Internal hydraulics, solitons and associated mixing in a stratified sound

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Abstract
Observations of tidally forced flow in a constricted and sill‐like region of a highly stratified sound are examined as a problem of two‐layer hydraulic exchange. Based on the current and salinity data, time and space dependent hydraulic conditions along the sound are discussed. It will be shown that the sound is subject to internal hydraulic controls at several locations, which account for the generation of strong internal hydraulic bores and drops in the upper layer. Internal solitary waves with 3–6 min period were found in the upstream propagating bore. Using results from different models of the solitary wave based on the Korteweg‐de Vries (KdV) equation, it is shown that the second‐order nonlinear term must be included in the two‐layer model. Results from a first‐order continuously stratified model, solved using the lowest mode eigenfunction, gave similar results as the second order two‐layer model. This implies that two‐layer models may ignore some properties of the real fluid and that internal solitons are also sensitive to the stratification characteristics of the water column. The vertical velocity shear, hydraulic characteristics of the flow and the presence of solitary waves were major contributors to vertical mixing in the sound.