Air-sea gas exchange

This chapter contains sections titled: Introduction Air-Sea Gas Exchange Models and Theory Laboratory Studies of Air-Water Gas Exchange Large-Scale Estimates of Air-Sea Gas Transfer Local Techniques and Measurements Micrometeorological Techniques and Measurements Parameterizations of Air-Sea Gas Transfer Future Work

Arctic Oceanography - Oceanography: Atmosphere-Ocean Exchange, Biogeochemistry & Physics

The Arctic Ocean is, on average, the shallowest of Earth’s oceans. Its vast continental shelf areas, which account for approximately half of the Arctic Ocean’s total area, are heavily influenced by the surrounding land masses through river run-off and coastal erosion. As a main area of deep water formation, the Arctic is one of the main «engines» of global ocean circulation, due to large freshwater inputs, it is also strongly stratified. The Arctic Ocean’s complex oceanographic configuration is tightly linked to the atmosphere, the land, and the cryosphere. The physical dynamics not only drive important climate and global circulation patterns, but also control biogeochemical cycles and ecosystem dynamics. Current changes in Arctic sea-ice thickness and distribution, air and water temperatures, and water column stability are resulting in measurable shifts in the properties and functioning of the ocean and its ecosystems. The Arctic Ocean is forecast to shift to a seasonally ice-free ocean resulting in changes to physical, chemical, and biological processes. These include the exchange of gases across the atmosphere-ocean interface, the wind-driven ciruclation and mixing regimes, light and nutrient availability for primary production, food web dynamics, and export of material to the deep ocean. In anticipation of these changes, extending our knowledge of the present Arctic oceanography and these complex changes has never been more urgent.

Air-sea exchange of methanol and acetone during HiWinGS: Estimation of air phase, water phase gas transfer velocities

The air-sea fluxes of methanol and acetone were measured concurrently using a proton-transfer-reaction mass spectrometer (PTR-MS) with the eddy covariance (EC) technique during the High Wind Gas Exchange Study (HiWinGS) in 2013. The seawater concentrations of these compounds were also measured twice daily with the same PTR-MS coupled to a membrane inlet. Dissolved concentrations near the surface ranged from 7 to 28 nM for methanol and from 3 to 9 nM for acetone. Both gases were consistently transported from the atmosphere to the ocean as a result of their low sea surface saturations. The largest influxes were observed in regions of high atmospheric concentrations and strong winds (up to 25 m s(-1)). Comparison of the total air-sea transfer velocity of these two gases (K-a), along with the in situ sensible heat transfer rate, allows us to constrain the individual gas transfer velocity in the air phase (k(a)) and water phase (k(w)). Among existing parameterizations, the scaling of k(a) from the COARE model is the most consistent with our observations. The k(w) we estimated is comparable to the tangential (shear driven) transfer velocity previously determined from measurements of dimethyl sulfide. Lastly, we estimate the wet deposition of methanol and acetone in our study region and evaluate the lifetimes of these compounds in the surface ocean and lower atmosphere with respect to total (dry plus wet) atmospheric deposition.

Southern Annular Mode and westerly-wind-driven changes in Indian-Atlantic exchange mechanisms

The dynamical link between the Indian Ocean and Atlantic Meridional Overturning Circulation (AMOC) remains poorly understood. This partly arises from the complex Agulhas leakage, which occurs via rings, cyclones, and non-eddy flux. Hindcast simulations suggest that leakage has recently increased but have not decomposed this signal into its constituent mechanisms. Here these are isolated in a realistic ocean model. Increases in simulated leakage are attributed to stronger eddy and non-eddy-driven transports, and a strong warming and salinification, especially within Agulhas rings. Variability in both regimes is associated with strengthening Indian Ocean westerly winds, reflecting an increasingly positive Southern Annular Mode. While eddy and non-eddy flux signals are tied through turbulent eddy dissipation, the ratio between the two varies decadally. Consequently, while altimetry suggests a recent increase in retroflection turbulence and implied leakage, non-eddy flux may also play a significant role in modulating the leakage AMOC connection.

Impacts of Climate Modes on Air-Sea Heat Exchange in the Red Sea

The impacts of various climate modes on the Red Sea surface heat exchange are investigated using the MERRA reanalysis and the OAFlux satellite reanalysis datasets. Seasonality in the atmospheric forcing is also explored. Mode impacts peak during boreal winter [December–February (DJF)] with average anomalies of 12–18 W m−2 to be found in the northern Red Sea. The North Atlantic Oscillation (NAO), the east Atlantic–west Russia (EAWR) pattern, and the Indian monsoon index (IMI) exhibit the strongest influence on the air–sea heat exchange during the winter. In this season, the largest negative anomalies of about −30 W m−2 are associated with the EAWR pattern over the central part of the Red Sea. In other seasons, mode-related anomalies are considerably lower, especially during spring when the mode impacts are negligible. The mode impacts are strongest over the northern half of the Red Sea during winter and autumn. In summer, the southern half of the basin is strongly influenced by the multivariate ENSO index (MEI). The winter mode–related anomalies are determined mostly by the latent heat flux component, while in summer the shortwave flux is also important. The influence of the modes on the Red Sea is found to be generally weaker than on the neighboring Mediterranean basin.

The Euro as an International Currency: explaining puzzling first evidence from the foreign exchange markets

This paper presents evidence that the bid-ask spreads in euro rates increased relative to the corresponding bid-ask spreads in the German mark (DM) prior to the creation of the currency union. This comes with a decrease in transaction volume in the euro rates relative to the previous DM rates. The starkest example is the DM(euro)/yen rate in which the spread has risen by almost two-thirds while the volume decreased by more than one third. This outcome is surprising because the common currency concentrated market liquidity in fewer external euro rates and higher volume tends to be associated with lower spreads. We propose a microstructure explanation based on a change in the information environment of the FX market. The elimination of many cross currency pairs increased the market transparency for order flow imbalances in the dealership market. It is argued that higher market transparency adversely affects the inventory risk sharing efficiency of the dealership market and induces the observed euro spread increase and transaction volume shortfall.

Charge exchange emission from solar wind helium ions

Charge exchange X-ray and far-ultraviolet (FUV) aurorae can provide detailed insight into the interaction between solar system plasmas. Using the two complementary experimental techniques of photon emission spectroscopy and translation energy spectroscopy, we have studied state-selective charge exchange in collisions between fully ionized helium and target gasses characteristic of cometary and planetary atmospheres (H2O, CO2, CO, and CH4). The experiments were performed at velocities typical for the solar wind (200-1500 km s(-1)). Data sets are produced that can be used for modeling the interaction of solar wind alpha particles with cometary and planetary atmospheres. These data sets are used to demonstrate the diagnostic potential of helium line emission. Existing Extreme Ultraviolet Explorer (EUVE) observations of comets Hyakutake and Hale-Bopp are analyzed in terms of solar wind and coma characteristics. The case of Hale-Bopp illustrates well the dependence of the helium line emission to the collision velocity. For Hale-Bopp, our model requires low velocities in the interaction zone. We interpret this as the effect of severe post-bow shock cooling in this extraordinary large comet.

Charge exchange and dissociative processes in collisions of slow He2+ ions with H2O molecules

Experimental and theoretical studies of one-electron capture in collisions of He2+ ions with H2O molecules have been carried out in the range 0.025-12 keV amu(-1) corresponding to typical solar wind velocities of 70-1523 km s(-1). Translational energy spectroscopy (TES), photon emission spectroscopy (PES), and fragment ion spectroscopy were employed to identify and quantify the collision mechanisms involved. Cross sections for selective single electron capture into n=1, 2, and 3 states of the He+ ion were obtained using TES while PES provided cross sections for capture into the He+(2p) and He+(3p) states. Our model calculations show that He+(n=2) and He+(n=3) formation proceeds via a single-electron process governed by the nucleus-electron interaction. In contrast, the He+(1s) formation mechanism involves an exothermic two-electron process driven by the electron-electron interaction, where the potential energy released by the electron capture is used to remove a second electron thereby resulting in fragmentation of the H2O molecule. This process is found to become increasingly important as the collision energy decreases. The experimental cross sections are found to be in reasonable agreement with cross sections calculated using the Demkov and Landau-Zener models.

Dissociative charge exchange and ionization of O-2 by fast H+ and O+ ions: Energetic ion interactions in Europa's oxygen atmosphere and neutral torus

Measurements of electron capture and ionization of O-2 molecules in collisions with H+ and O+ ions have been made over an energy range 10 - 100 keV. Cross sections for dissociative and nondissociative interactions have been separately determined using coincidence techniques. Nondissociative channels leading to O-2(+) product formation are shown to be dominant for both the H+ and the O+ projectiles in the capture collisions and only for the H+ projectiles in the ionization collisions. Dissociative channels are dominant for ionizing collisions involving O+ projectiles. The energy distributions of the O+ fragment products from collisions involving H+ and O+ have also been measured for the first time using time-of-flight methods, and the results are compared with those from other related studies. These measurements have been used to describe the interaction of the energetic ions trapped in Jupiter's magnetosphere with the very thin oxygen atmosphere of the icy satellite Europa. It is shown that the ionization of oxygen molecules is dominated by charge exchange plus ion impact ionization processes rather than photoionization. In addition, dissociation is predominately induced through excitation of electrons into high-lying repulsive energy states ( electronically) rather than arising from momentum transfer from knock-on collisions between colliding nuclei, which are the only processes included in current models. Future modeling will need to include both these processes.

Experimental investigation of the processes determining x-ray emission intensities from charge-exchange collisions

Absolute cross sections have been measured for single and double charge exchange and x-ray line emission for highly charged ions of C, N, 0, and Ne colliding with He, H-2 CO2, and H2O at collisions energies of 7q keV. Present results of charge exchange in He and H-2 compare favorably with previous results. For CO2 and H2O, where prior work is scarce, the classical overbarrier model is found to overestimate results by up to a factor of 3. An analysis of the relative intensities of the observed Lyman x-ray transitions indicates that capture into l states is not statistical, as collision velocities are insufficient to populate the highest angular-momentum states. The importance of autoionization following multiple capture is highlighted, and enhanced radiative stabilization following double capture is observed and compared to other studies. Present results are also discussed in terms of mechanisms likely to generate x-ray emission in comets.