The existence of the sea-surface microlayer (SML) is a global phenomenon, but exchange processes with the underlying near-surface water layer have received little scientific attention. Our motivation lies in the fact that the dynamic connection between the SML and the near-surface water layer via convection (driven by density anomalies) leads to heterogeneous properties of the SML. Our main objective is to obtain a mechanistic understanding of the dynamics between the SML and the near-surface layer. One of the main points of our proposal is that buoyancy-driven convection is a key component of the SML and air–sea interaction, despite its small-scaled nature. We will conduct a series of experiments in the laboratory, at the ICBM mesocosm facility SURF, and in the wind tunnel at the University of Hamburg, using a microprofiler system and miniature drifters to observe temperature and salinity changes (both determine the density) in a three-dimensional field from the SML to a few centimeters below the surface.

The experiments will be conducted under various physical, chemical, and biological conditions. In field studies, we will use our autonomous catamaran HALOBATES to collect SML and near-surface layers and observe temperature and salinity anomalies. We will deploy drifters to observe temperature and salinity gradients within the NSL while tracking various water masses in regions of fronts and slicks. We will integrate our observations in a mathematical framework to describe temperature and salinity profiles and their fluctuations under the influence of defined oceanic and atmospheric forcing. The models will capture the major physical processes in the SML and the underlying bulk water and provide a mechanistic description of the small-scale spatio-temporal SML structure as well as associated transport processes across the SML. Such mechanistic understanding of convection is essential, as the extent of bio-photochemical reactions and the air–sea exchange of trace gases, energy, and momentum will ultimately be determined by exchange processes between the SML and the near-surface layer, and ultimately with deeper layers.