Small Flows with Big Consequences: Wave-, Turbulence- and Shear current-Driven mixing under a water surface

The triple interactions of surface waves, turbulence, and shear currents (WTS) in the upper layer of the ocean play a key role in the Earth’s climate and ecology by controlling fluxes of heat, gas, and momentum between ocean and atmosphere. Climate simulations have large systematic errors because the mixing of waters due to WTS flow is not properly modelled, yet these flows remain little investigated and poorly understood. We urgently need to learn how WTS mixing depends on flow parameters, but none of today's research approaches can produce the empirical data which is needed. WaTurSheD presents the only practical way out of this stalemate: an extensive experimental campaign where each WTS parameter is individually controlled and systematically varied. I will make use of the new, large water channel laboratory at NTNU, the only facility where such an experimental campaign is currently possible, and combine experiments with new theory and a novel data analysis method. Through WaTurSheD the WTS-driven mixing in the upper ocean can for the first time be modelled based on direct empirical evidence. WaTurSheD is a unique opportunity for progress, combining my group's specialised expertise on wave-current interactions through both theory and experiment, and one-of-a-kind laboratory where my team can create a faithful, fully tuneable scale model of upper ocean WTS flow. The theory framework for ocean waves and currents must be advanced in order to accommodate the new insights, a task I will attend to myself. We will develop a completely new way to analyse near-surface turbulence: By detecting the imprints they leave on the surface using a computer vision technique, the most essential turbulent structures can be selected, allowing trends in WTS data to emerge which would otherwise be obscured by fast fluctuations. All WaTurSheD's components will unite towards its final goal: a universal scaling law for WTS flows valid from centimetres to hundreds of metres.