Boils and Turbulence in a Weakly Stratified Shallow Tidal Sea

Abstract

Abstract Measurements of turbulence are made in a weakly but variably stratified region of tidal straining in the eastern Irish Sea using turbulence sensors profiling vertically through the water column on the Fast Light Yo-yo (FLY) profiler and horizontally on the Autonomous Underwater Vehicle (AUV) Autosub. The tidal currents exceed 1 m s−1 at the location of the measurements in water of a depth of about 43.5 m, and result in turbulence extending from the seabed to the surface with a cycle period that is half that of the tides, as previously observed. The time of onset of enhanced turbulence that is measured by the sensors on FLY and Autosub as the speed of tidal currents increases are in good agreement, as are their mean levels. Boils on the sea surface are identified using the Autonomously Recording Inverted Echo Sounder, version 2 (ARIES II), a two-beam upward-pointing side-scan sonar mounted on a rig resting on the seabed. The boils have mean horizontal dimensions of about 25 m. They are continually present when turbulence within the water column near the surface is large, typically when the rate of dissipation of turbulent kinetic energy per unit mass ɛ exceeds about 3 × 10−6 W kg−1, compared to the rate of about 1 × 10−7 W kg−1 near times of slack tide when no boils are observed. The top of a region with a relatively high ɛ gradually extends upward from the seabed as the tidal flow increases and, in stably stratified conditions resulting from tidal straining during ebb tides, can be detected in the ARIES II sonographs as it approaches the sea surface, although only after sound reflection from the sea surface. Upward-moving bursts of enhanced turbulence below the surface with horizontal dimensions of about 5–9 m in the direction of the tidal flow are identified in the AUV record in periods of high tidal flow using conditional sampling. The bursts result in slight upward displacements of the AUV from its set operational depth. Boils first appear at times before the region of generally enhanced turbulence reaches the surface: large localized upward-moving bursts appear to precede the main advancing region of small-scale turbulence.