Observational based evaluation of air-sea gas fluxes and turbulence in the surface ocean boundary layer

Abstract

Turbulence within the ocean surface boundary layer (OSBL) is an important quantity for many processes as it mixes the ocean and transports various ocean quantities such as pollutants, heat, and dissolved gases. However, direct observations of the dissipation rate of turbulent kinetic energy \epsilon under open ocean conditions are limited. Consequently, our understanding on how to model turbulence and its related processes is constrained. Open ocean measurements from the Air-Sea Interaction Profiler (ASIP) from five cruises are combined with ship-based meteorological information, direct measurements of air-sea gas fluxes, and wave data from dedicated runs of the ECWAM wave model. This comprehensive data set allowed for an evaluation of commonly applied approaches to scale profiles of \epsilon, as well as to formulate a scaling relationship. During daytime conditions a relationship based on the friction velocity and wave age describes the observations best. During conditions when convection dominates over wind and wave-induced turbulence the scaling considers buoyancy forcing as additional source for turbulence. This data was also used to quantify the so-called small-eddy model under open-ocean conditions. This theoretical model relates air-sea gas transfer directly to turbulence, rather than often used empirical wind speed-based parameterisations. It can be shown that the agreement between the model and observations can be improved when using a variable Schmidt number exponent in the model, rather than a constant value of 1/2. Further analysis of a single deployment of ASIP in the Labrador Sea presents a unique situation where a stably stratified diurnally warmed OSBL is accompanied by a mixing event, which is most plausibly explained by a breaking internal wave. These results manifest the importance of observations in the upper ocean for understanding processes for oceanatmosphere exchange.