The linear and non-linear aspects of the internal tide in the NE Atlantic are discussed with particular reference to observations and modelling exercises conducted by the authors. Two important aspects of the internal tide are the generation of non-linear solitary internal waves in the thermocline and the enhancement of tidal currents at the sea-bed near the head of the shelf. Both are being studied using a combination of observations and linear / non-linear modelling. The three-dimensional nature of the internal tide is discussed from both a theoretical point of view and using a three-dimensional primitive equation model.
Internal tides are internal waves with tidal period that are found in many shelf edge regions, particularly in the north-east Atlantic where the barotropic tide (i.e. the surface tide, which has uniform currents with depth) are relatively large. Their generation is attributed to the interaction between bottom topography and stratification of the water column in the presence of barotropic tidal currents. They are associated with intensified currents at the sea bed, enhanced internal mixing at the shelf edge and non-linear solitary internal waves (SIWs) which can cause large depressions of the thermocline and strong currents over short periods of time. Knowledge of SIWs is important to the oil industry, prospecting for and extracting oil at the shelf edge, because the current surges can cause costly disruption to drilling operations (e.g. Osborne and Burch, 1980). They also have a significant impact on acoustic propagation and mixing of the thermocline at the shelf break. There is an extensive literature on the subject of internal tides, but it is considered inappropriate to attempt to summarise it here. A detailed discussion on other processes at the shelf edge is given in the comprehensive review by Huthnance (1995). The present paper aims to describe our own observational and modelling investigations of the internal tide in the NE Atlantic, ranging from the Iceland-Faeroes Ridge to the Portuguese shelf (Fig. 1).
