperature dominates in the subtropics. However, the same factors do not necessarily control ΔpCO2 on interannual time scales. Our results imply that the mechanisms driving seasonal variability in ΔpCO2 cannot necessarily be extrapolated to predict how ΔpCO2, and thus the North Atlantic CO2 sink, may respond to increases in anthropogenic CO2 over longer time scales.
Highlights
On multidecadal time scales, the ocean is a key route for removal of anthropogenic CO2 from the atmosphere, taking up approximately one third of emissions since preindustrial times (Khatiwala et al, 2013)
We demonstrate that on seasonal time scales, the subpolar North Atlantic ΔpCO2 signal is predominantly correlated with biological processes, whereas seawater temperature dominates in the subtropics
There is a significant degree of interannual variability in the relative importance of these effects on the annual mean ΔpCO2 (Figure S5), such as in the North Atlantic Subtropical Gyre (West), which varies from a slight dominance of temperature effects (2003) to a very strong dominance (2005)
Summary
The ocean is a key route for removal of anthropogenic CO2 from the atmosphere, taking up approximately one third of emissions since preindustrial times (Khatiwala et al, 2013). During air-sea gas exchange the CO2 concentration difference across the boundary layer determines the net direction of CO2 transfer (Woolf et al, 2016), that is, the difference between the partial pressure of CO2 (pCO2) in seawater and the overlying atmosphere (ΔpCO2). This approach ignores the impact of turbulent exchange and vertical temperature gradients near the sea surface but provides a useful broad-scale indicator of the direction of CO2 transfer.
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