Deep-Water Renewal in the Upper Basin of Loch Sunart, a Scottish Fjord

Abstract

Recording current meters were deployed near the surface and bottom in the upper basin of Loch Sunart during the summers of 1987, 1989, and 1990. The measurements revealed frequent, though irregular, deep-water renewal events when the basin water was replaced by density driven intrusions. The data suggested that these events were induced by wind-enhanced estuarine circulation during periods of low fresh water runoff. A semi-implicit, laterally integrated estuarine model was modified to allow variable vertical grid spacing and to permit the sea surface to move vertically through the grid, both modifications being necessary in order to apply the model to the sea loch environment. A turbulence closure scheme was also incorporated. The model can simulate the evolution of the salinity field over periods of several weeks and was used to examine the mechanisms that induce deep-water renewal. Model results suggested that very weak stratification in the basin is required for renewal to occur and indicated that recirculation of brackish surface water, caused by the very shallow sill, may increase the regularity of renewal by accelerating the rate of density reduction of the bottom water.