Direct measurement of hairpin‐like vortices in the bottom boundary layer of the coastal ocean

Abstract

<jats:title>Abstract</jats:title><jats:p>Laboratory measurements and numerical modeling at low Reynolds numbers (<jats:italic>R</jats:italic><jats:italic>e</jats:italic><jats:sub><jats:italic>θ</jats:italic></jats:sub>&lt;7700) indicate the energy‐containing turbulence of boundary layer flows comprises coherent packets of hairpin vortices. Here direct measurements in the bottom boundary layer of the coastal ocean at higher Reynolds numbers (<jats:italic>R</jats:italic><jats:italic>e</jats:italic><jats:sub><jats:italic>θ</jats:italic></jats:sub> = 266,150) show tidal flows also contain packets of large vortices separated by periods of more quiescent conditions. The 1452 vortices recorded within a 20 min period are typically aligned along stream (∼8.0° from the mean flow direction) and inclined to the horizontal (∼27.0° from the seabed), with a mean period of occurrence of 4.3 s. These results lend three‐dimensional, in situ support to an interpretation of the coastal ocean bottom boundary layer as comprising coherent packets of hairpin vortices. This demonstrates a direct linkage from low Reynolds number experiments to higher Reynolds number flows, permitting fine‐scale details of particle transport and pollutant dispersion to be inferred from lower Reynolds number data.</jats:p>