Late winter response to atmospheric and tidal forcing in the Førdefjord, inferred from mooring observations

Abstract

The Førdefjord is a narrow fjord on the western coast of Norway. A shallow sill is situated at the Ålasund, in the centre part of the fjord. A deep basin, 3 km north of the sill, has been approved as a deposit site for mine tailings from the Engebø mountain. The late winter conditions in the Førdefjord are investigated using data from five moorings covering the period from February 3 to March 5, 2017, in addition to vertical profiles of hydrography and microstructure obtained during the deployment cruise (February 2-10, 2017). The response of the Førdefjord to atmospheric and tidal forcing is discussed, with a special focus on the sill region and the future deposit site. The Førdefjord has a thin and fresh, dynamic surface layer with a deep, quiescent layer below. The currents in the fjord are generally weak, with the strongest currents recorded during spring tides at the surface, and near the Ålasund sill. The currents are mainly directed along the fjord. Tides dominate the current variability in the fjord, and the semidiurnal M_2 tide is the dominating constituent. In late winter, the Førdefjord is classified as a wave fjord. Semidiurnal internal tides are generated at the Ålasund sill, in addition to overtides of higher harmonics, such as the M_4 and M_6 tides. Synoptic sections collected across the sill show high dissipation rates and formation of hydraulic jumps, indicating occurrence of critical conditions during floods in spring tides. Northward winds together with changing stratification at the coast, were observed to lead to non-local effects into the Førdefjord, including an intermediate layer cold water intrusion. The tongue of cold water reaches the Ålasund sill region, contributing to the stratification variability in the inner fjord. The intermediate layer in the outer part of the Førdefjord is regularly ventilated during winter. Near the bottom, at the suggested deposit site, the currents are weak and directed out-fjord. However, large vertical isothermal displacements are observed during neap tides, and are consistent with an internal seiche event. Longer period observations, covering all seasons, are needed to quantify the possible effect of seiches, or the influence of sill processes on the deposit site.