How do the wind, internal and surface waves impact the bottom boundary layer in a temperate shelf sea?

Introduction:

Shelf seas play a significant role in the global cycling of carbon and nutrients. Fuelled by nutrients in the water column, carbon dioxide is removed from the atmosphere by marine phytoplankton and may be sequestered as particulate carbon into shelf sediments, exported off the shelf or re-mineralised in the water column. The particulate organic matter reaching the sea bed may be either reminerlised to dissolved inorganic forms, buried, re-suspended or advected. In a dynamic shelf sea where a large proportion of the mechanical energy input into the oceans by the tides, wind and waves is dissipated, often at the sea bed, the efficiency of carbon burial and the rate of release of nutrients to and from the sediments are closely linked to re-suspension and advection events.

Sediment re-suspension takes place when near-bed velocities and bed-shear stresses exceed a critical threshold. On the NW European Shelf the barotropic tide drives much of the variability in near-bed currents and sediment erosion. However, during the stratified summer months the internal tide and large amplitude high frequency waves impose a more complex depth varying velocity structure on top of these more predictable oscillatory motions. Non-linear wave packets for example drive large near bed velocities and increase bottom boundary layer energy dissipation. Inertial oscillations initiated by wind events also drive current variability and have already been shown to modify carbon and nutrient fluxes across the pycnocline. In shallow or fully mixed water columns episodic wind driven mixing events may penetrate the full depth and surface waves ‘feel’ the seabed. The spatially variable constructive and/or deconstructive coupling of these processes near the bed on intra-tidal to seasonal timescales across the shelf will play an important role in the timings and rates at which both carbon and nutrients are exchanged across the pelagic-benthic interface. In a climate where sea level, stratification, tidal conditions and storminess are all likely to change in the future an understanding of these couplings is essential.

Project Summary:

This project will focus on the response of bed shear stress and resuspension to the interaction between the barotropic tide, internal wave propagation, wind driven mixing and the surface wave climate in a seasonally stratifying shelf sea. The following questions will be addressed:

(1) How does the propagation of internal waves impact the magnitude and timing of resuspension events across the continental shelf?

(2) How does the development and evolution of seasonal stratification impact on the interaction between internal and barotropic tidal currents at the sea bed?

(3) When, where and how are wind and wave driven currents able to modify bed shear stresses across the shelf?

(4) What contribution to nutrient release do resuspension events make across a shelf sea?

The work will draw upon observational data sets collected during the recent FASTNEt and Shelf Sea Biogeochemistry programs. Three moored bedframes fitted with a range of acoustic Doppler instrumentation (Acoustic Doppler Velocimeter, Acoustic Doppler Current Profiler, high resolution Aquadop profilers) will provide measurements of velocity, stresses and dissipation and via the strength of the backscatter signals a relative measure of the material in suspension between the shelf edge and inner Malin Shelf. Full water column density structure will be provided by co-located moorings and ship based CTD sampling. The seasonal impact of internal wave, wind and wave dynamics on near-bed processes will be investigated using a 17 month long ADCP time series from the Celtic Sea configured to enable estimates of the rate of turbulent kinetic energy dissipation and shear production. This will be supported by a coincident record of full water column density and velocity structure. Wave climate and meteorological observations will be provided by moorings at both sites. A 1-d version of ERSEM, a modular ecosystem model of marine biogeochemistry, will be used to assess the release of nutrients back into the water column following re-suspension events initiated by various coupled and uncoupled scenarios.

The student will be trained in the processing and analysis of observational data sets, model setup, scientific writing and presentation. They will participate in the doctoral training programme of the University of Liverpool, attend both national and international conferences to present their work and will be encouraged to apply for international training schools.