Despite the huge extent of the ocean's surface, until now relatively little attention has been paid to the sea surface microlayer (SML) as the ultimate interface where heat, momentum and mass exchange between the ocean and the atmosphere takes place. Via the SML, large-scale environmental changes in the ocean such as warming, acidification, deoxygenation, and eutrophication potentially influence cloud formation, precipitation, and the global radiation balance. Due to the deep connectivity between biological, chemical, and physical processes, studies of the SML may reveal multiple sensitivities to global and regional changes. Understanding the processes at the ocean's surface, in particular involving the SML as an important and determinant interface, could therefore provide an essential contribution to the reduction of uncertainties regarding ocean-climate feedbacks. This review identifies gaps in our current knowledge of the SML and highlights a need to develop a holistic and mechanistic understanding of the diverse biological, chemical, and physical processes occurring at the ocean-atmosphere interface. We advocate the development of strong interdisciplinary expertise and collaboration in order to bridge between ocean and atmospheric sciences. Although this will pose significant methodological challenges, such an initiative would represent a new role model for interdisciplinary research in Earth System sciences.

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The Ocean's Vital Skin: Toward an Integrated Understanding of the Sea Surface Microlayer

[1] GasEx-2001, a 15-day air-sea carbon dioxide (CO 2 ) exchange study conducted in the equatorial Pacific, used a combination of ships, buoys, and drifters equipped with ocean and atmospheric sensors to assess variability and surface mechanisms controlling air-sea CO 2 fluxes. Direct covariance and profile method air-sea CO 2 fluxes were measured together with the surface ocean and marine boundary layer processes. The study took place in February 2001 near 125°W, 3°S in a region of high CO 2 . The diurnal variation in the air-sea CO 2 difference was 2.5%, driven predominantly by temperature effects on 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 CO 2 flux from the ocean to the atmosphere was determined to be 3.9 mol m -2 yr, with nighttime CO 2 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 CO 2 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 the amount and variability of air-sea gas exchange. This is in sharp contrast with 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.

Air-sea CO2 exchange in the equatorial Pacific : Air-Sea Exchange

High wind speeds prevent formation of a distinct bacterioneuston community in the sea-surface microlayer.

Dual emission LIF technique for pH and concentration field measurement around a rising bubble

Thermal signatures of turbulent airflows interacting with evaporating thin porous surfaces

. In this study we present gas-exchange measurements conducted in a large-scale wind–wave tank. Fourteen chemical species spanning a wide range of solubility (dimensionless solubility, α = 0.4 to 5470) and diffusivity (Schmidt number in water, Scw = 594 to 1194) were examined under various turbulent (u10 = 0.73 to 13.2 m s−1) conditions. Additional experiments were performed under different surfactant modulated (two different concentration levels of Triton X-100) surface states. This paper details the complete methodology, experimental procedure and instrumentation used to derive the total transfer velocity for all examined tracers. The results presented here demonstrate the efficacy of the proposed method, and the derived gas-exchange velocities are shown to be comparable to previous investigations. The gas transfer behaviour is exemplified by contrasting two species at the two solubility extremes, namely nitrous oxide (N2O) and methanol (CH3OH). Interestingly, a strong transfer velocity reduction (up to a factor of 3) was observed for the relatively insoluble N2O under a surfactant covered water surface. In contrast, the surfactant effect for CH3OH, the high solubility tracer, was significantly weaker.

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Measuring air-sea gas exchange velocities in a large scale annular wind-wave tank

The mechanisms of air-sea gas transfer are studied in a large annular air-sea interaction facility, the Heidelberg Aeolotron. A novel visualization technique is used, in which an alkaline gas (ammonia) in ppm concentration levels is absorbed into slightly acid water (pH = 4). The concentration gradient in the aqueous mass boundary layer is transformed into a sharp pH gradient, which is made visible by a fluorescent pH indicator (Pyranine). Regulating the gas flux into the water allows controlling the thickness of the fluorescent layer. By observing the fluorescence induced by high power LEDs on a 2D surface patch ((21 x 25) cm²) with a camera from above at 100 fps and a spatial resolution of 150 μm, the processes creating near-surface turbulence can be studied even at a wavy water surface. Results from an extensive set of experiments are presented, covering wind speeds from 0.4 - 8.6 m/s, different concentrations of a soluble surfactant, and waves with limited and unlimited fetch. The fluorescence footprints of different processes known to contribute
to gas exchange, e.g. Langmuir circulations and microscale breaking, are identified. It is found that the transition of the Schmidt number exponent, which is essential for
the scaling of transfer velocities of different gases, from 2/3 to 1/2 is correlated with the frequency of microscale breaking. Surfactants shift the onset of surface renewal
events to higher wind speeds. Direct comparison to simultaneously captured thermal imagery shows that despite the large difference in diffusion coefficients between ammonia and heat by a factor of 100, the transport of both tracers is governed by the same mechanisms.

Visualization of air-water gas exchange

A novel 2-D fluorescence imaging technique has been developed to visualize the thickness of the aqueous mass boundary layer at a free water surface. Fluorescence is stimulated by high-power LEDs and is observed from above with a low noise, high resolution and high-speed camera. The invasion of ammonia into water leads to an increase in pH (from a starting value of 4), which is visualized with the fluorescent dye pyranine. The flux of ammonia can be controlled by controlling its air side concentration. A higher flux leads to basic pH values (pH > 7) in a thicker layer at the water surface from which fluorescent light is emitted. This allows the investigation of processes affecting the transport of gases in different depths in the aqueous mass boundary layer. In this paper, the chemical system and optical components of the measurement method are presented and its applicability to a wind-wave tank experiment is demonstrated.

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High resolution 2-D fluorescence imaging of the mass boundary layer thickness at free water surfaces

Der Schwerpunkt der Arbeit ist die Untersuchung des Austauschs volatiler Stoff e mittlerer Loslichkeit zwischen Luft und Wasser. Dazu wurden simultan zehn Transferwiderstande von Tracern aus einem breiten Spektrum an Loslichkeiten in Experimenten am Aeolotron, einem ringformigen Wind-Wellen-Kanal, gemessen. Die Abhangigkeit der Transferwiderstande von der Schubspannungsgeschwindigkeit und der mittleren quadratischen Neigung wird sowohl fur sauberes als auch fur oberfachenfi lmbedecktes Wasser untersucht. Dabei wird deutlich, dass kein einzelner Parameter ausreichend ist, um den Gasaustausch fur beide Falle einheitlich zu beschreiben. Des Weiteren wurde die Schmidtzahlskalierung fur Tracer mittlerer Loslichkeiten untersucht. Es ist ublich von dem Transferwiderstand eines Tracers mittels dieser Skalierung auf den Transferwiderstand eines anderen Tracers zu schliesen. Dies setzt voraus, dass entweder der luft- oder wasserseitige Transferwiderstand fur beide Tracer vernachlassigt werden kann, welches fur Substanzen mittlerer Loslichkeit nicht gegeben ist. Hier wird daher erstmals eine erweiterte Schmidtzahl-Skalierungsmethode experimentell getestet. Der luftseitige Anteil des Transferwiderstands wird durch Schmidtzahlskalierung mit einem sehr gut loslichen Referenz-Tracer (Methanol) berechnet. Entsprechend wird der wasserseitige Anteil mit einem vollstandig wasserseitig kontrollierten Referenz-Tracer (N2O) berechnet. Der Gesamtwiderstand kann mit der Partitionierungsgleichung von Liss und Slater (1974) aus beiden Anteilen zusammengesetzt werden. Die Ergebnisse dieser erweiterten Skalierungsmethode zeigen sehr gute Ubereinstimmung mit den direkt gemessenen Transferwiderstanden. Schlieslich wird eine einfache Funktion angegeben, die das gemessene prozentuale Verhaltnis des luftseitigen Widerstands zum Gesamtwiderstand in Abhangigkeit der Schubspannungsgeschwindigkeit und Loslichkeit empirisch beschreibt.

Aufteilung des Transferwiderstands zwischen Luft und Wasser beim Austausch flüchtiger Substanzen mittlerer Löslichkeit zwischen Ozean und Atmosphäre

A Comparative Lab Study of Transfer Velocities of A Comparative Lab Study of Transfer Velocities of Volatile Tracers with Widely Varying Solubilities Volatile Tracers with Widely Varying Solubilities

Christine Kräuter

Papers

Measuring air-sea gas exchange velocities in a large scale annular wind-wave tank

Visualization of air-water gas exchange

High resolution 2-D fluorescence imaging of the mass boundary layer thickness at free water surfaces

Aufteilung des Transferwiderstands zwischen Luft und Wasser beim Austausch flüchtiger Substanzen mittlerer Löslichkeit zwischen Ozean und Atmosphäre

A Comparative Lab Study of Transfer Velocities of A Comparative Lab Study of Transfer Velocities of Volatile Tracers with Widely Varying Solubilities Volatile Tracers with Widely Varying Solubilities