Publication Date:
2023-01-26
Description:
Glider observations show a subsurface chlorophyll maximum (SCM) at the base of the seasonal pycnocline in the North Sea during stable summer conditions. A colocated peak in the dissipation rate of turbulent kinetic energy suggests the presence of active turbulence that potentially generates a nutrient flux 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 base of the pycnocline (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: (a) only turbulence generated inside the stratified PCB is able to entrain a tracer (e.g., nutrients) from the bottom mixed layer into the SCM region; (b) surface wind forcing only plays a secondary role during stable summer conditions; (c) interfacial shear from the tide accounts for the majority of turbulence production at the PCB; (d) in stable summer conditions, the strength of the turbulent diapycnal fluxes at the PCB is set by the strength of the anticyclonic component of the tidal currents.
Description:
Plain Language Summary:
Many midlatitude shelf seas are vertically stratified in summer, where a warm surface layer sits on top of a cold, dense bottom layer. Both of these layers are unproductive environments for phytoplankton—the bottom layer is light limited, and the surface layer is nutrient‐limited. However, abundant phytoplankton is observed directly at the interface between surface and bottom layers. In order to sustain this phytoplankton, nutrient‐rich bottom water needs to be mixed with interface water. While both wind and tides are major causes for mixing in the coastal ocean, we find that the tides alone provide sufficient stirring at the right place to potentially act as an effective fuel pump for the phytoplankton. Interestingly, it is not the strength of the tides alone that counts, rather the sense of rotation of the tidal currents; rotation opposite to the Earth's spin causes more stirring than rotation along with it.
Description:
Key Points:
Turbulence and chlorophyll both peak at the base of the pycnocline on a mid‐latitude shelf.
Locally generated turbulence at the pycnocline base is a fuel pump for the subsurface chlorophyll maximum.
Amplitude and polarity of the M2 tide govern the local generation of turbulence at the pycnocline base.
Description:
Helmholtz Association
Description:
https://doi.org/10.5281/zenodo.3525787
Description:
https://oceancolor.gsfc.nasa.gov/l3/
Description:
https://www.cen.uni-hamburg.de/icdc/data/ocean/nsbc.html
Keywords:
ddc:551.46
;
shelf seas
;
storms
;
North Sea
;
turbulence
;
straification
;
marginal stability
;
subsurface chlorophyll maximum
;
fuel pump
;
modeling
Language:
English
Type:
doc-type:article
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