On-shelf transport of slope water lenses within the seasonal pycnocline

We show that discrete lenses of anomalously high-salinity water, originating from the shelf edge and trapped within the seasonal pycnocline, are advected 100 km or more onto the Celtic Sea continental shelf. We propose that the lenses are created by increased diapycnal mixing at the shelf edge associated with breaking high-frequency internal wave packets. Quasi-synoptic hydrography sections show the lenses to be 3–5 km wide, their temporal persistence confirmed by moored instrumentation and a series of CTD casts. Estimates of the propagation speed of these features (∼0.020 m s−1) compare favorably with the magnitude of observed residual currents. Residual current variability within the pycnocline is dominated by vertical structures most consistent with the second baroclinic mode. The residual flow is therefore thought to be predominantly driven by non-linear second mode internal tidal waves. These are observations of a shelf edge exchange process not previously identified.