Freshwater Fluxes over the Ocean I – Evaporative Fluxes (FreshOcean)

• Applicant: Professor Oliver Wurl, Ph.D.
• Subject Area: Oceanography
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 510639210

Alterations in the global water cycle with climate change are one of the greatest challenges for society as dry regions get drier and wet regions to become wetter. The problem is that 85% of evaporation and 77% of precipitation occurs over the oceans, and the global hydrological cycle is poorly understood due to observational challenges over the oceans. However, any exchange of freshwater between the ocean and atmosphere occurs at an upper thin sea surface layer, the so-called skin layer. Evaporation of water vapor from the skin layer increases its salinity, whereas precipitation decreases the salinity in the skin layer. The main objective of this research project is a comprehensive understanding of the dynamics and alterations in saline and related thermal fields in the oceanic skin and the near-surface layer (NSL), and their link to evaporative freshwater fluxes. One of the main points of this work is that the freshwater flux (i.e. evaporation minus precipitation) acts directly on the actual sea surface and therefore predominantly forces skin salinity quasi-instantaneously, whereas current approaches using salinity of the mixed layer relates to freshwater fluxes on decadal scales. A comprehensive set of experiments will be conducted in a large-scale mesocosm facility at the University of Oldenburg, in which driving forces for evaporation can be controlled (water temperature, wind forces, turbulent mixing, air temperature, and humidity). In addition, an expedition in the central Atlantic with its feature of high surface salinity, i.e. evaporation rates exceed precipitation rates, using a radio-controlled catamaran with the capability to collect skin layers. The work will complement ongoing activities to investigate the link between skin salinity and precipitation. This work is a first step to understand how the skin and near-surface salinity can be used to integrate dynamic freshwater fluxes and to develop parameterizations to extrapolate freshwater fluxes using satellite-based salinity data.

• DFG Programme: Research Grants