Control of a fjord basin's dynamics by tidal mixing in embracing sill zones

Abstract

A common configuration in fjords is a basin embraced by sills. This paper addresses the dynamics of a large fjord basin lying between comparatively deep sills where tidal mixing is vigorous: Puget Sound's Main Basin (∼ 10 km × 90 km × 0·3 km) between 44 m (landward) and 66 m (seaward) depth sills. Tidal action over the sills causes a vigorous two-layer circulation in the basin where no net motion occurs near the average depth of the embracing sills. On the flood tide lower-layer water is upwelled at the landward sill and upper-layer water is downwelled at the seaward sill. The resultant circulation in the basin is quite active at all depths throughout the year. Currents computed from differences in dissolved oxygen between hydrographic stations compare favorably with measured currents. Based on differences between the ends of the basin, the bulk residence time in the lower layer is about three weeks—a short time for a large fjord basin. Experiments in a hydraulic model demonstrate the marked sensitivity of basin circulation to tidal action in the sill zones: transport in the upper layer is directly proportional to the tidal prism inland of the landward sill zone, and there is an exponential type of response to abrupt changes in fresh-water flowing into the seaward sill zone (approximately 60% of equilibrium attained in 2 months). The rapid response causes the basin's lower-layer water properties to follow closely both the primary and secondary features of the seasonal cycles of local air temperature and runoff.