Structure, seasonal development and sunglint spatial coherence of the internal tide on the Celtic and Armorican shelves and in the Bay of Biscay

The generation and propagation of internal tides from the shelf break in the Bay of Biscay is now a well-documented process, but a description of the spatial coherence of the internal tides has so far been impossible with conventional in situ observations. This paper first analyses the shelf measurements of internal tides and shows that by studying available remotely sensed images over a number of years, particularly in the visible band (which we term “sunglint” images), it is possible to gain significant insight into the spatial coverage, long-crestedness, and seasonal development of these features. The sunglint images provide a synoptic description of the internal tides, and show that they may occur up to 250 km onshelf from the shelf break with coherent crests extending over 400 km in the along shelf direction. The oceanic signal was observed to extend from the shelf break right across the Bay of Biscay (∼300 km). The images allow the tidal wavelengths to be reliably estimated both onshelf and offshelf from the shelf break, without complications arising from advection by the barotropic tide which occur when in situ measurements are made. A strong seasonal signal was found which results from the development of the stratification in the upper water column. By approximating observed temperature profiles, a simple two-layer model is developed for the onshelf waves which provides a relationship between the tidal wavelengths, and the thickness of and the temperature difference across, the upper layer. It is possible to use this relationship in combination with remote sensed images, providing sea-surface temperature and tidal wavelength, to infer the depth of the upper layer, which generally increases as the stratification develops, so allowing a simple method for estimating the stratification in the upper water column. The effect of nonlinearity is important in determining wave structure but has only a small effect on phase speed or wavelength in the presence of Earthʹs rotation.