Abstract
Eddy-covariance measurements made in the marine atmospheric boundary layer above a high Arctic fjord (Adventfjorden, Svalbard) are analyzed. When conditions are unstable, but close to neutral −0.1 < z/L < 0, where z is the height, and L is the Obukhov length, the exchange coefficient for sensible heat CH is significantly enhanced compared with that expected from classical surface-layer theory. Cospectra of the vertical velocity component (w) and temperature (T) reveal that a high-frequency peak develops at f ≈ 1 Hz for z/L > − 0.15. A quadrant analysis reveals that the contribution from downdrafts to the vertical heat flux increases as conditions become close to neutral. These findings are the signature of the evolving unstable very-close-to-neutral (UVCN) regime previously shown to enhance the magnitude of sensible and latent heat fluxes in the marine surface layer over the Baltic Sea. Our data reveal the significance of the UVCN regime for the vertical flux of the carbon dioxide (CO2) concentration (C). The cospectrum of w and C clearly shows how the high-frequency peak grows in magnitude for z/L > − 0.15, while the high-frequency peak dominates for z/L > − 0.02. As found for the heat flux, the quadrant analysis of the CO2 flux shows a connection between the additional small-scale turbulence and downdrafts from above. In contrast to the vertical fluxes of sensible and latent heat, which are primarily enhanced by the very different properties of the air from aloft (colder and drier) during UVCN conditions, the increase in the air–sea transfer of CO2 is possibly a result of the additional small-scale turbulence causing an increase in the water-side turbulence. The data indicate an increase in the gas-transfer velocity for CO2 for z/L > − 0.15 but with a large scatter. During the nearly 2 months of continuous measurements (March–April 2013), as much as 36% of all data are associated with the stability range −0.15 < z/L < 0, suggesting that the UVCN regime is of significance in the wintertime Arctic for the air–sea transfer of heat and possibly also CO2.
Highlights
Oceans have taken up approximately 25% of the anthropogenically emitted carbon dioxide (CO2) into the atmosphere, causing ocean acidification (Le Quéré et al 2015), while limiting the climate impact
We investigate the turbulent characteristics of the vertical fluxes of both heat and CO2 during unstable, but near-neutral conditions
We present data indicating a possible impact of the additional turbulence related to the unstable very-close-to-neutral (UVCN) regime on the air–sea gas-transfer velocity for CO2
Summary
Oceans have taken up approximately 25% of the anthropogenically emitted carbon dioxide (CO2) into the atmosphere, causing ocean acidification (Le Quéré et al 2015), while limiting the climate impact. Studies of air–sea gas exchange in the high Arctic environment are, few and a detailed description of the processes controlling the transfer efficiency in these areas is lacking. In the presence of turbulence, the molecular diffusion layer separating air and water deforms, which in turn enhances the air–sea gas-transfer velocity, an increase in turbulence results in higher values of kc and, a larger air–sea CO2 flux. Former studies over land and the open sea during unstable very-close-to-neutral (UVCN) conditions in combination with relatively high wind speeds have shown an enhancement of the vertical fluxes of temperature (T) and humidity (Smedman et al 2007a, b; Sahlée et al 2008a, b). We present data indicating a possible impact of the additional turbulence (found for unstable, but near-neutral conditions) related to the UVCN regime on the air–sea gas-transfer velocity for CO2. Enhanced Air–Sea Exchange of Heat and Carbon Dioxide Over a
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