Deep water renewal of Loch Etive: A three basin Scottish fjord

Abstract

Loch Etive is a fjord with three basins. Innermost bottom water stagnates for months or years, with slowly changing temperature and salinity, falling deep oxygen concentration and with a secondary pycnocline below sill depth. A bottom water renewal is described and shown to be caused by low freshwater runoff. The renewal is a series of overflows of sill water during spring flood tides. During overflow, dense fluid forms a turbulent plume whose observed behaviour is similar to that expected from theory: an entrainment constant of 0.013 is found on a bottom slope of 6 °. Observations of salinity in the sill region show that the flushing time there is 4 weeks and that this is the response time of the renewal mechanism to changes in runoff. A model of the non-linear dependence of renewal upon runoff is made and used to hindcast during 1964–1975. The hindcast is verified and shows that renewal is aperiodic with a mean repetition time of 1 1 3 years.