The Internal Seiches in Gullmar Fjord. Part I: Dynamics

Abstract

Internal seiche motions with period 1?3 days in Gullmar Fjord are investigated based on mooring data with high vertical and time resolution. The period and structure of the internal seiches are well described by a simple analytical three-layer model taking into account blocking of the basin water at the sill. Energy budgets for forcing and damping of the seiches are estimated. The internal seiches are forced by direct wind stress on the surface and by internal coastal waves of frequency close to the seiches, the latter contribution being a little larger. The e-folding timescale for the internal seiche damping is approximately equal to the seiche periods, which means that the seiches are effectively damped, but also that the forcing must be in near resonance in order to maintain the observed seiche motions. In the basin water the seiches manifest themselves with large vertical motions and clear upward phase propagation. The phase propagation is related with a continuous, rather than strictly layered stratification, and can therefore not be described with the three-layer model. It is proposed that the downward energy radiation inherent in the upward phase propagation is caused by removal of energy in the basin water, probably due to dissipation after near-critical reflection from the sloping fjord bottom. The energy flux related to this process is only 2% of the total seiche damping, and therefore not important for the seiche dynamics, which are dominated by the pycnocline movements. In a companion paper, however, it is shown that this energy flux is an important contribution to the increase of potential energy in the basin water due to mixing.