The Internal Seiches in Gullmar Fjord. Part II: Contribution to Basin Water Mixing

Abstract

The mixing in the basin water (the water below sill level) of Gullmar Fjord has been investigated with the main focus on the contribution from internal seiches. A companion paper reports evidence for dissipation of internal seiche energy in the basin water after near-critical reflection from the bottom. In the present paper the magnitude and variation of basin water mixing is investigated, using the budget method. The results are related to variations of three energy sources, namely (i) the internal seiches, (ii) the internal tides, and (iii) the internal waves generated by the external seiche. The mixing efficiency, defined as the irreversible work against buoyancy forces due to mixing divided by the total mechanical energy loss from the sources mentioned above, is about 7%, similar to results obtained for other fjords. Large variations in mixing are shown to be related to large variations of the energy sources. The internal seiches are found to be important for the mixing, with a contribution that is 144% of the internal tide contribution during the most energetic period and 92% on average over the investigated periods. Including contributions from the external seiche, the wind forcing is responsible for 61% of the basin water mixing, while tidal forcing is responsible for 39%.