The role of turbulence in fueling the subsurface chlorophyll maximum in
tidally dominated shelf seas
Abstract
Glider observations show a subsurface chlorophyll maximum (SCM) at the
base of the seasonal pycnocline (PCB) in the central North Sea during
stable summer conditions. A co-located peak in the dissipation rate of
turbulent kinetic energy suggests the presence of active turbulence that
generates the nutrient fluxes necessary to fuel the SCM. A
one-dimensional turbulence closure model is used to investigate the
dynamics behind this local maximum in turbulent dissipation at the PCB
as well as its associated nutrient fluxes. Based on a number of
increasingly idealized forcing setups of the model, we are able to draw
the following conclusions: (1) only turbulence generated inside the
stratified PCB is able to entrain nutrients from the bottom mixed layer
into the SCM region; (2) surface wind forcing only plays a secondary
role during stable summer conditions; (3) interfacial shear from the
tide accounts for the majority of turbulence production at the PCB; (4)
in stable summer conditions the strength of the turbulent nutrient
fluxes at the PCB is set by the strength of the anticyclonic component
of the tidal currents.