J.C.B. da Silva

Role of surface films in ERS SAR signatures of internal waves on the shelf: 1. Short‐period internal waves

Analysis of synthetic aperture radar (SAR) images of internal waves (IWs) on the Iberian shelf and classification of the IW surface signatures is presented. There are three types of IW signatures in the form of bright/dark, dark, and bright bands that correspond to positive/negative, negative, or positive variations of radar backscatter, respectively. Strong positive sign IW signatures occur at very low wind velocities (less than 2m/s). At winds higher than 2 m/s both positive/negative and negative signatures are observed for range-propagating IWs. For azimuth-propagating IWs, negative signatures prevail. Two groups of experiments carried out in the frame of the Multidisciplinary Oceanographic Research in the Eastern North Atlantic (MORENA) project on the Iberian shelf in August 1994 are described: (1) IW measurements simultaneous with ERS-1 SAR overpasses and (2) IW measurements, visual observations of the surface, and sampling of films from the sea surface. IW manifestations at low to moderate winds took the form of slicks locating over IW troughs; at the near-threshold wind velocity they were of the form of intensified decimeter-scale waves (antislicks) located over IW crests. Measurements of wave damping due to films collected from the slick and nonslick areas showed the concentration of surfactants and retrieved film elasticity to be higher in the slicks than in the nonslick areas. A theoretical model of the surface wave modulation by IWs is developed to include surface wave straining by the IW current and surface wave damping due to surfactant films. Pressure-area curves for real marine films are used in the model. The relation between the film and the straining effects on the surface wave spectrum in the IW field is shown to depend on film parameters, surface wave-length, and wind velocity. The model predicts strong damping of centimeter-scale waves over IW troughs mainly due to the film effect and intensification and depression of decimeter-scale waves due to both straining and film effects. Dependencies of the model spectrum variations as a function of wind velocity for range and azimuth propagating IWs are obtained, the negative sign contrast being shown to predominate for azimuth propagating IWs. Model variations of the spectrum of decimeter-scale waves are shown to be significantly sensitive to surfactants (film pressure), the negative sign contrasts due to films being superimposed on the straining effects, and these are eventually capable of suppressing the positive sign contrasts. It is concluded that the occurrence of the different classes of IW signatures in the SAR images can be explained by the action of surface-active films.

Remote-sensing evidence for the local generation of internal soliton packets in the central Bay of Biscay

Large-amplitude internal solitary waves (or “solitons”) occurring in packets near the shelf break in the Bay of Biscay are well-documented and understood. The presence of similar features has now also been reported in the central Bay, ≈150 km from the nearest shelf break topography. The present paper analyses available remote-sensing synthetic aperture radar (SAR) data from the ERS satellites in this region. By doing so, we are able to provide convincing support for the hypothesis that these waves, instead of having travelled along the thermocline from the shelf break, are instead generated locally in the central Bay by the surfacing of a beam of internal tidal energy originating from the shelf break. This reinforces the results of a previous independent study, while at the same time providing a much more extensive investigation than was then possible. We have also exploited the large swath width (100 km) and high spatial resolution (100 m×100 m) of the SAR to examine for the first time the full surface structure of the internal waves in the central Bay, which are found to have a mean wavelength of 1.35 km, and a mean along-crest “coherence” length of 21.55 km.

Internal solitary waves in the Mozambique Channel: Observations and interpretation

This paper presents new results showing that the Sofala shelf in the Mozambique Channel (20°S, 36°E) is a previously unknown “hot spot” for the generation of internal tides and internal waves. We investigate available Envisat advanced synthetic aperture radar imagery of the region, which is capable of showing the surface signatures of the internal waves. This is complemented by modeling of the ray pathways of internal tidal energy propagation, and of the P. G. Baines (1982) barotropic body force, which drives the generation of internal tides near the shelf break. The hot spot region is localized between 20° and 21°S because of the particular nature of the bathymetry there. Farther north and south, the forcing is reduced and insufficient to generate internal solitary waves in the synthetic aperture radar images. The analysis reveals two distinct types of internal wave trains that are observed traveling oceanward away from the shelf break, and we suggest that these result from direct generation at the shelf break and from “local” generation at about 80 km from the shelf break, respectively, because of the surfacing of internal tidal rays at the thermocline. Finally, we have investigated seasonal differences in the wave patterns, which penetrate more extensively into the channel during the southern summer and appear slightly farther to the south during the southern winter. We also conclude that the local generation process is more likely to occur during the winter when the stratification is reduced.

On the observability of internal tidal waves in remotely‐sensed ocean colour data

This paper reveals bands of enhanced levels of near-surface chlorophyll in the central Bay of Biscay in remotely-sensed images from the SeaWiFS ocean colour sensor, and shows that these are most probably associated with the crests of internal tidal waves travelling away from the shelf break. We are able to explain this effect as likely to result from the uplifting of a subsurface chlorophyll maximum by the passing internal tides, to such a level as may be “seen” by the satellite sensor, and have quantified the effect with a simple model. We believe this is the first time that internal tidal waves have been reported as having a direct and observable effect in such remotely-sensed ocean colour images.

Role of surface films in ERS SAR signatures of internal waves on the shelf: 2. Internal tidal waves

Analysis of ERS-1 synthetic aperture radar (SAR) images over the Iberian shelf shows that they contain the signatures of large-scale banded structures with characteristic wavelengths of 10–20 km, parallel to the shelf break. Their consistency with the structure of internal tides leads to the conclusion that they are the surface manifestations of internal tidal waves. The effect of imaging of such large-scale phenomena by SAR operating in L, C and X bands cannot be explained by conventional hydrodynamic modulation models based on the effect of straining of surface wind waves due to the internal tidal currents. An alternative explanation is described by an analysis of modulation of centimeter- and decimeter-scale surface waves caused by the redistribution of surface-active substances by internal tidal currents. Model calculations of variations of the wind wave spectrum across the internal tide profile are presented; an empirical film pressure-area equation and the results of thermistor chain measurements of internal tidal waves in the study area were used in the modeling. The effect of additional modulation of short surface waves due to variations of wind velocity relative to the internal tidal current is also estimated. It is demonstrated that modulation of centimeter-decimeter wind waves in the presence of surfactant films is the dominant factor and can explain the effect of internal tidal wave imaging by SAR.

Role of surface films in ERS SAR signatures of internal waves on the shelf 3. Mode transitions

It was shown recently that three different types of short-period internal wave (IW) synthetic aperture radar (SAR) signatures occur on the Iberian Shelf depending on parameters of surface active films. Large-scale modulation of film characteristics by currents associated with internal tidal waves can also produce a signature in ERS SAR images. On the western Iberian Shelf short-period IWs typically propagate in groups of packets that are believed to be generated by the evolution of internal tidal waves over the shelf. Here onshore propagating short-period IWs can exhibit different SAR signatures within the same wave packet according to their position relative to the phase of the internal tidal wave. These transitions of signature mode can be explained by large-scale variations of film parameters produced by the internal tidal waves. At low-to-moderate winds, leading waves in the IW packets usually exhibit double sign signatures, which evolve to negative sign signatures for the subsequent waves in the packets. At very low, near-threshold winds the IW signatures can be in the form of the double sign type undergoing transition to positive sign signatures when compared to the background behind the IW packet. This is a consequence of different threshold wind speeds for excitation of Bragg waves at different film pressures. An increase of unperturbed film pressure in the direction of the ranking order of solitons in a packet of short-period IWs, which is predicted by a theoretical model and in agreement with measurements of film elasticity, is demonstrated. Observations of SAR signature mode transitions are concluded to be consistent with previous model results, and a diagram describing mode transitions of the IW signatures in relation to film pressure and wind speed is presented.

Remote sensing of water quality parameters over Alqueva Reservoir in the south of Portugal

In this study, the potential of MEdium Resolution Imaging Spectrometer (MERIS) to describe variations of optically active substances over Alqueva artificial lake is investigated. Limnological laboratory analyses of the water samples collected monthly, from 2003 to 2006, are used in combination with MERIS. The water surface spectral reflectance is derived from Level1b MERIS data, using radiative transfer calculations to account for the atmospheric effects. The lake water spectral surface reflectance is combined with laboratory analyses of cyanobacteria total densities as well as chlorophyll a concentrations and empirical algorithms for both quantities are derived. The results obtained are compared with independent laboratory analyses from 2007, with good correlation coefficients obtained both for cyanobacteria (R = 0.93) and chlorophyll a(R = 0.80). The methodology proposed here has been developed to inexpensively monitor Alqueva Reservoir water quality in terms of cyanobacteria and chlorophyll a on a regular basis, and to provide useful information to the authorities.

On the role of wind direction in ERS SAR signatures of internal waves on the Iberian shelf

Results of an investigation of ERS SAR signatures of internal waves (IWs) on the Western Iberian shelf are shown. The mode of signature is defined by a parameter that evaluates the IW intensity profiles in relation to the mean backscatter of the image background. The signature mode parameter is found to be correlated with the angle between the IW propagation direction and the wind velocity. A simple model which takes into account modulation of short-scale surface waves by relative wind velocity to the surface current generated by the IWs is considered and was found to be in good qualitative agreement with the experimental results.

Satellite Altimetry Observations of Large-Scale Internal Solitary Waves

High sampling rate altimetry and near-simultaneous synthetic aperture radar image data reveal short-period oscillations, which are identified for the first time as internal solitary waves. Their rough and slick patterns introduce mixed contributions in the altimeter’s footprint, contradicting the assumption of a uniform Brown surface. The resulting geophysical parameters are significantly changed in the waves’ surroundings, yielding unrealistic estimates when compared with the unperturbed background. A statistical analysis is presented for the South China Sea to infer the frequency of the phenomena, whereas possible implications are briefly discussed in the framework of present and forthcoming altimeter missions.

The role of synergy in developing a marine SAR analysis and interpretation system

The development of a Marine SAR Analysis and Interpretation System (MARSAIS) for application in the coastal zone is the objective of a current project, funded by the European Union. The system is aimed at the non-expert SAR user and integrates several state-of-the-art algorithms and multiple sensor synergy for quantitative interpretation of SAR data. In this work we focus on the part of the project that is concerned with the combined interpretation of SAR images and data from other sensors, and the ways in which this synergy improves our understanding of SAR monitoring of coastal processes.