Turbulent mixing in the seasonally-stratified western Irish Sea: a Thorpe Scale perspective

The seasonal thermocline in shelf-seas represents an important biogeophysical barrier to the vertical flux of nutrients into the photic zone. Episodic weakening of this barrier plays an important role in sustaining the sub-surface chlorophyll maximum in summer and hence impacts the carbon draw-down in the seasonally-stratified zones of the shelf seas. Here we present estimates of the rate of turbulent kinetic energy dissipation inferred from microstructure shear probes and compare them with dissipation rates inferred from a standard conductivity-temperature-depth instrument and from a fast thermistor (Thorpe Scale methodology) at a site in the seasonally-stratified Irish Sea. All methods show strong dissipation rates in response to tidal stresses near the bed (order 10−2 Wm−3) with qualitatively similar temporal and spatial patterns. In the interior of the water column, however, only the microstructure shear probe estimates resolve the mixing in the region of the thermocline.