Vertical mixing at the thermocline determines transport pathways for heat, nutrients and carbon diox-ide, which are crucial to ecosystems and to the Earths climate. Current models are unable to predictthe correct levels of mixing in the thermocline region, falling short by orders of magnitude. This the-sis examines two candidates for this anomalous internal mixing; inertial shear spikes driven by windstress and internal waves generated by tidal currents interacting with underwater topography.50 days of vertical current and temperature measurements were made in the Western Irish Sea inspring and early summer are complemented by a 48 hour time series of turbulent dissipation. Inertialcurrents generated by strong winds were observed in the surface mixed layer with magnitudes ofover 0.3ms−1, while internal tidal currents of 0.1ms−1were observed at spring tides. Analysis of thebulk shear vector revealed that the frequency of the shear rotation switched many times during theobservations depending on wind and tidal forcing.When wind stress was high, inertial shear spikes were observed, with maximum shear productionoccurring when the bulk shear vector and wind vector aligned. Maximum shear resulted 3.75 hourslater when the surface motion was at 90◦to the right of the wind direction. Inertial shear spikes werethe most energetic baroclinic process but were found not responsible for the anomalous thermoclinemixing because their dynamics were well reproduced by a 1D turbulence closure model and becauseof to the way in which they modified the vertical temperature structure. High vertical resolution ob-servations showed that the spikes driven by the wind generated sufficient shear to reduce the gradientRichardson number below one quarter and sustain mixing at the base of the surface mixed layer. Eachspike generated instabilities which eroded the top of the thermocline, deepening the mixed layer in aseries of steps until the buoyancy frequency increased sufficiently to suppress mixing.
The observations of the nature of the internal tidal revealed that it evolved from a distinct mode 1 struc-ture to a vertical structure with mode 2 characteristics. The mode 1 internal tide was found to have anenergy flux of 34Wm−1which was insufficient to account for the 1mWm−2of dissipation which wasobserved in the thermocline region. However one third of this energy was found to come from thepassage of a packet of non-linear internal waves. These waves generated strong shear and drove insta-bilities in the centre of the thermocline, elevating dissipation rates toεth=1×10−2mWm−3whichwas up to 3 orders of magnitude higher than the background level ofεth=1×10−5mWm−3.
The transition from 2 layer to 3 layer during a period of internal tidal activity lead to the hypothesisthat the diffusion of the thermocline could be used to quantify internal mixing rates. Based on obser-vations of vertical temperature structure an internal mixing parameter∆Φwas developed to quantifythe thermocline diffusion.∆Φrevealed that in the Irish Sea more energy was required to cause thethermocline diffusion than was directly measured. A simple mixing model revealed that the this wasdue to heat input at the surface combined with low wind stress.∆Φproved more useful when appliedto spatial datasets where dissipation rates were higher, leading to a quantification of the level of inter-nal mixing with distance from the Celtic Sea shelf break. Replicating this spatial signal in∆Φusingthe internal mixing model required an energy flux of 400Wm−1and an e-folding scale of 50km.2
The transition from 2 layer to 3 layer during a period of internal tidal activity lead to the hypothesisthat the diffusion of the thermocline could be used to quantify internal mixing rates. Based on obser-vations of vertical temperature structure an internal mixing parameter∆Φwas developed to quantifythe thermocline diffusion.∆Φrevealed that in the Irish Sea more energy was required to cause thethermocline diffusion than was directly measured. A simple mixing model revealed that the this wasdue to heat input at the surface combined with low wind stress.∆Φproved more useful when appliedto spatial datasets where dissipation rates were higher, leading to a quantification of the level of inter-nal mixing with distance from the Celtic Sea shelf break. Replicating this spatial signal in∆Φusingthe internal mixing model required an energy flux of 400Wm−1and an e-folding scale of 50km.2.