Estimation of bubbled-mediated air/sea gas exchange from concurrent DMS and CO<sub>2</sub> transfer velocities at intermediate-high wind speeds

Abstract

<p><strong>Abstract.</strong> Simultaneous air/sea fluxes and concentration differences of dimethylsulfide (DMS) and carbon dioxide (CO<sub>2</sub>) were measured during a summertime North Atlantic cruise in 2011. This dataset reveals significant differences between the gas transfer velocities of these two gases (Δ<i>k<sub>w</sub></i>) over a range of wind speeds up to 21 m  s<sup>−1</sup>. These differences occur at and above the approximate wind speed threshold when waves begin breaking. Whitecap fraction (a proxy for bubbles) was also measured and has a positive relationship with Δ<i>k<sub>w</sub></i>, consistent with enhanced bubble-mediated transfer of the less soluble CO<sub>2</sub> relative to that of the more soluble DMS. However, the correlation of Δ<i>k<sub>w</sub></i> with whitecap fraction is no stronger than with wind speed. Models used to estimate bubble-mediated transfer from in situ whitecap fraction under-predict the observations, particularly at intermediate wind speeds. Examining the differences between gas transfer velocities of gases with different solubilities is a useful way to detect the impact of bubble-mediated exchange. More simultaneous gas transfer measurements of different solubility gases across a wide range of oceanic conditions are needed to understand the factors controlling the magnitude and scaling of bubble-mediated gas exchange.</p>

Wave‐Related Reynolds Number Parameterizations of CO2 and DMS Transfer Velocities

“…Eddy covariance measurements of air‐sea CO 2 and DMS fluxes were also made during the Knorr11 cruise, which took place in the North Atlantic Ocean, during wind speeds ranging from ~2 to 20 m s −1 (Bell et al, , ). Omnidirectional surface wave spectra were obtained using an ultrasonic altimeter (Christensen et al, ).…”

Gas Transfer by Breaking Waves

“…State‐of‐the‐art ocean‐atmosphere parameterizations for the transfer velocity, used in ocean and climate models (Fairall et al, , ), include a Schmidt number scaling (typically, , S c = ν / D , where ν is the kinematic viscosity) but depend solely on wind speed to account for turbulent and wave breaking‐induced transfers. Recent open ocean gas transfer velocity measurements of CO 2 and dimethyl sulfide (DMS) display very large scatter, when analyzed as a function of wind speed (Bell et al, ; Brumer et al, ; Edson et al, ; Garbe et al, ; Miller et al, ), exposing the failure of wind speed parameterization at intermediate to high wind speeds, which is associated with bubble‐mediated gas transfer. Separating the contribution to the total flux into the bubble gas transfer and the diffusive transfer at an unbroken surface, bubble‐mediated gas transfer is introduced through the whitecap coverage (Asher et al, ; Keeling, ; Woolf, , ; Woolf & Thorpe, ), which is ultimately described by a function of the wind speed.…”

“…Figure shows examples of the variables computed in our model, typical of the range covered in our calculations, using field measurements of the wave statistics (Sutherland & Melville, ), and the synthetic data reproducing the HiWings and Knorr 2011 conditions from Bell et al () and Brumer et al (). Figure a shows the wave spectrum, ϕ ( k ), and the associated saturation spectrum B ( k ) = ϕ ( k ) k 3 .…”

Upper Ocean Response to Rain Observed From a Vertical Profiler

“…The ship went from Woods Hole MA toward the south of Greenland (Figure ). The main goal of this campaign was to acquire an observational data set of surface ocean properties and air‐sea fluxes during phytoplankton blooms through a combination of meteorological, wave, and whitecap measurements, as well as direct measurements of temperature, salinity, and turbulence in the near‐surface layer of the ocean (see also Bell et al, , ; Esters et al, ; Scanlon & Ward, ; Scanlon et al, ; Sutherland et al, ).…”

“…An eddy covariance system was installed on the bow mast of the research vessel, consisting of two CSat3 sonic anemometers and a 3‐D inertial motion unit, to correct the wind measurements for ship motion (Landwehr et al, ; Miller et al, ). The setup resulted in direct estimates of momentum, heat, and gas fluxes (Bell et al, , ). Meteorological observations were made with a Vaisala WXT520 weather system, which was also mounted on the bow mast.…”