Evidence of tidal straining in well-mixed channel flow from micro-structure observations

Abstract

This study presents, for the first time, micro-structure observations in tidally energetic, weakly stratified regimes obtained in the Wadden Sea, a tidal shallow coastal area in the South-Eastern North Sea characterised by barrier islands separated by tidal gulleys. The tidal currents are typically overlaid by a weak horizontal density gradient due to freshwater run-off from land. The observations in an energetic tidal channel clearly show the expected effects of tidal straining: destratification during flood and increased stratification during ebb. Microstructure observations are consistent with the tidal straining dynamics: during the flood relatively high values of viscous dissipation are observed whilst during ebb the values are substantially smaller. It is also shown that the tidal cycle of stratification and destratification depends on the position in the tidal channel. In parts of the channel, increased stratification occurs during full flood, a phenomenon which can only be explained by advection of stratified water masses formed outside the tidal channel. The observations presented here show the general significance of the tidal straining process for tidally energetic weakly stratified regimes. Copyright 2011 by the American Geophysical Union.