Analysis of Supercritical Stratified Tidal Flow in a Scottish Fjord

Abstract

Abstract The baroclinic tidal regime of the fjord Loch Etive (Scotland) is studied. Analysis is performed on the basis of both in situ data and numerical simulations, with the use of a fully nonlinear nonhydrostatic fine-resolution model. It was found that the crest of the sill that separates Loch Etive into inner and outer parts is subjected to a supercritical flow regime with maximum Froude numbers in excess of 5. Strong supercritical conditions lead to the formation of flow separation just above the sill’s crest. As is inherent to jet-type fjordic systems, this, in turn, leads to a weak nonlinear baroclinic wave response. On the other hand, observations and numerical results also revealed the presence of propagating internal tidal waves with amplitudes up to 10 m, several kilometers from the constriction. It is shown that these baroclinic tidal waves are excited during the ebb phase over a section of the inner flank of the sill, at a depth below 30 m, where the local Froude number is substantially less than unity. Thus Loch Etive simultaneously exhibits both weak nonlinear response due to strong supercritical conditions with flow separation over the sill and a significant linear baroclinic tidal response due to the deeper flank of the sill. In this respect, exhibiting both jet- and wave-type behavior, Loch Etive can be referred to as a “hybrid type” fjord: the authors suggest that many jet-type fjords will also generate a significant baroclinic response and should correctly fit into this new category.