THE VERTICAL STRUCTURE OF TURBULENT DISSIPATION IN SHELF SEAS

The free-fall FLY profiler has been used to determine the variation in energy dissipation epsilon in the water column over a tidal cycle at mixed and stratified sites in the Irish Sea. It was found that epsilon exhibits a strong M(4) variation with a pronounced phase lag that inc reases with height above the bed. In mixed conditions this M(4) signal , which extends throughout the water column, is reasonably well reprod uced by turbulent closure models of the vertical exchange, In the summ er stratified situation, the M(4) signal in epsilon is confined to abo ut 40 m above the seabed with phase delays of more than 4 h relative t o the seabed. The lowest levels of dissipation (similar to-10(-5) W m( -3)), measured in the pycnocline, are significantly above the system n oise 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 t he diffusivity K-q = 0.2ql. With K-q set equal to vertical eddy viscos ity N-z, which depends on the Richardson number Ri, the model underest imates dissipation in the pycnocline by two decades, which would imply the possibility of a midwater source of TKE. The observed depth-integ rated dissipation is found to be consistent with estimates based on th e energy lost from the tidal wave when adjustment is made for the unsa mpled high energy region close to the bed.