Abstract
Abstract. The oceanic sink of carbon dioxide (CO2) is an important part of the global carbon budget. Understanding uncertainties in the calculation of this net flux into the ocean is crucial for climate research. One of the sources of the uncertainty within this calculation is the parameterization chosen for the CO2 gas-transfer velocity. We used a recently developed software toolbox, called the FluxEngine (Shutler et al., 2016), to estimate the monthly air–sea CO2 fluxes for the extratropical North Atlantic Ocean, including the European Arctic, and for the global ocean using several published quadratic and cubic wind speed parameterizations of the gas-transfer velocity. The aim of the study is to constrain the uncertainty caused by the choice of parameterization in the North Atlantic Ocean. This region is a large oceanic sink of CO2, and it is also a region characterized by strong winds, especially in winter but with good in situ data coverage. We show that the uncertainty in the parameterization is smaller in the North Atlantic Ocean and the Arctic than in the global ocean. It is as little as 5 % in the North Atlantic and 4 % in the European Arctic, in comparison to 9 % for the global ocean when restricted to parameterizations with quadratic wind dependence. This uncertainty becomes 46, 44, and 65 %, respectively, when all parameterizations are considered. We suggest that this smaller uncertainty (5 and 4 %) is caused by a combination of higher than global average wind speeds in the North Atlantic (> 7 ms−1) and lack of any seasonal changes in the direction of the flux direction within most of the region. We also compare the impact of using two different in situ pCO2 data sets (Takahashi et al. (2009) and Surface Ocean CO2 Atlas (SOCAT) v1.5 and v2.0, for the flux calculation. The annual fluxes using the two data sets differ by 8 % in the North Atlantic and 19 % in the European Arctic. The seasonal fluxes in the Arctic computed from the two data sets disagree with each other possibly due to insufficient spatial and temporal data coverage, especially in winter.
Highlights
The region of extratropical North Atlantic Ocean, including the European Arctic, is a region responsible for the formation of deep ocean waters
Using the FluxEngine software, we have produced global gridded monthly net CO2 air–sea fluxes and from these we have extracted the values for the two study regions, the extratropical North Atlantic Ocean and separately for its subset – the European Arctic seas
In this paper we have studied the effect of the choice of gas-transfer velocity parameterization on the net CO2 air–sea gas fluxes in the North Atlantic and the European Arctic using the recently developed FluxEngine software
Summary
The region of extratropical North Atlantic Ocean, including the European Arctic, is a region responsible for the formation of deep ocean waters (see Talley, 2013, for a recent review) This process, part of the global overturning circulation, makes the area a large sink of atmospheric carbon dioxide (CO2) (Takahashi et al, 2002, 2009; Landschützer et al, 2014; Le Quéré et al, 2015; Orr et al, 2001). The trend and variations in the North Atlantic CO2 sinks has been intensively studied since observations have shown it appeared to be decreasing (Lefèvre et al, 2004) This decrease on interannual timescales has been confirmed by further studies (Schuster and Watson, 2007) and this trend has continued in recent years north of 40◦ N (Landschützer et al, 2013). Recent assessments of the Atlantic and the Arctic net sea-air CO2 fluxes (Schuster et al, 2013) and the global ocean net carbon uptake (Wanninkhof et al, 2013) show that the cause is still unknown
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