Net circulation and salinity variations in an open-ended Swedish fjord system

Abstract

Water circulation and salinity variations above sill levels in the Orust Tjöm fjords on the Swedish west coast, are investigated by means of hydrographic observations and a process-oriented box model. This fjord system, which is situated in the outer part of the Baltic estuary, consists of several basins separated by a number of sills and narrow passages. In contrast to most fjords, it is connected to the sea at both ends. Current measurements at different locations indicate a net, counterclockwise (mainly northward) circulation with an average of about 100 m³ s⁻¹. Model results and analyses of observations show that this net subtidal through-flow is primarily forced by the difference in steric height between the open ends. Salinity variations in the coastal waters give rise to density driven currents, mainly from the south which dominate the fjord exchange at large. The subtidal net circulation may be important for the properties in the northern parts of the system, particularly during periods of reversal. Validation against measurements shows that the model captures variations in the net circulation, as well as variations of the basin salinities above sill level. A seven-year model run, using monthly hydrographic data as forcing, yielded that the net circulation was counterclockwise during 81% of the time with a long-term average flow of about 70 m³ s⁻¹. Tidal choking in the narrow, northern end increases the flow resistance substantially, thereby decreasing the net through-flow.