Abstract
AbstractAir‐sea carbon dioxide (CO2) flux is generally estimated by the bulk method using upper ocean CO2 fugacity measurements. In the summertime Arctic, sea‐ice melt results in stratification within the upper ocean (top ∼10 m), which can bias bulk CO2 flux estimates when the seawater CO2 fugacity is taken from a ship's seawater inlet at ∼6 m depth (fCO2w_bulk). Direct flux measurements by eddy covariance are unaffected by near‐surface stratification. We use eddy covariance CO2 flux measurements to infer sea surface CO2 fugacity (fCO2w_surface) in the Arctic Ocean. In sea‐ice melt regions, fCO2w_surface values are consistently lower than fCO2w_bulk by an average of 39 μatm. Lower fCO2w_surface can be partially accounted for by fresher (≥27%) and colder (17%) melt waters. A back‐of‐the‐envelope calculation shows that neglecting the summertime sea‐ice melt could lead to a 6%–17% underestimate of the annual Arctic Ocean CO2 uptake.
Highlights
The Arctic Ocean is a strong sink of atmospheric CO2 due to the active biological production and high CO2 solubility in cold waters (Anderson et al, 1998; Takahashi et al, 2009)
The results show that the summertime near-surface stratification due to seaice melt could lead to an ∼10% underestimation of the annual Arctic Ocean CO2 uptake
Stratified areas were located at the edge of or within the sea ice (Figure S1 in Supporting Information S1), with relatively low near-surface salinity and temperature (Figure 1) suggesting that sea-ice melt is the principal reason for near-surface stratification
Summary
The Arctic Ocean is a strong sink of atmospheric CO2 due to the active biological production and high CO2 solubility in cold waters (Anderson et al, 1998; Takahashi et al, 2009). While only accounting for 4% of the world ocean by area and seasonally covered by sea ice, the Arctic Ocean contributes 5%–14% (66–199 Tg C yr−1, Bates & Mathis, 2009; Yasunaka et al, 2018) of mean global atmospheric CO2 removal every year (∼1,400 Tg C yr−1; Landschützer et al, 2014; Takahashi et al, 2009). This Arctic carbon sink is rapidly changing due to climate change. Sea-ice melt causes near-surface stratification and suppresses water mixing between the surface and sub-surface, which likely generates upper-ocean gradients in temperature, salinity, dissolved inorganic carbon (DIC), total alkalinity (TA) and seawater CO2
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