Rd. Pingree et Al. New, STRUCTURE, SEASONAL DEVELOPMENT AND SUNGLINT SPATIAL COHERENCE OF THEINTERNAL TIDE ON THE CELTIC AND ARMORICAN SHELVES AND IN THE BAY OF BISCAY, Deep-sea research. Part 1. Oceanographic research papers, 42(2), 1995, pp. 245-284
The generation and propagation of internal tides from the shelf break
in the Bay of Biscay is now a well-documented process, but a descripti
on of the spatial coherence of the internal tides has so far been impo
ssible with conventional in situ observations. This paper first analys
es 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 possib
le to gain significant insight into the spatial coverage, long-crested
ness, and seasonal development of these features. The sunglint images
provide a synoptic description of the internal tides, and show that th
ey may occur up to 250 km onshelf from the shelf break with coherent c
rests extending over 400 km in the along shelf direction. The oceanic
signal was observed to extend from the shelf break right across the Ba
y of Biscay (approximately 300 km). The images allow the tidal wavelen
gths 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 seasona
l signal was found which results from the development of the stratific
ation 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. I
t is possible to use this relationship in combination with remote sens
ed images, providing sea-surface temperature and tidal wavelength, to
infer the depth of the upper layer, which generally increases as the s
tratification 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 Earth's rotation.