Internal Kelvin Waves in a Broad, Mid-Latitude Fjord

Abstract

Broad embayment, such as some Fjords, can host the generation and propagation of internal Kelvin waves should the ratio of their Rossby radius to shelf-slope width be large. Here, we report on such a process in a broad, mid-latitude, fjord where the upper-layer dynamics are dominated by sub-inertial baroclinic motions. This fjord is of particular interest due to recent development of the aquaculture industry and to being historical fishing grounds. Using a fully non-linear, three-dimensional primitive equation numerical model we show that the large majority of the thermocline vertical fluctuations (80%) and much of the along-shore currents (50%) can be reproduced with a flat-bottom assumption. We use the model to determine the origin of the process, which is expressed by upwelling and downwelling disturbances traveling cyclonically around the bay. We find that the wind generates local and remote, from a large and deep neighboring bay, responses which travel as forced and freely propagating long, coastally trapped, internal waves into and around the fjord. The resulting dynamics within the fjord can therefore be complex, particularly in the fall season during which nearly continuous and directionally variable winds occur. Under such forcing conditions and given the irregularity of the regional coastline, signals combine to form temporally and spatially varying responses, enhancing or diminishing transient upwelling or downwelling and their associated along-shore current pulses.