Parameterization of gas transfer velocity at a gas-liquid interface in the presence of counter-rotating roller vortices, such as Langmuir Circulation.

Counter-rotating roller vortices, known as Langmuir circulation, are observed on the ocean’s surface as lines of seaweed or bubbles approximately parallel to the wind direction. These vortices are caused by flow instability due to Stokes drift and wind stress conditions common in the ocean. These secondary currents induce turbulent upwelling in the mixed layer, leading to higher surface velocity divergence. Their impact on air-sea gas transfer is not yet quantified. Recent laboratory experiments set up an analogous circulation using span-wise periodic bed bathymetry in a recirculating open channel. Results show that the counter-rotating vortices increase gas transfer velocity by 9-15% over the control case without the vortices (Re~10⁴). Furthermore, we explore how the aspect ratio of the circulation cells relates to gas transfer velocity. The surface flow velocity field is measured using Surface Particle Image Velocimetry (SPIV), and gas transfer is directly measured using a dissolved oxygen probe. Direct oxygen transfer measurements are compared to transfer velocity models based on second order spatiotemporal structure function and surface velocity divergence. In most cases, estimates from both models reasonably agree with direct measurements.