World oceans cover more than 70% of the earth surface and constitutes a major sink of atmospheric CO2. Two of the most important gases in the marine carbon cycling are O2 and CO2 and hence accurate descriptions of the air-sea gas exchange of these gases are crucial. Still there is a lack of knowledge of the relative importance of processes controlling the efficiency of the air-sea gas transfer. This is especially true for Arctic and high latitude seas were studies on air-sea gas exchange are few. By studying processes causing water-side turbulence, using gases of different solubility and various measurement techniques, more knowledge on the governing processes can be obtained. Here we present the very first air-sea fluxes of O2 using atmospheric eddy covariance measurements and investigate the dependence between the gas transfer velocity of O2 and turbulence generated by the mean wind. The instrument was found to suffer from the limited precision and time response, causing significant corrections on the O2 flux. After correcting for this, the O2 fluxes displays an anti-correlation with the air-sea fluxes of CO2 in agreement with the measured air-sea gradient of O2. The transfer velocities for O2 indicates a stronger wind dependence than other commonly used parameterizations of the transfer velocity for CO2 and O2, this especially for wind speeds > 5 m s-1 where the typical onset of wave breaking occur. During two winter months eddy covariance measurements were taken over a high Arctic fjord. The data revealed a significant enhancement of the gas transfer velocity for CO2 from water-side convection, generated by cooling of surface waters. The dependence between water-side convection and gas transfer velocity were found for winds as high as 9 m s-1, but were strongest for wind speeds< 7  m s-1.  The data also showed on enhanced air-sea gas transfer of CO2 when conditions were unstable very close to neutral. This enhanced transfer were associated to increased contribution to the CO2 flux from downdraft of air with higher concentrations of CO2.  The combined effect of water-side convection and turbulence generated by wind results in a very effective transfer, thus the air-sea gas exchange at these latitudes may be significantly underestimated.

Turbulent fluxes govern the exchange of momentum, heat and moisture between the Earth’s surface and the overlying air. Computations of these fluxes are crucial, particularly over lakes and seas because most of the earth’s surface consists of water. One of the most common methods of calculating turbu- lent fluxes is the bulk method, where the fluxes are expressed with exchange coefficients. With more knowledge of these coefficients, the fluxes can be determined with a higher accuracy. Consequently, the turbulence structure and the exchange of moisture, momentum and heat between the surface and the overlying air can be better understood. The goal of this study was to compute the neutral exchange co- efficients for drag (CDN), heat (CHN) and moisture (CEN) and investigate their dependency on various atmospheric conditions, based on four years of measurements from Lake Erken, located about 70 km east of Uppsala. The coefficients were evaluated against the wind speed, stratification and time over water TOW (the time that the air is above the water before it reaches the tower). A special analysis was done by studying the variation of the coefficients with the wind speed during the UVCN-regime. Another analysis was done to see if the coefficients may have been influenced by non-local processes, e.g. advection from the surroundings. Additionally, normalized standard deviations for the temperature and humidity were evaluated for different stabilities. The results were compared with estimations by the COARE3.0 algorithm (for the dependency on the wind speed and the stability) in a previous report and other earlier studies.  The results indicated that the neutral exchange coefficients were higher and more dispersed during near neutral stratification and low TOWs. The normalized standard deviations also increased during neutral conditions. The explanation for this could be related to the presence of the UVCN-regime or non-local effects such as advection or entrainment from the surroundings. The wind speed had no ob- vious impact on the coefficients. However, the drag coefficient was larger and more spread out in the wind speed range 1-3 m/s. In comparison to earlier studies, the exchange coefficients were higher and scattered to a greater extent. This may be because of a strong UVCN-regime, sustainable non-local influences, relatively steeper waves than open-sea conditions or outliers in the data.