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Wave breaking induced bubbles contribute a significant part of air‐sea gas fluxes. Recent modeling of the sea state dependent CO2 flux found that bubbles contribute up to ∼40% of the total CO2 air‐sea fluxes (Reichl & Deike, 2020, https://doi.org/10.1029/2020gl087267). In this study, we implement the sea state dependent bubble gas transfer formulation of Deike and Melville (2018, https://doi.org/10.1029/2018gl078758) into a spectral wave model (WAVEWATCH III) incorporating the spectral modeling of the wave breaking distribution from Romero (2019, https://doi.org/10.1029/2019gl083408). We evaluate the accuracy of the sea state dependent gas transfer parameterization against available measurements of CO2 gas transfer velocity from 9 data sets (11 research cruises, see Yang et al. (2022, https://doi.org/10.3389/fmars.2022.826421)). The sea state dependent parameterization for CO2 gas transfer velocity is consistent with observations, while the traditional wind‐only parameterization used in most global models slightly underestimates the observations of gas transfer velocity. We produce a climatology of the sea state dependent gas transfer velocity using reanalysis wind and wave data spanning 1980–2017. The climatology shows that the enhanced gas transfer velocity occurs frequently in regions with developed sea states (with strong wave breaking and high significant wave height). The present study provides a general sea state dependent parameterization for gas transfer, which can be implemented in global coupled models. Plain Language Summary: The atmosphere‐ocean gas exchange influences Earth's climate, including local and global sinks and sources of CO2. We implement a physics based bubble mediated gas exchange model using state of the art gas transfer velocity models dependent on wind and waves, together with the various gases physicochemical parameters to better constrain the trend of the ocean‐atmosphere gas fluxes, validated against extensive field measurements of CO2 fluxes. In current climate models, the CO2 air‐sea fluxes are parameterized solely based on wind speed, and we discuss the high frequency variability induced by the waves. The climatology of gas transfer velocity for both a simple bulk formula and a complete sea state models are provided for the community and can be used in global ocean or climate models. Key Points: The sea state dependent CO2 gas transfer velocity models that are evaluated against existing data setsTwo models are examined that are consistent with the observations, with differences at low wind speed due to the representation of swellA database of both sea state dependent gas transfer velocities for the period 1980–2017 are archived for future CO2 studies [ABSTRACT FROM AUTHOR]

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The gas transfer velocity, k, modulates gas fluxes across air‐water interfaces in rivers. While the theory postulates a local scaling law between k and the turbulent kinetic energy dissipation rate ε, empirical studies usually interpret this relation at the reach‐scale. Here, we investigate how local k(ε) laws can be integrated along heterogeneous reaches exploiting a simple hydrodynamic model, which links stage and velocity to the local slope. The model is used to quantify the relative difference between the gas transfer velocity of a heterogeneous stream and that of an equivalent homogeneous system. We show that this aggregation bias depends on the exponent of the local scaling law, b, and internal slope variations. In high‐energy streams, where b>1, spatial heterogeneity of ε significantly enhances reach‐scale values of k as compared to homogeneous settings. We conclude that small‐scale hydro‐morphological traits bear a profound impact on gas evasion from inland waters. Plain Language Summary: Gas emissions from rivers and streams are modulated by the gas transfer velocity at the water‐air interface, k, which is physically related to the energy dissipated by the flow field, ε. Here, we study how a local relation between gas transfer rate and energy dissipation can be spatially averaged in presence of heterogeneous flow fields induced by changes in the local slope. Our results indicate that reach‐scale relations between k and ε in general differ from the corresponding local scaling laws. In particular, we show that high‐energy heterogeneous streams are characterized by a gas transfer velocity significantly higher than that of an equivalent homogeneous stream. These results offer a clue for the interpretation of empirical data about stream outgassing in river networks. Key Points: We analyze local and reach‐wise relations between gas transfer velocity (k) and energy dissipation rate (ε)Reach‐scale k(ε) laws depend on the exponent that modulates the local relation between k and ε and the heterogeneity of the reachReach‐scale k(ε) laws in high‐energy heterogeneous streams are affected by a positive aggregation bias [ABSTRACT FROM AUTHOR]

Streams in high‐elevation tropical ecosystems known as páramos may be significant sources of carbon dioxide (CO2) to the atmosphere by transforming terrestrial carbon to gaseous CO2. Studies of these environments are scarce, and estimates of CO2 fluxes are poorly constrained. In this study, we use two independent methods for measuring gas transfer velocity (k), a critical variable in the estimation of CO2 evasion and other biogeochemical processes. The first method, kinematic k600 (k600‐K), is derived from an empirical relationship between temperature‐adjusted k (k600) and the physical characteristics of the stream. The second method, measured k600 (k600‐M), estimates gas transfer velocity in the stream by in situ measurements of dissolved CO2 (pCO2) and CO2 evasion to the atmosphere, adjusting for temperature. Measurements were collected throughout a 5‐week period during the wet season of a peatland‐stream transition within a páramo ecosystem located above 4000 m in elevation in northeastern Ecuador. We characterized the spatial heterogeneity of the 250‐m reach on five occasions, and both methods showed a wide range of variability in k600 at small spatial scales. Values of k600‐K ranged from 7.42 to 330 m/d (mean = 116 ± 95.1 m/d), whereas values of k600‐M ranged from 23.5 to 444 m/d (mean = 121 ± 127 m/d). Temporal variability in k600 was driven by increases in stream discharge caused by rain events, whereas spatial variability was driven by channel morphology, including stream width and slope. The two methods were in good agreement (less than 16% difference) at high and medium stream discharge (above 7.0 L/s). However, the two methods considerably differed from one another (up to 73% difference) at low stream discharge (below 7.0 L/s, which represents 60% of the observations collected). Our study provides the first estimates of k600 values in a high‐elevation tropical catchment across steep environmental gradients and highlights the combined effects of hydrology and stream morphology in co‐regulating gas transfer velocities in páramo streams. [ABSTRACT FROM AUTHOR]

The air‐water exchange of trace gases such as CO2 is usually parameterized in terms of a gas transfer velocity, which can be derived from direct measurements of the air‐sea gas flux. The transfer velocity of poorly soluble gases is driven by near‐surface ocean turbulence, which may be enhanced or suppressed by the presence of sea ice. A lack of measurements means that air‐sea fluxes in polar regions, where the oceanic sink of CO2 is poorly known, are generally estimated using open‐ocean transfer velocities scaled by ice fraction. Here, we describe direct determinations of CO2 gas transfer velocity from eddy covariance flux measurements from a mast fixed to ice adjacent to a sea‐ice lead during the summer‐autumn transition in the central Arctic Ocean. Lead water CO2 uptake is determined using flux footprint analysis of water‐atmosphere and ice‐atmosphere flux measurements made under conditions (low humidity and high CO2 signal) that minimize errors due to humidity cross‐talk. The mean gas transfer velocity is found to have a quadratic dependence on wind speed: k660 = 0.179 U102, which is 30% lower than commonly used open‐ocean parameterizations. As such, current estimates of polar ocean carbon uptake likely overestimate gas exchange rates in typical summertime conditions of weak convective turbulence. Depending on the footprint model chosen, the gas transfer velocities also exhibit a dependence on the dimension of the lead, via its impact on fetch length and hence sea state. Scaling transfer velocity parameterizations for regional gas exchange estimates may therefore require incorporating lead width data. Plain Language Summary: Polar oceans absorb large amounts of carbon dioxide from the atmosphere, but there is a lot of uncertainty over exactly how much is taken up. The amount the oceans absorb depends on both the concentration of carbon dioxide in the water, and the rate of gas exchange between the ocean and the atmosphere. This rate itself depends mostly on wind speed. In sea ice‐regions, which even in winter have some areas of open water, the gas exchange rate is often estimated by using an ocean gas exchange rate, multiplied by the fraction of the sea‐ice area, that is, open water. However, there are very few measurements of the gas exchange rate in sea‐ice areas, and there is an on‐going debate about whether the sea ice itself increases or decreases the exchange rate. Here, direct measurements of the gas exchange rate were made in an area of water surrounded by sea ice in the Arctic during summer and the beginning of autumn. The measured gas exchange rate was lower than typical ocean rates, suggesting that the absorption of carbon dioxide by polar oceans has previously been overestimated. Key Points: CO2 uptake in lead waters is determined using water‐atmosphere and ice‐atmosphere flux measurements from an on‐ice mastThe wind‐speed dependent gas transfer velocity in the lead is suppressed by 30% relative to the open oceanDependent on choice of footprint model, k varies with fetch, hence polar ocean carbon uptake estimates should incorporate lead width data [ABSTRACT FROM AUTHOR]

The North Atlantic is an important basin for the global ocean's uptake of anthropogenic and natural carbon dioxide (CO2), but the mechanisms controlling this carbon flux are not fully understood. The air-sea flux of CO2, F, is the product of a gas transfer velocity, k, the air-sea CO2 concentration gradient, Δ pCO2, and the temperature- and salinity-dependent solubility coefficient, α. k is difficult to constrain, representing the dominant uncertainty in F on short (instantaneous to interannual) timescales. Previous work shows that in the North Atlantic, Δ pCO2 and k both contribute significantly to interannual F variability but that k is unimportant for multidecadal variability. On some timescale between interannual and multidecadal, gas transfer velocity variability and its associated uncertainty become negligible. Here we quantify this critical timescale for the first time. Using an ocean model, we determine the importance of k, Δ pCO2, and α on a range of timescales. On interannual and shorter timescales, both Δ pCO2 and k are important controls on F. In contrast, pentadal to multidecadal North Atlantic flux variability is driven almost entirely by Δ pCO2; k contributes less than 25%. Finally, we explore how accurately one can estimate North Atlantic F without a knowledge of nonseasonal k variability, finding it possible for interannual and longer timescales. These findings suggest that continued efforts to better constrain gas transfer velocities are necessary to quantify interannual variability in the North Atlantic carbon sink. However, uncertainty in k variability is unlikely to limit the accuracy of estimates of longer-term flux variability. [ABSTRACT FROM AUTHOR]

The significance of the water-side gas transfer velocity for air–sea CO2 gas exchange (k) and its non-linear dependence on wind speed (U) is well accepted. What remains a subject of inquiry are biases associated with the form of the non-linear relation linking k to U (hereafter labeled as f(U), where f(.) stands for an arbitrary function of U), the distributional properties of U (treated as a random variable) along with other external factors influencing k, and the time-averaging period used to determine k from U. To address the latter issue, a Taylor series expansion is applied to separate f(U) into a term derived from time-averaging wind speed (labeled as 〈 U 〉 , where 〈. 〉   indicates averaging over a monthly time scale) as currently employed in climate models and additive bias corrections that vary with the statistics of U. The method was explored for nine widely used f(U) parameterizations based on remotely-sensed 6-hourly global wind products at 10 m above the sea-surface. The bias in k of monthly estimates compared to the reference 6-hourly product was shown to be mainly associated with wind variability captured by the standard deviation σ U around 〈 U 〉 or, more preferably, a dimensionless coefficient of variation I u = σ U / 〈 U 〉 . The proposed correction outperforms previous methodologies that adjusted k when using 〈 U 〉 only. An unexpected outcome was that upon setting I u 2 = 0.15 to correct biases when using monthly wind speed averages, the new model produced superior results at the global and regional scale compared to prior correction methodologies. Finally, an equation relating I u 2 to the time-averaging interval (spanning from 6 h to a month) is presented to enable other sub-monthly averaging periods to be used. While the focus here is on CO2, the theoretical tactic employed can be applied to other slightly soluble gases. As monthly and climatological wind data are often used in climate models for gas transfer estimates, the proposed approach provides a robust scheme that can be readily implemented in current climate models. [ABSTRACT FROM AUTHOR]

Carbon dioxide (CO2) fluxes between the ocean and atmosphere (FCO2) are commonly computed from differences between their partial pressures of CO2 (ΔpCO2) and the gas transfer velocity (k). Commonly used wind-based parameterizations for k imply a zero intercept, although in situ field data below 4 m s−1 are scarce. Considering a global average wind speed over the ocean of 6.6 m s−1, a nonzero intercept might have a significant impact on global FCO2. Here, we present a database of 245 in situ measurements of k obtained with the floating chamber technique (Sniffle), 190 of which have wind speeds lower than 4 m s−1. A quadratic parameterization with wind speed and a nonzero intercept resulted in the best fit for k. We further tested FCO2 calculated with a different parameterization with a complementary pCO2 observation-based product. Furthermore, we ran a simulation in a well-tested ocean model of intermediate complexity to test the implications of different gas transfer velocity parameterizations for the natural carbon cycle. The global ocean observation-based analysis suggests that ignoring a nonzero intercept results in an ocean-sink increase of 0.73 Gt C yr−1. This corresponds to a 28% higher uptake of CO2 compared with the flux calculated from a parameterization with a nonzero intercept. The differences in FCO2 were higher in the case of low wind conditions and large ΔpCO2 between the ocean and atmosphere. Such conditions occur frequently in the Tropics. [ABSTRACT FROM AUTHOR]