Sean P. McKenna

Air-sea CO2 exchange in the equatorial Pacific

surface solubility. The wind speed was 6.0 ± 1.3 m s � 1 , and the atmospheric boundary layer was unstable with conditions over the range � 1 < z/L < 0. Diurnal heat fluxes generated daytime surface ocean stratification and subsequent large nighttime buoyancy fluxes. The average CO2 flux from the ocean to the atmosphere was determined to be 3.9 mol m � 2 yr � 1 , with nighttime CO2 fluxes increasing by 40% over daytime values because of a strong nighttime increase in (vertical) convective velocities. The 15 days of air-sea flux measurements taken during GasEx-2001 demonstrate some of the systematic environmental trends of the eastern equatorial Pacific Ocean. The fact that other physical processes, in addition to wind, were observed to control the rate of CO2 transfer from the ocean to the atmosphere indicates that these processes need to be taken into account in local and global biogeochemical models. These local processes can vary on regional and global scales. The GasEx-2001 results show a weak wind dependence but a strong variability in processes governed by the diurnal heating cycle. This implies that any changes in the incident radiation, including atmospheric cloud dynamics, phytoplankton biomass, and surface ocean stratification may have significant feedbacks on theamount andvariability ofair-sea gasexchange.Thisisinsharp contrastwith previous field studies of air-sea gas exchange, which showed that wind was the dominating forcing function. The results suggest that gas transfer parameterizations that rely solely on wind will be insufficient for regions with low to intermediate winds and strong insolation. INDEX TERMS: 0312 Atmospheric Composition and Structure: Air/sea constituent fluxes (3339,4504);3307MeteorologyandAtmosphericDynamics:Boundarylayerprocesses;3339Meteorologyand Atmospheric Dynamics: Ocean/atmosphere interactions (0312, 4504); 4231 Oceanography: General: Equatorial oceanography; 4227 Oceanography: General: Diurnal, seasonal, and annual cycles; KEYWORDS: air-sea carbon dioxide fluxes, equatorial Pacific, direct covariance technique, profile flux technique, diurnal surface layer

Air‐sea gas transfer: Its dependence on wind stress, small‐scale roughness, and surface films

[1]xa0The influence of wind stress, small-scale waves, and surface films on air-sea gas exchange at low to moderate wind speeds (<10 m s−1) is examined. Coincident observations of wind stress, heat transfer velocity, surface wave slope, and surface film enrichments were made in coastal and offshore waters south of Cape Cod, New England, in July 1997 as part of the NSF-CoOP Coastal Air-Sea Chemical Fluxes study. Gas transfer velocities have been extrapolated from aqueous heat transfer velocities derived from infrared imagery and direct covariance and bulk heat flux estimates. Gas transfer velocity is found to follow a quadratic relationship with wind speed, which accounts for ∼75–77% of the variance but which overpredicts transfer velocity in the presence of surface films. The dependence on wind stress as represented by the friction velocity is also nonlinear, reflecting a wave field-dependent transition between limiting transport regimes. In contrast, the dependence on mean square slope computed for the wave number range of 40–800 rad m−1 is found to be linear and in agreement with results from previous laboratory wind wave studies. The slope spectrum of the small-scale waves and the gas transfer velocity are attenuated in the presence of surface films. Observations over large-scale gradients of biological productivity and dissolved organic matter show that the reduction in slope and transfer velocity are more clearly correlated with surface film enrichments than with bulk organic matter concentrations. The mean square slope parameterization explains ∼89–95% of the observed variance in the data and does not overpredict transfer velocities where films are present. While the specific relationships between gas transfer velocity and wind speed or mean square slope vary slightly with the choice of Schmidt number exponent used to scale the heat transfer velocities to gas transfer velocities, the correlation of heat or gas transfer velocity with mean square slope is consistently better than with wind speed.

Observations of flow repeatability and secondary circulation in an oscillating grid-stirred tank

An oscillating grid-stirred tank was studied for flow repeatability and the existence of secondary circulations. For the particular tank studied, results indicate that mean flow values may vary by up to 25% and turbulent fluctuations may vary by up to 15% from one run to another. This result was observed to exhibit a potential grid geometry dependence. More importantly, there is evidence of significant flow field sensitivity to initial conditions. Particle image velocimetry results were used to reveal secondary mean flows in the grid-stirred tank. Because these characteristics are believed to be intrinsic to grid-stirred tanks, studies using such tanks must recognize and consider these effects.

Physicochemical effects of the marine microlayer on air-sea gas transport

Much progress has been made in understanding the role of surface films in air-sea gas transfer. Advances in both laboratory and field techniques have provided improved measurements and needed insight into this complex and important process. Current efforts focused toward better understanding the key physical mechanisms involved in interfacial gas transfer and the role of surfactants, along with innovative and interdisciplinary field and laboratory measurements, will help to further our present knowledge.

The influence of surface films on near-surface vortical flows

Abstract An experimental apparatus was designed and constructed to generate reproducible vortex rings in the laboratory. These rings were produced in a small test tank and were allowed to propagate upward where they then interacted with an air/water interface. Tests were performed on the following surfaces: a clean free distilled water surface, a solid polycarbonate wall, and the surface of a 1 μm solution of Triton X-100 in distilled water. The vortex rings studied in the present investigation ranged in Froude number from 0.042 to 0.164 and in Reynolds number from 1772 to 6785. Measurements of the two-dimensional near-surface flow field were made using the technique of particle image velocimetry (PIV). Preliminary results show, (i) that the experimental set-up employed in this study is able to generate repeatable vortex rings with varying strengths and propagation speeds, (ii) that the PIV system used to quantify the experimental observations is capable of resolving spatial velocity fields of the flow accurately, and (iii) that the presence of the surface active film studied here profoundly affects the dynamics of the interaction between the vortex rings and the surface of the solution. These effects were observed to manifest themselves in several ways which included vortex path modification, production of secondary and tertiary vortices, and a measured impact on total circulation and vorticity.