AUV observations of mixing in the tidal outflow from a Scottish sea loch

Freshwater outflows from estuarine systems carry nutrients, sediments, larvae and other materials into the coastal ocean where they mix into ambient ocean water. Loch Etive is one such estuary on the west coast of Scotland that is fed by a large rainfall catchment area, and from which the outflow is strongly modulated on a tidal basis. Freshwater leaves the loch on the ebbing tide in a pulse released into the coastal ocean as a thin layer, the leading edge of which develops into an undular bore or a train of internal solitary waves with strong mixing and associated entrainment. A Hydroid 600 m depth-rated AUV equipped with a forward-mounted microstructure sensing package has been used in conjunction with moored measurements to study the dynamics of thin freshwater layers that are regularly released into the coastal ocean. During observations conducted in winter 2009 and spring 2010 we observed turbulent bores and internal solitary waves in Ardmucknish Bay, near the entrance to Loch Etive. The passage of the bores and internal solitary waves of depression were associated with downward vertical movement of near-surface water and turbulent mixing that was enhanced above background by over 2 orders of magnitude.