Abstract
Oceanic internal waves occur within stratified water along the boundary between water layers of different density and are generated when strong tidal currents flow over seabed topography. Their amplitude can exceed 50 m and they transport energy over long distances and cause vertical mixing when the waves break. This study presents the first fully automated methodology for the mapping of internal waves using satellite synthetic aperture radar (SAR) data and applies this to explore their spatial and temporal distribution within UK shelf seas. The new algorithm includes enhanced edge detection and spatial processing to target the appearance of these features on satellite images. We acquired and processed over 7000 ENVISAT ASAR scenes covering the UK continental shelf between 2006 and 2012, to automatically generate detailed maps of internal waves. Monthly and annual internal wave climatology maps of the continental shelf were produced showing spatial and temporal variability, which can be used to predict where internal waves have the most impact on the seabed environment and ecology in UK shelf seas. These observations revealed correlations between the temporal patterns of internal waves and the seasons when the continental shelf waters were more stratified. The maps were validated using well-known seabed topographic features. Concentrations of internal waves were automatically identified at Wyville-Thomson Ridge in June 2008, at the continental shelf break to the east of Rosemary Bank in January 2010 and in the Faroe-Shetland Channel in June 2011. This new automated methodology has been shown to be robust for mapping internal waves using a large SAR dataset and is recommended for studies in other regions worldwide and for SAR data acquired by other sensors.
Highlights
Oceanic internal waves (IWs) usually develop in stratified water, in which pycnoclines form boundaries that separate water layers of different density
Based on the Canny algorithm, we have developed a methodology for automatic processing of ENVISAT advanced synthetic aperture radar (ASAR) sensor data and applied this methodology to investigate internal waves on the UK continental shelf (UKCS) in 2006–2012
This study describes two wave processes developing in this area: a tidal beam generated at the shelf break and the bottom trapped internal waves generated by the tidal flow over local seabed features
Summary
Oceanic internal waves (IWs) usually develop in stratified water, in which pycnoclines form boundaries that separate water layers of different density. IWs are common in continental shelf regions, and where brackish water overlies saltwater at the mouths of large rivers. IWs are responsible for transferring energy between the large-scale tides and small-scale mixing. On the continental shelf, when their energy reaches the seabed, the shear stress causes sediment disturbance and resuspension. Areas prone to such disturbance form an ideal habitat for certain benthic animals, and should be considered for protection when planning offshore developments such as renewable energy devices and oil and gas platforms
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