Table of contents

The 7^th International Symposium on Gas Transfer at Water Surfaces was held 18-21 May 2015 in Seattle, Washington. These symposia, which have occurred approximately every five years since 1983, bring together the international scientific and engineering community investigating the mechanisms, parameterizations, and applications of gas transfer at water surfaces. The focus in part is on the physical processes that govern the gas flux, which include air entrainment (by breaking waves, flow over hydraulic structures, or direct injection), turbulence (generated by mechanical agitation, wind, wave breaking, rainfall, or currents), and the effect of natural and synthetic surfactants. Of equal importance to the treatment of the forcing mechanisms are the discussions on how to model or parameterize air-water gas transfer relevant to biogeochemical cycling on global, regional, and local scales. Furthermore, because the mechanisms controlling the gas flux also govern the air-sea flux of heat and momentum, the conference is attended by many studying a wide range of air- and water-side mixing processes that occur at or near the ocean surface. This provides opportunities for crossfertilization of ideas between the classical air-sea interaction community and those focused on biogeochemical cycling.

The first symposium was held at Cornell University, Ithaca, New York in 1983 [1]. The second symposium took place in Minneapolis in 1990 [2] and the third symposium was held at the University of Heidelberg in Heidelberg, Germany in 1995 [3]. The fourth symposium took place in Miami in 2000 [4] and the fifth symposium was held in Liège, Belgium in 2005 in conjunction with the 37^th International Liège Colloquium on Ocean Dynamics [5]. The sixth symposium was held in Kyoto, Japan in 2010 [6].

Gas transfer at air-water interfaces encompasses a wide variety of research, including fundamental fluid dynamics, biogeochemistry, and oceanography across a range of spatial and temporal scales. For example, the exchange of greenhouse gases between the atmosphere and natural bodies of water is critical to characterize and quantify global climate change. The establishment of a regular series of international symposia with a periodicity of five years was motivated by the increasing societal interest in the consequences of gas transfer at air-water interfaces, the size and productivity of the research community, and the wide geographic distribution of active researchers. The symposium has established a reputation as an important and influential venue for presenting and disseminating research progress to the community. The five-year period between gatherings ensures that significant progress since the last occurrence will be reported.

The research problems involve extremely challenging and complex issues associated with turbulent flow over a wide range of spatial and temporal scales that include the effects of wind and currents, breaking waves, rainfall, and two-phase flow effects due to the presence of bubbles generated by breaking waves and raindrops. The complexity is increased by the occurrence of both natural and anthropogenic surface films, which can modify the chemical and physical characteristics of the interface. The challenge of understanding gas transfer and the effect of gas exchange on biogeochemistry has led to the need for multidisciplinary, collaborative efforts and the development of a wide variety of innovative observational and experimental techniques. As many dedicated past attendees can attest, the symposia have fostered productive and long-lasting collaborations that have made new and pivotal contributions over the past three decades.

Professor Sergei A. Kitaigorodskii, one of the pioneers of air-sea interaction studies, passed away on 4 December 2014 in Helsinki, at the age of 80. He had a clear vision of the fundamental problems of marine boundary layer physics. His bold ideas opened new paths more than a half century ago, and they continue to stimulate active research. In particular, he made significant progress on the difficult problem of estimating gas transfer rates through gas-liquid interfaces and contributed to many of the Gas Transfer at Water Surfaces conferences.

Kitaigorodskii was born in Moscow on 13 September 1934. His father, Professor Alexander Kitaigorodskii, was a well-known physicist. Sergei Kitaigorodskii completed his undergraduate studies at Moscow State University in 1956, and he continued his postgraduate studies on the theory of turbulent mixing at the Academy of Sciences, Moscow, where he was awarded his Ph.D. from the Institute of Physics of the Atmosphere in 1960.

As a post-doctoral fellow at the Institute of Oceanology of the Academy of Sciences, he formulated the Kitaigorodskii similarity hypothesis for the wind-generated wave spectrum in 1961. He later recounted how, as a young student, he had been impressed by the simple and at the same time fundamental results of O. M. Phillips, and he had wanted to follow that path by combining it with the Russian tradition, the Kolmogorov similarity hypothesis of turbulence. The Kitaigorodskii hypothesis immediately showed its value when W. J. Pierson Jr. and L. Moskowitz were able in 1964 to collapse their empirical data into a single dimensionless curve, and to formulate the Pierson-Moskowitz spectrum for fully developed seas. At the same time Kitaigorodskii continued his studies of the atmospheric side and showed how swell could influence the marine boundary layer, a topic upon which his ideas remain important.

In 1962 Kitaigorodskii became a Senior Scientist at the institute and turned his main interest back to turbulence of the oceanic mixed layer. Here he again made fundamental contributions. These, combined with his earlier studies of waves and the air-sea boundary layer, were summarized in his thesis for the Russian degree of Doctor of Science in 1968. This work was the basis of a monograph, Physics of Air- Sea Interaction, published in 1970 and translated into English in 1973; it was used widely as a textbook. In the same year, 1973, he received the Rosenstiel gold medal, and in 1978 the Liege University Award.

Between 1968 and 1977 Kitaigorodskii was head of the Laboratory of the Physics of Ocean- Atmosphere Interaction in the Institute of Oceanology.

During the 1970s Kitaigorodskii established contacts with the Finnish geophysics community. His Finnish wife had completed her term as a press correspondent in Moscow, and she returned to Helsinki with their twin daughters. The Soviet system allowed Kitaigorodskii to visit his family every other year, and he used this opportunity to lecture and conduct research at several Finnish institutes.

On a day early last autumn, Ed Andreas went out, as was his wont, for a run from his home in Lebanon, New Hampshire. While on this run, Ed, a contributor to this volume and a collaborator and friend of many of us in the air-sea exchange community, suffered a cardiac event, from which he succumbed on 30 September 2015.

Mention of Ed Andreas will immediately bring to many readers' minds his protracted effort to incrementally improve the existing parameterizations of the sea surface spray flux (e.g., Andreas et al. 1995; Andreas 1998; Andreas 2001; Andreas et al. 2015) and his recent efforts to combine this modeling with concepts from atmospheric chemistry to assess the potential significance of spray-mediated air-sea gas flux (Andreas et al., this volume). Since 2007 Ed worked on these topics from his home, which served as his outpost of NorthWest Research Associates. But it should be noted that many of his earlier papers dealt with the Arctic marine boundary layer, and sea-air fluxes at high latitudes (e.g., Andreas et al. 1979; Makshtas et al. 1986), reflecting his almost 30-year long tenure (1978-2006) at the U.S. Army's Cold Regions Research and Engineering Laboratory of the U.S. Army Corps of Engineers, located just down the road from Lebanon, in Hanover, New Hampshire.

Having been born in Sterling, Illinois, Ed went off to nearby Knox College for his B.S. in Physics, and then on to Michigan State University for an M.S. in Physics, before deserting the Midwest for Corvallis, where he earned his Ph.D. in Physical Oceanography at Oregon State University in 1977. While Ed certainly became an active member of our global scientific community, he never lost his Midwestern virtues. He could write well, and received numerous awards for his technical communication, but he was often refreshingly candid in speech. Ed would on occasion provide honest "unvarnished" critiques of his and his collaborators' efforts, but he did not hesitate to praise others' efforts in print. Ed was a principled person, and this was manifest in his professional dealings with others. Working on a manuscript with Ed was always, as it should be, an ultimately enlightening intellectual exercise.

His approach to any task combined both the rigor of the physicist with the practicality of an engineer, and as a result of his efficient nature, he has left our air-sea interaction community with a trove of no less than 140 valuable scientific publications, with others yet to appear.

Ed Andreas was a Fellow of the American Meteorological Society and of the Royal Meteorological Society (UK). He was recipient of the Antarctica Service Medal, and of numerous performance awards from the Cold Regions Research and Engineering Laboratory.

This brief tribute just touches upon Ed Andreas as a scientist. But Ed was a truly multi-faceted individual, as is apparent to any reader of the posthumous biographical sketch that appeared in his local newspaper (Jurgens 2015), which you may want to access. We would like to acknowledge with gratitude the biographical materials we received from Ed's immediate family, and from his former associates at NorthWest Research Associates, Inc. in Redmond, Washington.

All papers published in this volume of IOP Conference Series: Earth and Environmental Science have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

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.

The utility of a satellite-based whitecap database for estimates of surface sea spray production and bubble-mediated gas transfer on a global scale is presented. Existing formulations of sea spray production and bubble-mediated CO₂ transfer velocity involve whitecap fraction parametrization as a function of wind speed at 10 m reference height W(U₁₀) based on photographic measurements of whitecaps. Microwave radiometric measurements of whitecaps from satellites provide whitecap fraction data over the world oceans for all seasons. Parametrizations W(U₁₀) based on such radiometric data are thus applicable for a wide range of conditions and can account for influences secondary to the primary forcing factor, the wind speed. Radiometric satellite-based W(U₁₀) relationship was used as input to: (i) the Coupled Ocean-Atmosphere Response Experiment Gas transfer (COAREG) algorithm to obtain CO₂ transfer velocity and total CO₂ flux; and (ii) the sea spray source function (SSSF) recommended by Andreas in 2002 to obtain fluxes of sea spray number and mass. The outputs of COAREG and SSSF obtained with satellite-based W(U₁₀) are compared with respective outputs obtained with the nominal W(U₁₀) relationship based on photographic data. Good comparisons of the gas and sea spray fluxes with direct measurements and previous estimates imply that the satellite- based whitecap database can be useful to obtain surface fluxes of particles and gases in regions and conditions difficult to access and sample in situ. Satellite and in situ estimates of surface sea spray production and bubble-mediated gas transfer thus complement each other: accurate in situ observations can constrain radiometric whitecap fraction and mass flux estimates, while satellite observations can provide global coverage of whitecap fraction and mass flux estimates.

For over 30 years, air-sea interaction specialists have been evaluating and parameterizing the role of whitecap bubbles in air-sea gas exchange. To our knowledge, no one, however, has studied the mirror image process of whether sea spray droplets can facilitate air-sea gas exchange. We are therefore using theory, data analysis, and numerical modeling to quantify the role of spray on air-sea gas transfer. In this, our first formal work on this subject, we seek the rate-limiting step in spray-mediated gas transfer by evaluating the three time scales that govern the exchange: τ_air, which quantifies the rate of transfer between the atmospheric gas reservoir and the surface of the droplet; τ_int, which quantifies the exchange rate across the air-droplet interface; and τ_aq, which quantifies gas mixing within the aqueous solution droplet.

Simultaneous measurements of sea spray aerosol (SSA), wind, wave, and microwave brightness temperature are obtained in the open ocean on-board Floating Instrument Platform (FLIP). These data are analysed to clarify the ocean surface processes important to SSA production. Parameters are formulated to represent surface processes with characteristic length scales spanning a broad range. The investigation reveals distinct differences of the SSA properties in rising winds and falling winds, with higher SSA volume in falling winds. Also, in closely related measurements of whitecap coverage, higher whitecap fraction as a function of wind speed is found in falling winds than in rising winds or in older seas than in younger seas. Similar trend is found in the short scale roughness reflected in the microwave brightness temperature data. In the research of length and velocity scales of breaking waves, it has been observed that the length scale of wave breaking is shorter in mixed seas than in wind seas. For example, source function analysis of short surface waves shows that the characteristic length scale of the dissipation function shifts toward higher wavenumber (shorter wavelength) in mixed seas than in wind seas. Similarly, results from feature tracking or Doppler analysis of microwave radar sea spikes, which are closely associated with breaking waves, show that the magnitude of the average breaking wave velocity is smaller in mixed seas than in wind seas. Furthermore, breaking waves are observed to possess geometric similarity. Applying the results of breaking wave analyses to the SSA and whitecap observations described above, it is suggestive that larger air cavities resulting from the longer breakers are entrained in rising high winds. The larger air cavities escape rapidly due to buoyancy before they can be fully broken down into small bubbles for the subsequent SSA production or whitecap manifestation. In contrast, in falling winds (with mixed seas more likely), the shorter breaker entrains smaller and more numerous air cavities that stay underwater longer for more efficient bubble breakup by turbulence and prolonging the surface disturbances attributable to wave breaking. For low winds, the breaking scale is small and with high efficiency for SSA or whitecap generation; the trend of rising or falling wind is less important.

Although the air-sea gas transfer velocity k is usually parameterized with wind speed, the so-called small-eddy model suggests a relationship between k and ocean surface dissipation of turbulent kinetic energy . Laboratory and field measurements of k and have shown that this model holds in various ecosystems. Here, field observations are presented supporting the theoretical model in the open ocean. These observations are based on measurements from the Air-Sea Interaction Profiler and eddy covariance CO₂ and DMS air-sea flux data collected during the Knorr11 cruise. We show that the model results can be improved when applying a variable Schmidt number exponent compared to a commonly used constant value of 1/2. Scaling to the viscous sublayer allows us to investigate the model at different depths and to expand its applicability for more extensive data sets.

Direct numerical simulations (DNS) of an initially quiescent coupled air-water interface driven by an air flow with free stream speed of 5 m/s have been conducted and scalar transfer from the air side to the water side and subsequent vertical transport in the water column have been analysed. Two simulations are compared: one with a freely deforming interface, giving rise to gravity-capillary waves and aqueous Langmuir turbulence (LT) characterized by small-scale (centimeter-scale) Langmuir cells (LC), and the other with the interface intentionally held flat, i.e., without LC. It is concluded that LT serves to enhance vertical transport of the scalar in the water side and in the process increases scalar transfer efficiency from the air side to the water side relative to the shear-dominated turbulence in the flat interface case. Furthermore, transition to LT was observed to be accompanied by a spike in scalar flux characterized by an order of magnitude increase. These episodic flux increases, if linked to gusts and overall unsteadiness in the wind field, are expected to be an important contributor in determining the long-term average of the air-sea gas fluxes.

The gas flux at the water surface is affected by physical processes including turbulence from wind shear, microscale wave breaking, large-scale breaking, and convection due to heat loss at the surface. The main route in the parameterizations of the gas flux has been to use the wind speed as a proxy for the gas flux velocity, indirectly taking into account the dependency of the wind shear and the wave processes. The interest in the contributions from convection processes has increased as the gas flux from inland waters (with typically lower wind and sheltered conditions) now is believed to play a substantial role in the air-water gas flux budget. The gas flux is enhanced by convection through the mixing of the mixed layer as well as by decreasing the diffusive boundary layer thickness. The direct numerical simulations performed in this study are shown to be a valuable tool to enhance the understanding of this flow configuration often present in nature.

Characterizing the vertical distribution of large spray particles (i.e., spume) in high wind conditions is necessary for better understanding of the development of the atmospheric boundary layer in extreme conditions. To this end a laboratory experiment was designed to observe the droplet concentration in the air above actively breaking waves. The experiments were carried out in hurricane force conditions (U₁₀ equivalent wind speed of 36 to 54 m/s) and using both fresh water and salt water. While small differences between fresh and salt water were observed in profiles of radius-integrated spray volume fraction, the profiles tend to converge as the wind forcing increases. This supports the assumption that the physical mechanism for spume production is not sensitive to salinity and its corresponding link to the bubble size distribution.

The equilibrium range of wind-waves at normal and extremely high wind speeds was investigated experimentally using a high-speed wind-wave tank together with field measurements at normal wind speeds. Water level fluctuations at normal and extremely high wind speeds were measured with resistance-type wave gauges, and the wind-wave spectrum and significant phase velocity were calculated. The equilibrium range constant was estimated from the wind-wave spectrum and showed the strong relationship with inverse wave age at normal and extremely high wind speeds. Using the strong relation between the equilibrium range constant and inverse wave age, a new method for estimating the wind speed at 10-m height (U₁₀) and friction velocity (u*) was proposed. The results suggest that U₁₀ and u* can be estimated from wave measurements alone at extremely high wind speeds in oceans under tropical cyclones.

At the land-based marine measuring site Östergarnsholm in the Baltic Sea, the eddy covariance technique was used to measure air-sea fluxes of carbon dioxide and oxygen. High- frequency measurements of oxygen were taken with a Microx TX3 optode using the luminescence lifetime technique. The system gives reasonable oxygen fluxes after the limited frequency response of the sensor was corrected for. For fluxes of carbon dioxide the LICOR-7500 instrument was used. Using flux data to estimate transfer velocities indicates higher transfer velocity for oxygen compared to carbon dioxide for winds above 5 m/s. There are too few data for any extensive conclusions, but a least-square fit of the data gives a cubic wind speed dependence of oxygen corresponding to k₆₆₀ = 0.074U³₁₀. The more effective transfer for oxygen compared to carbon dioxide above 5 m/s is most likely due to enhanced efficiency of oxygen exchange across the surface. Oxygen has lower solubility compared with carbon dioxide and might be more influenced by near surface processes such as microscale wave breaking or sea spray.

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.

The influence of wave-associated parameters controlling turbulent CO₂ fluxes through the air-sea water interface is evaluated in a coastal region. The study area, located within the Todos Santos Bay, Baja California, México, was found to be a weak sink of CO₂ with a mean flux of -1.32 µmol m^-2s^-1. The low correlation found between flux and wind speed (r = 0.09), suggests that the influence of other forcing mechanisms, besides wind, is important for gas transfer modulation through the sea surface, at least for the conditions found in this study. In addition, the results suggest that for short periods where an intensification of the wave conditions occurs, a CO₂ flux response increases the transport of gas to the ocean.

With the ever-growing interest in satellite remote sensing, direct observations of short wave characteristics are needed along coastal margins. These zones are characterized by a diversity of physical processes that can affect sea surface topography. Here we present connections made between ocean wave spectral shape and wind forcing in coastal waters using polarimetric slope sensing and eddy covariance methods; this is based on data collected in the vicinity of the mouth of the Columbia River (MCR) on the Oregon-Washington border. These results provide insights into the behavior of short waves in coastal environments under variable wind forcing; this characterization of wave spectra is an important step towards improving the use of radar remote sensing to sample these dynamic coastal waters. High wavenumber spectral peaks are found to appear for U₁₀ > 6 m/s but vanish for τ > 0.1 N/m², indicating a stark difference between how wind speed and wind stress are related to the short-scale structure of the ocean surface. Near-capillary regime spectral shape is found to be less steep than in past observations and to show no discernable sensitivity to wind forcing.

Methane (CH₄) and carbon dioxide (CO₂) are two important greenhouse gases. Previous studies have shown that lakes can be important natural sources of atmospheric CH₄ and CO₂. It is therefore important to monitor the fluxes of these gases between lakes and the atmosphere in order to understand the processes that govern the exchange. Most previous lake flux studies are based on chamber measurements, by using the eddy covariance method, the resolution in time and in space of the fluxes is increased, which gives more information on the governing processes. Eddy covariance measurements at a Swedish lake show that both methane fluxes (FCH₄) and carbon dioxide fluxes (FCO₂) experience high nighttime fluxes for a large part of the data set (largest median FCH_4night ≈ 13 nmol m² s^-1 and smallest median FCH_4day ≈ 4.0 nmol m^-2 s^-1, largest median FCO_2night ≈ 0.2 μmol m² s^-1 and smallest median FCO_2day ≈ 0.02 μmol m^-2 s^-1, with larger variability during night). For the diel cycle of the CH₄ fluxes it is suggested that water side convection could enhance the transfer velocity, transport CH₄ rich water to the surface, as well as trigger ebullition. The high nighttime CO₂ fluxes could to a large extent be explained with enhanced transfer velocities due to water side convection. If gas fluxes are not measured during nighttime, when water side convection normally is generated, periods of potential high gas flux might be missed and estimations of the total amount of gas released from lakes to the atmosphere will be biased.

The present study investigates the effects of strip roughness on surface velocity divergence (SD) to develop physical modelling of gas transfer mechanisms in natural rivers. Particularly, turbulence measurements were conducted by PIV in a computer-controlled laboratory flume with varying water discharge and roughness spacing systematically, in order to obtain the space and time distributions of surface velocity divergence, turbulent kinetic energy and dissipation rate on the horizontal plane. Finally, a new empirical model for the surface velocity divergence was proposed considering turbulence microscales.

Velocity and gas concentration measurements were carried out to reveal gas transfer phenomena in open-channel turbulent flows with flat bottom and submerged vegetation bottom conditions. A large-scale coherent vortex appears near the vegetation top due to shear instability, and the submerged vegetation was found to promote gas transfer beneath the air- water interface. Furthermore, we revealed a great dependency of gas transfer on vegetation density. The present measurement results propose a new surface divergence model with wide generality, connecting reasonably the gas transfer velocity and the surface divergence intensity in open-channel flows, irrespective of bottom roughness conditions.

The diffusive and bubble-mediated components of air-sea gas exchange can be quantified separately using time-series measurements of a suite of dissolved inert gases. We have evaluated the performance of four published air-sea gas exchange parameterizations using a five-day time-series of dissolved He, Ne, Ar, Kr, and Xe concentration in Monterey Bay, CA. We constructed a vertical model including surface air-sea gas exchange and vertical diffusion. Diffusivity was measured throughout the cruise from profiles of turbulent microstructure. We corrected the mixed layer gas concentrations for an upwelling event that occurred partway through the cruise. All tested parameterizations gave similar results for Ar, Kr, and Xe; their air-sea fluxes were dominated by diffusive gas exchange during our study. For He and Ne, which are less soluble, and therefore more sensitive to differences in the treatment of bubble-mediated exchange, the parameterizations gave widely different results with respect to the net gas exchange flux and the bubble flux. This study demonstrates the value of using a suite of inert gases, especially the lower solubility ones, to parameterize air-sea gas exchange.

Sea surface CO₂ dynamics are not well characterized in the Arctic Ocean (AO). Most data are from ship-based studies during the low-ice period (May-September) and obtained from near shore areas because of accessibility. More CO₂ data are needed to improve models for predicting the future of the carbon cycle in the region and its relationship to ocean acidification. Air-sea gas exchange rates are complicated by the presence of ice. Consequently gas exchange rates have a larger uncertainty in the AO compared to other ocean regions. To provide more information about CO₂ dynamics and gas exchange in the AO, in situ time-series data have been collected from the Canada Basin during late summer to autumn of 2012. Partial pressure of CO₂ (pCO₂), dissolved O₂ (DO) concentration, temperature, and salinity were measured at ∼6-m depth under little ice and multi-year ice on two ice-tethered profilers (ITPs) for 40-50 days. The pCO₂ levels were always below atmospheric saturation whereas DO was almost always slightly above saturation. Although the two ITPs were on an average only 222 km apart, one was further south and had 14 ± 12% ice cover; whereas the more northern ITP had 63 ± 16% ice cover. Consequently the two data sets differed significantly in external forcings. Modeled variability of CO₂ and DO estimate that gas exchange would significantly alter sea surface pCO₂ in the low ice cover record but minimally in the more extensively ice-covered region. If these conditions extended over the entire AO, the total uptake of atmospheric CO₂ would be 28.6 Tg C yr^-1 and 15.4 Tg C yr^-1 under low and high ice-covered conditions, respectively.

To explore the dynamics and implications of incomplete air-sea equilibration during the formation of abyssal water masses, we simulated noble gases in the Estimating the Circulation & Climate of the Ocean (ECCO) global ocean state estimate. A novel computation approach utilizing a matrix-free Newton-Krylov (MFNK) scheme was applied to quickly compute the periodic seasonal solutions for noble gas tracers. MFNK allows for quick computation of a cyclo-stationary solution for tracers (i.e., a spun-up, repeating seasonal cycle), which would otherwise be computationally infeasible due to the long time scale of dynamic adjustment of the abyssal ocean (1000's of years). A suite of experiments isolates individual processes, including atmospheric pressure effects, the solubility pump and air-sea bubble fluxes. In addition to these modeled processes, a volumetric contribution of 0.28 ± 0.07% of glacial melt water is required to reconcile deep-water observations in the Weddell Sea. Another primary finding of our work is that the saturation anomaly of heavy noble gases in model simulations is in excess of two-fold more negative than is suggested from Weddell Sea observations. This result suggests that model water masses are insufficiently ventilated prior to subduction and thus there is insufficient communication between atmosphere and ocean at high latitudes. The discrepancy between noble gas observations and ECCO simulations highlights that important inadequacies remain in how we model high-latitude ventilation with large implications for the oceanic uptake and storage of carbon.

The present work reports simultaneous bubble size and gas transfer measurements in a bubbly wake flow of a hydrofoil, designed to be similar to a hydroturbine blade. Bubble size was measured by a shadow imaging technique and found to have a Sauter mean diameter of 0.9 mm for a reference case. A lower gas flow rate, greater liquid velocities, and a larger angle of attack all resulted in an increased number of small size bubbles and a reduced weighted mean bubble size. Bubble-water gas transfer is measured by the disturbed equilibrium technique. The gas transfer model of Azbel (1981) is utilized to characterize the liquid film coefficient for gas transfer, with one scaling coefficient to reflect the fact that characteristic turbulent velocity is replaced by cross-sectional mean velocity. The coefficient was found to stay constant at a particular hydrofoil configuration while it varied within a narrow range of 0.52-0.60 for different gas/water flow conditions.

Particle image velocimetry (PIV) is a state-of-the-art non-intrusive technique for velocity and fluid flow measurements. Due to ongoing improvements in image hardware and processing techniques, the diversity of applications of the PIV method continues to increase. This study presents an accurate thermal image velocimetry (TIV) technique using a CO₂ laser source to measure the surface wave particle velocity using infrared imagery. Experiments were carried out in a 2-D wind wave flume with glass side walls for deep-water monochromatic and group waves. It was shown that the TIV technique is robust for both unforced and wind-forced group wave studies. Surface wave particles attain their highest velocity at the group crest maximum and slow down thereafter. As previously observed, each wave crest slows down as it approaches its crest maximum but this study demonstrates that the minimum crest speed coincides with maximum water velocity at the wave crest. Present results indicate that breaking is initiated once the water surface particle velocity at the wave crest exceeds a set proportion of the velocity of the slowing crest as it passes through the maximum of a wave group.

Prolonged periods of drought and consequent evaporation from open water bodies in arid parts of Australia continue to be a threat to water availability for agricultural production. Over many parts of Australia, the annual average evaporation exceeds the annual precipitation by more than 5 times. Given its significance, it is surprising that no evaporation mitigation technique has gained widespread adoption to date. High capital and maintenance costs of manufactured products are a significant barrier to implementation. The use of directly recycled clean plastic containers as floating modular devices to mitigate evaporation has been investigated for the first time. A six-month trial at an arid zone site in Australia of this potential cost effective solution has been undertaken. The experiment was performed using clean conventional drinking water bottles as floating modules on the open water surface of 240-L tanks with three varying degrees of covering (nil, 34% and 68%). A systematic reduction in evaporation is demonstrated during the whole study period that is approximately linearly proportional to the covered surface. These results provide a potential foundation for robust evaporation mitigation with the prospect of implementing a cost-optimal design.