Synthetic Aperture Radar Observations of Internal Solitary Waves in the Southern Bay of Biscay

A. Azevedo, J. C. B. da Silva
Department of Physics and Institute of Oceanography, Faculdade de Ciências da Universidade de Lisboa
Rua Ernesto de Vasconcelos, Campo Grande, 1700 Lisboa, Portugal
jdasilva@fc.ul.pt

A. L. New
James Rennell Division for Ocean Circulation and Climate, Southampton Oceanography Centre
European Way, Southampton SO14 3ZH, UK.

ABSTRACT

Internal Waves have long been recognized as a classical mechanism for surface film formation. The surface current disturbances induced by internal wave propagation generates convergence and divergence patterns near the surface that can produce inhomogeneities in surface film concentration, if surfactant materials are present at the surface. The elastic properties of the surface film strongly change the damping characteristics of surface capillary waves, which produce a distinct internal wave signature in spaceborne radar images of the ocean. In the present paper, we analyse signatures of available synthetic aperture radar (SAR) images of the Bay of Biscay to evaluate if surface films play a role in the internal wave imaging mechanism. We report on a series of SAR observations and reveal that the southern Bay of Biscay is a “hotspot” region of internal wave generation, with a high level of Internal Solitary Wave (ISW) activity. In the southern Bay of Biscay, the ISWs travel towards the East-North-East from the Cape Finisterre region off North-West Spain. In fact, we reveal the presence of two sets of wave-trains travelling in slightly different directions (050° T and 060° T). By calculating the strength of the barotropic tidal forcing in the region, and identifying the likely propagation pathways (rays) of internal tidal energy, we identify the generation sites for these wave-trains as lying on either side of the Ortegal Promontory. This is an undersea “headland” projecting towards the Northwest from the northwestern-most coast of Spain (near 44° N, 8.5° W), and over which the barotropic tides are forced to flow. For each generation site, internal tidal rays emanating from “critical” (where the ray slope is equal to the topographic slope) deep topography, in regions of strong barotropic forcing, rise to the surface and pass through the thermocline close to the earliest occurrences of the ISWs in the respective wavetrains. These rays would then produce, through non-linear processes, the ISWs through the same “local generation” mechanism that has been used to explain the occurrence of the ISWs in the central Bay of Biscay. The methods we have used to deduce the generation sites for these waves are expected to prove equally useful for studies in other areas of the world’s oceans, in particular off the West coast of Portugal.

Last Update: 2005-03-16