Sea-to-air and diapycnal nitrous oxide fluxes in the eastern tropical North Atlantic Ocean

Abstract

<jats:p>Abstract. Sea-to-air and diapycnal fluxes of nitrous oxide (N2O) into the mixed layer were determined during three cruises to the upwelling region off Mauritania. Sea-to-air fluxes as well as diapycnal fluxes were elevated close to the shelf break, but elevated sea-to-air fluxes reached further offshore as a result of the offshore transport of upwelled water masses. To calculate a mixed layer budget for N2O we compared the regionally averaged sea-to-air and diapycnal fluxes and estimated the potential contribution of other processes, such as vertical advection and biological N2O production in the mixed layer. Using common parameterizations for the gas transfer velocity, the comparison of the average sea-to-air and diapycnal N2O fluxes indicated that the mean sea-to-air flux is about three to four times larger than the diapycnal flux. Neither vertical and horizontal advection nor biological production were found sufficient to close the mixed layer budget. Instead, the sea-to-air flux, calculated using a parameterization that takes into account the attenuating effect of surfactants on gas exchange, is in the same range as the diapycnal flux. From our observations we conclude that common parameterizations for the gas transfer velocity likely overestimate the air-sea gas exchange within highly productive upwelling zones.</jats:p>

Technical Note: A simple method for air–sea gas exchange measurements in mesocosms and its application in carbon budgeting

“…Measured transfer velocities are within the range of zero wind open-ocean baseline velocities assumed to be dominated by chemical enhancement and buoyancy fluxes (Wanninkhof et al, 2009). The influence of sea surface microlayers of surface active organic molecules is discussed to be responsible for large discrepancies in gas exchange between productive coastal waters and open-ocean conditions (Frew, 1997;Kock et al, 2012). The decrease in open-ocean k by 20-50 %, due to surfactants smoothening wind effects on the water surface (Tsai and Liu, 2003), can be expected to be of minor relevance to mesocosm gas exchange where wind stress is not the dominating energy input.…”

Diapycnal oxygen supply to the tropical North Atlantic oxygen minimum zone

“…Turbulence profiles and nutrient profiles have been used in various shelf regions to estimate nutrient supply to surface waters (Sharples et al, 2001(Sharples et al, , 2007Rippeth et al, 2009;Hales et al, 2005Hales et al, , 2009Schafstall et al, 2010). Similarly, Kock et al (2012) used microstructure turbulence and nitrous oxide gradients on the shelf and in the open ocean of the tropical Atlantic to estimate gas flux to surface waters.…”

Air-Sea Interactions of Natural Long-Lived Greenhouse Gases (CO2, N2O, CH4) in a Changing Climate

“…Neither vertical advection nor biological production could explain this discrepancy. Kock et al (2012) speculated that surfactants may dampen air-sea gas exchange of N 2 O and other gases such as CO 2 (see for example Tsai and Liu 2003) in areas with a high biological productivity.…”