The Vertical Structure of Turbulent Dissipation in Shelf Seas

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Abstract
The free-fall FLY profiler has been used to determine the variation in energy dissipation ϵ in the water column over a tidal cycle at mixed and stratified sites in the Irish Sea. It was found that ϵ exhibits a strong M4 variation with a pronounced phase lag that increases with height above the bed. In mixed conditions this M4 signal, which extends throughout the water column, is reasonably well reproduced by turbulent closure models of the vertical exchange. In the summer stratified situation, the M4, signal in ϵ is confined to about 40 m above the seabed with phase delays of more than 4 h relative to the seabed. The lowest levels of dissipation (∼10−5 W m−3), measured in the pycnocline, are significantly above the system noise level and much higher than predicted by a model using the Mellor-Yamada level 2 closure scheme (MY2.0). However, when allowance is made for the diffusion of TKE, the model (MY2.2) simulates the depth-time distribution of dissipation in the stratified case satisfactorily if the diffusivity Kq = 0.2ql. With Kq set equal to vertical eddy viscosity Nz,. which depends on the Richardson number Ri, the model underestimates dissipation in the pycnocline by two decades, which would imply the possibility of a midwater source of TKE. The observed depth-integrated dissipation is found to be consistent with estimates based on the energy lost from the tidal wave when adjustment is made for the unsampled high energy region close to the bed.

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