Stratification and internal temperature oscillations near a coastal inlet

A vertical array of fast-response temperature sensors was deployed to investigate interior ocean temperature and thermal stratification variability during Spring, 2018 near an inlet entrance on the Otago inner continental shelf (15 m water depth). Over the 24-day observation period, the background water column structure evolved from supporting a near-surface pycnocline, to being characterised by a deep surface mixed layer. Periods of mostly low stratification due to atmospheric- and surface wave-driven mixing are also observed. Time-varying normal-mode projection demonstrates that the vertical structure of semi-diurnal temperature oscillations evolves in response to this background stratification in a manner consistent with the interpretation of internal waves. At higher frequencies (supratidal) the projection is less consistent as vertically coherent oscillations from non-linear internal solitons co-exist with non-coherent oscillations, a potential signature of stratified mixing. Although both processes are likely important contributors to the transport of coastal tracers and inlet-ocean exchange, further measurements are needed to determine the origin and fate of internal waves on the Otago continental shelf.