Abstract
We present an analysis of internal solitary waves (ISWs) on the SE Brazilian continental shelf using a set of Envisat/ASAR satellite images. For the 17-month observation period, 467 ISW packets were detected. Most of observed solitons were associated to 4–6 ms-1 wind. The number of ISW packets shows a seasonal signal with a peak in summer, with higher concentration in the outer shelf in all seasons, followed by midshelf during the summer. Propagation direction of ISWs was predominantly onshore with packets separated by typical M2 internal tide wavelengths (~10–40 km). The highest values of the barotropic tidal forcing F are concentrated at the shelf break between 200 and 500 m isobaths. These characteristics suggest that ISWs are formed from nonlinear disintegration of internal tides generated at the shelf break that propagate shoreward as interfacial internal waves. No significant change in the number of ISWs from spring to neap tides was observed in spite of significant tidal current variation (60%). Even not being a region of strong tides, this study shows that ISWs are a frequent and widespread feature, possibly playing a significant dynamic role, affecting biological production, sediment dispersion, and transport.
Highlights
Since the launch of SEASAT in 1978, numerous studies of oceanic internal solitary waves (ISWs) have been made using synthetic aperture radar (SAR) images
According to [7], for C band radar images, internal waves can still be seen as bright bands in a dark background in winds under 2 ms−1 due to a resonance mechanism of dm-waves with the ISW
Average wind speeds above 8 ms−1 are observed south of 28∘S. These wintertime higher winds are normally associated with the passage of cold fronts which brings S/SE average winds of 8 ms−1 [47]. These results indicate that typical wind speeds observed over the continental shelf at Southeast Continental Shelf (SECS) are adequate for SAR observations of ISWs, and except for extreme meteorological events or after the passage of strong cold fronts, wind should not be a limiting factor
Summary
Since the launch of SEASAT in 1978, numerous studies of oceanic internal solitary waves (ISWs) have been made using synthetic aperture radar (SAR) images. The availability of a great number of SAR satellite images has shown that ISWs are an ubiquitous oceanic phenomenon. They are frequently observed wherever tidal currents and stratification occur near significant seafloor topographic features, such as shelf break zones, plateaus, or sills [1]. In a pioneer work, [6] suggested as the main mechanism the hydrodynamic modulating effect of convergences/divergences of ISWs surface currents on the Bragg waves Another mechanism was proposed by [7, 8], who showed that in low wind speed regions (less than 5 ms−1), the ISW signatures in SAR images are strongly controlled by the effects of surface films or surfactants. [9] showed that wave breaking is a major mechanism leading to the formation of surface signatures of ISWs in SAR images
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