Tidal shelf seas are energetic areas of intense biological activity and turbulent motion. This turbulence is largely generated at the water surface from wind stress and wave breaking and in the bottom boundary layer through friction generated by tidal motion. When stratification is formed, a thermocline often emerges and separates both boundary regions, controlling the vertical transport of heat and other scalars. Whilst diapycnal mixing has broad implications on the marine ecosystem functioning, unraveling the key processes triggering this mixing remains an active area of research.
Conducting turbulence measurements in the field is a challenging task, which until recently was constrained by short term measurements collected in areas of increased mixing, potentially introducing a bias in the current notion of vertical transport. Moreover, because of the large-scale impact of small-scale turbulent mixing, the latter has to be inevitably parameterized in ocean models. It is therefore essential to improve understanding of the natural variability of turbulent motion to improve its representation in large-scale models.
The present work aims to assess turbulence levels in an energetic tidal shelf sea under different stratification and weather conditions in order to gain an insight of the local natural variability of mixing. This aim was enabled through the use of autonomous underwater gliders, which are able to collect measurements uninterrupted for several weeks and to record data during adverse meteorological events. The vertical structure of the water column during the summer months in the study area, the German Bight of the North Sea, is variable and may range from a fully mixed regime to a stably stratified thermocline. Whilst the vertical structure of turbulence dissipation in well-mixed regimes is found to be close to homogeneous, the presence of a thermocline generates a low turbulence layer where active turbulent mixing takes place only sporadically. Despite their intermittency, such mixing events are shown to play an important role in heat transport. Within the framework of this study, turbulence measurements during a storm event were collected, from which the rate of dissipation of turbulent kinetic energy was obtained. The storm consisted of 2 major pulses of elevated wind speeds (> Beaufort 6), which generated strong shear across the sharp thermocline and increased dissipation levels by nearly an order of magnitude, rapidly overturning the water column. Rough estimates suggest that such events of strong mixing could play an important role on the average seasonal fluxes.
In addition to naturally occurring mixing mechanisms, advances in offshore wind farm technology have enabled their construction and operation in deeper areas of shelf seas, in which stratification forms. Wind turbine foundations extract power from strong tidal currents and generate turbulence additional to background levels. Field measurements and large-eddy simulations were used to assess the role of single foundation structures in mixing local temperature gradients. The wake of single turbine foundations is characterized by strong turbulence localized within a narrow region of 50 - 100 m up to 200 - 300 m downstream, after which turbulence levels off towards background levels. The signature of temperature anomalies and that of other scalars due to the pylon reaches farther out and is observed until the end of the domain at 600 m downstream the obstacle. The additional mixing generated by a single foundation at current pylon spacings is low, however simplified estimates suggest that the effect of multiple structures on local stratification could be important. Lastly, shelf seas are dynamic regions and it is essential to advance understanding of their natural state and variability to improve the predictability of mixing events, as well as changes that may affect these areas of great societal relevance.