Observations of intratidal variability of flows over a sill/contraction combination in a Chilean fjord

Underway velocity measurements were carried out for the first time in a Chilean fjord using an acoustic Doppler velocimeter with the purpose of elucidating the intratidal variability of flows through a pass, Paso Galvarino. The pass included a sill, where the bottom sloped by roughly 30%, and a coastline contraction of ∼90%. The relatively small dimensions of the pass allowed for rapid sampling of the flow evolution throughout the tidal cycle. The backscattered sound signal from the velocimeter and from an echo sounder were used to describe the vertical excursions of the pycnocline throughout the domain and to identify regions of enhanced vertical mixing within the pass. The spatial variability of the flow in the pass was consistent with uniform two‐layer flow. At the narrowest section of the contraction the pycnocline dropped sharply around both maximum flood and maximum ebb, while the flow accelerated downstream relative to the tidal flow. The slope of the pycnocline changed sign from flood to ebb, which was atypical of other fjord observations but could be explained by the transitions from subcritical to supercritical flow. These transitions switched location at either side of the narrowest section of the contraction. Leeward of this section, increased sound backscatter suggested intensified turbulence that extended over a greater area during ebb than during flood because the distance between the point of pycnocline drop and the end of the pass was longer during ebb. Enhanced vertical mixing within the pass was reflected in the tidally averaged fields by a three‐layer flow that consisted of near‐surface and near‐bottom flow converging toward the pass and flow around the pycnocline diverging away from the pass.