Direct covariance air‐sea CO<sub>2</sub> fluxes

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<jats:p>Direct covariance air‐sea CO<jats:sub>2</jats:sub> flux measurements over the open ocean are reported. These measurements were performed during June 1998 in the North Atlantic within a significant CO<jats:sub>2</jats:sub> sink. These direct estimates are in general agreement with the traditional geochemical isotope constraints. The covariance, or eddy correlation, technique directly measures the air‐sea CO<jats:sub>2</jats:sub> flux over hour timescales by correlating the fluctuations of CO<jats:sub>2</jats:sub> with the turbulent vertical velocity fluctuations in the atmospheric surface layer. These measurements quantify the transfer of CO<jats:sub>2</jats:sub> between the atmosphere and ocean over a range of wind speeds and improve the understanding of the environmental factors controlling the flux. The relatively large flux of CO<jats:sub>2</jats:sub> in the study region, together with improved analytical techniques, facilitated the measurements. The half‐hour mean wind speeds varied from 0.9 to 16.3 m s<jats:sup>−1</jats:sup> over the month‐long experiment. The mean <jats:italic>p</jats:italic>CO<jats:sub>2</jats:sub> during the study period was −85.8±16.0 μatm, and the mean covariance CO<jats:sub>2</jats:sub> flux was estimated at 4.6 mol m<jats:sup>−2</jats:sup> yr<jats:sup>−1</jats:sup>. The average observed wind speed was 7.7 m s<jats:sup>−1</jats:sup>. This is in close agreement with 3.9 mol m<jats:sup>−2</jats:sup> yr<jats:sup>−1</jats:sup>, the approximate CO<jats:sub>2</jats:sub> flux based on <jats:sup>14</jats:sup>C parameterizations at this wind speed. At high winds, where the relationship between gas physical properties, surface processes, and air‐sea gas exchange is still elusive, direct CO<jats:sub>2</jats:sub> flux measurements are crucial. The measurements for winds in excess of 11 m s<jats:sup>−1</jats:sup> show a general enhancement of gas transfer velocity over previous indirect measurements, and it is believed that this enhancement can be explained by the fact that the indirect methods cannot discriminate surface process variability such as atmospheric stability, upper ocean mixing, wave age, wave breaking, or surface films.</jats:p>

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