Deep Water Inflows to Upper Loch Linnhe

The frequency and nature of deep water inflows to Upper Loch Linnhe, a shallow-silled fjord on the west coast of Scotland have been elucidated by a study based on two current meter moorings and ship-mounted ADCP and CTD surveys. From November 1992 to June 1993, 11 inflows to the bottom of the loch are identified from changes in temperature, salinity and along-loch currents. Although inflows happen throughout the observational period, the most significant events occur between late February and early March. During such major events the deep water is completely replaced by water from the adjacent Loch Linnhe. Further deep water inflows occur in April to June with a sufficient cumulative effect to again replace the deep water of Upper Loch Linnhe. Over the winter (November to March), deep water inflows are of small volumes. Although each of these winter inflows is insufficient to replace the deep water, they do have a significant effect on the thermal structure of the loch. At times with no inflow, vertical mixing brings about significant changes in the deep water. Deep water inflows occur as flood tide pulses of denser water advected across the sill by the barotropic tidal exchange. The density of the incoming tidal flux and associated deep water inflows are correlated with low freshwater flows. Large tidal range and seaward blowing winds promote deep water inflows during periods of low freshwater input.