Air-sea Transfer Velocity Research Articles

On the short-term response of entrained air bubbles in the upper ocean: a case study in the north Adriatic Sea

Abstract. Air bubbles in the upper ocean are generated mainly by waves breaking at the air–sea interface. As such, after the waves break, entrained air bubbles evolve in the form of plumes in the turbulent flow, exchange gas with the surrounding water, and may eventually rise to the surface. To shed light on the short-term response of entrained bubbles in different stormy conditions and to assess the link between bubble plume penetration depth, mechanical and thermal forcings, and the air–sea transfer velocity of CO2, a field experiment was conducted from an oceanographic research tower in the north Adriatic Sea. Air bubble plumes were observed using high-resolution echosounder data from an upward-looking 1000 kHz sonar. The backscatter signal strength was sampled at a high resolution, 0.5 s in time and 2.5 cm along the vertical direction. Time series profiles of the bubble plume depth were established using a variable threshold procedure applied to the backscatter strength. The data show the occurrence of bubbles organized into vertical plume-like structures, drawn downwards by wave-generated turbulence and other near-surface circulations, and reaching the seabed at 17 m depth under strong forcing. We verify that bubble plumes adapt rapidly to wind and wave conditions and have depths that scale approximately linearly with wind speed. Scaling with the wind–wave Reynolds number is also proposed to account for the sea-state severity in the plume depth prediction. Results show a correlation between measured bubble depths and theoretical air–sea CO2 transfer velocity parametrized with wind-only and wind/wave formulations. Further, our measurements corroborate previous results suggesting that the sinking of newly formed cold-water masses helps bring bubbles to greater depths than those reached in stable conditions for the water column. The temperature difference between air and sea seems sufficient for describing this intensification at the leading order of magnitude. The results presented in this study are relevant for air–sea interaction studies and pave the way for progress in CO2 gas exchange formulations.

A quantitative evaluation of the factors influencing the air-sea carbon dioxide transfer velocity

The numerous factors influencing the air-sea carbon dioxide (CO2) transfer velocity have been discussed for many years, yet the contributions of various factors have undergone little quantitative estimation. To better understand the mechanism of air-sea transfer, the effects of different factors are discussed on the air-sea transfer velocity and the various parametric models describing the phenomenon are classified and compared. Then, based on GAS EX-98 and ASGAMAGE data, wind models are evaluated and the effects of some factors are discussed quantitatively, including bubbles, waves, wind and so on by considering their interaction through a piecewise average approach. It is found that the air-sea CO2 transfer velocity is not only the function of the wind speed, but is also affected by bubbles, wave parameters and other factors. Stepwise and linear regressions are used. When considering the wind speed, bubbles mediated and the significant wave height, the root mean square error is reduced from 34.53 cm/h to 16.96 cm/h. Discussing the various factors quantitatively can be useful in future assessments of a large spatial scale and long-term air-sea CO2 flux and global change.

Air-sea transfer of gas phase controlled compounds

Gases in the atmosphere/ocean have solubility that spans several orders of magnitude. Resistance in the molecular sublayer on the waterside limits the air-sea exchange of sparingly soluble gases such as SF6 and CO2. In contrast, both aerodynamic and molecular diffusive resistances on the airside limit the exchange of highly soluble gases (as well as heat). Here we present direct measurements of air-sea methanol and acetone transfer from two open cruises: the Atlantic Meridional Transect in 2012 and the High Wind Gas Exchange Study in 2013. The transfer of the highly soluble methanol is essentially completely airside controlled, while the less soluble acetone is subject to both airside and waterside resistances. Both compounds were measured concurrently using a proton-transfer-reaction mass spectrometer, with their fluxes quantified by the eddy covariance method. Up to a wind speed of 15 m s-1, observed air-sea transfer velocities of these two gases are largely consistent with the expected near linear wind speed dependence. Measured acetone transfer velocity is ∼30% lower than that of methanol, which is primarily due to the lower solubility of acetone. From this difference we estimate the “zero bubble” waterside transfer velocity, which agrees fairly well with interfacial gas transfer velocities predicted by the COARE model. At wind speeds above 15 m s-1, the transfer velocities of both compounds are lower than expected in the mean. Air-sea transfer of sensible heat (also airside controlled) also appears to be reduced at wind speeds over 20 m s-1. During these conditions, large waves and abundant whitecaps generate large amounts of sea spray, which is predicted to alter heat transfer and could also affect the air-sea exchange of soluble trace gases. We make an order of magnitude estimate for the impacts of sea spray on air-sea methanol transfer.

A mixing length model for the aqueous boundary layer including the effect of wave breaking on enhancing gas transfer

A mixing length model for air-water gas transfer is developed to include the effects of wave breaking. The model requires both the shear velocity induced by the wind and the integrated wave dissipation. Both of these can be calculated for tanks and oceans by a full spectrum wave model. The gas transfer model is calibrated, with laboratory tank measurements of carbon dioxide flux, and transported to oceanic conditions to yield air-sea transfer velocity versus wind speed.

Modelling marine emissions and atmospheric distributions of halocarbons and dimethyl sulfide: the influence of prescribed water concentration vs. prescribed emissions

Abstract. Marine-produced short-lived trace gases such as dibromomethane (CH2Br2), bromoform (CHBr3), methyliodide (CH3I) and dimethyl sulfide (DMS) significantly impact tropospheric and stratospheric chemistry. Describing their marine emissions in atmospheric chemistry models as accurately as possible is necessary to quantify their impact on ozone depletion and Earth's radiative budget. So far, marine emissions of trace gases have mainly been prescribed from emission climatologies, thus lacking the interaction between the actual state of the atmosphere and the ocean. Here we present simulations with the chemistry climate model EMAC (ECHAM5/MESSy Atmospheric Chemistry) with online calculation of emissions based on surface water concentrations, in contrast to directly prescribed emissions. Considering the actual state of the model atmosphere results in a concentration gradient consistent with model real-time conditions at the ocean surface and in the atmosphere, which determine the direction and magnitude of the computed flux. This method has a number of conceptual and practical benefits, as the modelled emission can respond consistently to changes in sea surface temperature, surface wind speed, sea ice cover and especially atmospheric mixing ratio. This online calculation could enhance, dampen or even invert the fluxes (i.e. deposition instead of emissions) of very short-lived substances (VSLS). We show that differences between prescribing emissions and prescribing concentrations (−28 % for CH2Br2 to +11 % for CHBr3) result mainly from consideration of the actual, time-varying state of the atmosphere. The absolute magnitude of the differences depends mainly on the surface ocean saturation of each particular gas. Comparison to observations from aircraft, ships and ground stations reveals that computing the air–sea flux interactively leads in most of the cases to more accurate atmospheric mixing ratios in the model compared to the computation from prescribed emissions. Calculating emissions online also enables effective testing of different air–sea transfer velocity (k) parameterizations, which was performed here for eight different parameterizations. The testing of these different k values is of special interest for DMS, as recently published parameterizations derived by direct flux measurements using eddy covariance measurements suggest decreasing k values at high wind speeds or a linear relationship with wind speed. Implementing these parameterizations reduces discrepancies in modelled DMS atmospheric mixing ratios and observations by a factor of 1.5 compared to parameterizations with a quadratic or cubic relationship to wind speed.

Air–Sea $$\mathrm{CO}_{2}$$ CO 2 Gas Transfer Velocity in a Shallow Estuary

The air–sea transfer velocity of \(\mathrm{CO}_{2}\, (k_{\mathrm{CO}_{2}})\) was investigated in a shallow estuary in March to July 2012, using eddy-covariance measurements of \(\mathrm{CO}_{2}\) fluxes and measured air–sea \(\mathrm{CO}_{2}\) partial-pressure differences. A data evaluation method that eliminates data by nine rejection criteria in order to heighten parametrization certainty is proposed. We tested the data evaluation method by comparing two datasets: one derived using quality criteria related solely to the eddy-covariance method, and the other derived using quality criteria based on both eddy-covariance and cospectral peak methods. The best parametrization of transfer velocity normalized to a Schmidt number of 600 \((k_{600})\) was determined to be: \(k_{600} = 0.3\,{U_{10}}^{2.5}\) where \(U_{10}\) is the wind speed in m \(\mathrm{s}^{-1}\) at 10 m; \(k_{600}\) is based on \(\mathrm{CO}_{2}\) fluxes calculated by the eddy-covariance method and including the cospectral peak method criteria. At low wind speeds, the transfer velocity in the shallow water estuary was lower than in other coastal waters, possibly a symptom of low tidal amplitude leading to low intensity water turbulence. High transfer velocities were recorded above wind speeds of 5 m \(\mathrm{s}^{-1}\), believed to be caused by early-breaking waves and the large fetch (6.5 km) of the estuary. These findings indicate that turbulence in both air and water influences the transfer velocity.

Global air-sea surface carbon dioxide transfer velocity and flux estimated using 17 a altimeter data and a new algorithm

The global distributions of the air-seaCO(2) transfer velocity and flux are retrieved from TOPEX/Poseidon and Jason altimeter data from October 1992 to December 2009 using a combined algorithm. The 17 a average global, area-weighted, Schmidt number-corrected mean gas transfer velocity is 21.26 cm/h, and the full exploration of the uncertainty of this estimate awaits further data. The average total CO2 flux (calculated by carbon) from atmosphere to ocean during the 17 a was 2.58 Pg/a. The highest transfer velocity is in the circumpolar current area, because of constant high wind speeds and currents there. This results in strongCO(2) fluxes. CO2 fluxes are strong but opposite direction in the equatorial east Pacific Ocean, because the air-sea CO2 partial pressure difference is the largest in the global oceans. The results differ from the previous studies calculated using the wind speed. It is demonstrated that the air-sea transfer velocity is very important for estimating air-sea CO2 flux. It is critical to have an accurate estimation for improving calculation of CO2 flux within climate change studies.

The effects of experimental uncertainty in parameterizing air-sea gas exchange using tracer experiment data

Abstract. It is not practical to measure air-sea gas fluxes in the open ocean for all conditions and areas of interest. Therefore, in many cases fluxes are estimated from measurements of air-phase and water-phase gas concentrations, a measured environmental forcing function such as wind speed, and a parameterization of the air-sea transfer velocity in terms of the environmental forcing function. One problem with this approach is that when direct measurements of the transfer velocity are plotted versus the most commonly used forcing function, wind speed, there is considerable scatter, leading to a relatively large uncertainty in the flux. Because it is known that multiple processes can affect gas transfer, it is commonly assumed that this scatter is caused by single-forcing function parameterizations being incomplete in a physical sense. However, scatter in the experimental data can also result from experimental uncertainty (i.e., measurement error). Here, results from field and laboratory results are used to estimate how experimental uncertainty contributes to the observed scatter in the measured fluxes and transfer velocities as a function of environmental forcing. The results show that experimental uncertainty could explain half of the observed scatter in field and laboratory measurements of air-sea gas transfer velocity.

Evidence from the Baltic Sea for an enhanced CO2 air-sea transfer velocity

Evidence from the Baltic Sea for an enhanced CO2 air-sea transfer velocity

Evidence from the Baltic Sea for an enhanced CO2 air—sea transfer velocity

Surface water total CO2 concentrations (CT) and the CO2 partial pressure of the surface water and in the atmosphere were measured in the eastern Gotland Sea at approximately monthly intervals during five cruises in the winter of 1999/2000. Taking into account vertical/lateral exchange processes and the decomposition of organic matter, the monthly changes in CT were used to determine CO2evasion fluxes. In addition, the CO2 fluxes were calculated on the basis of the CO2 partial pressure differences using local wind speed (u) records and different currently applied parametrizations of the gas exchange transfer velocity (k). The latter resulted in substantially lower monthly fluxes that indicated a considerable underestimation of k from the k(u) functions used. To achieve an optimal agreement between the flux calculations and the balance-derived CO2 fluxes, the coefficients of both a simple quadratic and cubic function k(u) were iterated using a least-squares fitting procedure. The resulting equations, which refer to short-term wind data and to the CO2 exchange at 20 ¼C, were k=(0.45 ± 0.10)u2 and k=(0.037 ± 0.008)u3 (k, cm h−1; u, m s−1). These yielded higher k values than most of the previously proposed parametrizations. Unfortunately, our data did not allow us to decide whether the quadratic or cubic function is more appropriate to describe the gas exchange dynamics.

Possible errors in CO2air-sea transfer velocity from deliberate tracer releases and eddy covariance measurements due to near-surface concentration gradients

[1] The air-sea transfer velocity of CO2, kCO2, from a deliberate tracer experiment (DTE) and from eddy covariance measurements (ECM) during ASGAMAGE differed on average by a factor of ∼2.5, in spite of the fact that both methods were applied with maximum care. In an attempt to explain this difference, implicit assumptions were investigated critically using complementary one-dimensional (1-D) and 2-D models for the upper oceanic boundary layer and atmospheric surface layer. Simulations of a DTE and ECM facilitate examination of the possible impact of near-surface vertical gradients, horizontal inhomogeneity, and instationarity of the tracer concentration on the results from field experiments. The simulations suggest that vertical tracer concentration gradients due to physical processes might occur outside the diffusive boundary and that these might have escaped prior observation due to their confinement to the upper meter of the water column. Such gradients could then cause DTE to typically underestimate kCO2 by 10–25%, depending on ambient conditions, such as light penetration. Buffering of the seawater CO2 system renders ECM much less sensitive to near-surface gradients. However, inhomogeneity of the CO2 concentration in water might cause scatter in ECM due to inhomogeneity-related flux divergence in air. Additionally, in the case of instationarity a complex and strongly varying fine structure of vertical CO2 concentration profiles in water could cause inconsistencies with CO2 flux observations in air. Both mechanisms might explain several tens of percents of the scatter in the ECM, while systematic effects are ∼10%.

SULFUR: THE PLANKTON/CLIMATE CONNECTION

Question (Aiken et al.): Given the uncertainties involved in the calculation of air-sea exchange of gases such as DMS, is it possible to assess by other methods the importance of natural versus anthropogenic sulfur emissions? Answer: In our paper we discuss various approaches that have been taken to estimate the rate of emission of DMS from the oceans including models (Erikson et al. 1990, Thompson et al. 1990) and the use of observed concentration fields of DMS combined with knowledge of air-sea transfer velocity (Andreae 1986, Bates et al. 1987b). In addition measurements of MSA in atmospheric aerosols can be used to infer the emission of DMS into the atmosphere (J. Prospero, University of Miami, pers. commun.). Many studies (not reviewed in this paper) have investigated the emissions of sulfur from manmade sources. In all these attempts, an assessment is often made of the relative importance of the two sources, although the methodology used to calculate biogenic and anthropogenic sources are fundamentally different. The only consistent approach we are aware of, which has the potential to directly ascribe sulfur in the atmosphere to its major sources, is through the use of sulfur isotope signature measurements. The method relies on the sulfur isotope signature of fossil fuels being significantly different from that of DMS and its oxidation products. This approach shows great promise and is currently being investigated in our laboratory.

Simulation of the global air‐sea transfer velocity field of helium

Air‐sea exchange is important in constructing a budget of He in the upper oceanic water column. This exchange of He is controlled by the transfer velocity (KwHe) and the air‐sea concentration gradient. Here, the global transfer velocity field of He is computed for January and July with user‐defined derived fields based on control runs of the NCAR (National Center for Atmospheric Research) CCM1 (Community Climate Model), a 12 layer atmospheric general circulation model. The spatial resolution of these calculations is 4.5° × 7.5° latitude‐longitude. The range of KwHe variation over the globe is over an order of magnitude, varying from 5 to 75 cm hr−1. Equatorial values are generally 10‐20 cm hr−1 and high latitude regions experience values in excess of 60 cm hr−1. The annual variability of the transfer velocity in the high‐latitude northern hemisphere is larger than in the high‐latitude southern hemisphere. This work shows that the spatial and temporal variation in the global transfer velocity field of He is significant. This result should be included in future high‐resolution 3‐dimensional coupled ocean‐atmosphere tracer models when attempting to quantitatively deconvolute vertical transport and air‐sea exchange in surface ocean He budgets. This approach may allow high‐resolution global estimates of nitrate flux into the euphotic zone, an important component in oceanic biogeochemical modeling efforts.

Tracers of air-sea gas exchange

The flux of gas across the air-sea interface is determined by the product of the interfacial concentration difference driving the exchange and a rate constant, often termed the transfer velocity. The concentration-difference term is generally obtained by direct measurement, whereas more indirect approaches are required to estimate the transfer velocity and its variation as a function of controlling parameters such as wind and sea state. Radioactive tracers have proved particularly useful in the estimation of air-sea transfer velocities and, recently, stable purposeful tracers have also started to be used. In this paper the use of the following tracers to determine transfer velocities at the sea surface is discussed: natural and bomb-produced 14 C, dissolved oxygen, 222 Rn and sulphur hexafluoride. Other topics covered include the relation between transfer velocity and wind speed as deduced from tracer and wind-tunnel studies, and the discrepancy between transfer velocities determined by using tracers and from eddy correlation measurements in the atmosphere.