J.M. Magalhaes

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.

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 EUFAR transnational access project A.NEW (Airborne observations of Nonlinear Evolution of internal Waves generated by internal tidal beams)

Internal Solitary Waves (ISWs) are ubiquitous features in the coastal oceans. The propagation and breaking of ISWs contribute significantly to turbulent mixing in the near-surface layers, through the continual triggering of instabilities as they propagate into shallow water over the continental shelf. We report on the first results of an EU funded project denominated A.NEW (Airborne observations of Nonlinear Evolution of internal Waves generated by internal tidal beams). Here we show, for the first time, coincident multi-sensor airborne observations off the Portuguese coast, which reveal the 3D structure of air bubble entrainment in the internal wave field. Coincident thermal infrared imaging shows the surface thermal signatures of ISWs and reveals the turbulent character of some internal waves on the shelf.

Internal Solitary Waves in the Andaman Sea: New Insights from SAR Imagery

The Andaman Sea in the Indian Ocean has been a classical study region for Internal Solitary Waves (ISWs) for several decades. Papers such as Osborne and Burch (1980) usually describe mode-1 packets of ISWs propagating eastwards, separated by distances of around 100 km. In this paper, we report on shorter period solitary-like waves that are consistent with a mode-2 vertical structure, which are observed along the Ten Degree Channel, and propagate side-by-side the usual large mode-1 solitary wave packets. The mode-2 waves are identified in TerraSAR-X images because of their distinct surface signatures, which are reversed when compared to those that are typical of mode-1 ISWs in the ocean. These newly observed regularly-spaced packets of ISW-like waves are characterized by average separations of roughly 30 km, which are far from the nominal mode-1 or even the mode-2 internal tidal wavelengths. On some occasions, five consecutive and regularly spaced mode-2 ISW-like wave envelopes were observed simultaneously in the same TerraSAR-X image. This fact points to a tidal generation mechanism somewhere in the west shallow ridges, south of the Nicobar Islands. Furthermore, it implies that unusually long-lived mode-2 waves can be found throughout the majority of the fortnightly tidal cycle. Ray tracing techniques are used to identify internal tidal beams as a possible explanation for the generation of the mode-2 solitary-like waves when the internal tidal beam interacts with the ocean pycnocline. Linear theory suggests that resonant coupling with long internal waves of higher-mode could explain the longevity of the mode-2 waves, which propagate for more than 100 km. Owing to their small-scale dimensions, the mode-2 waves may have been overlooked in previous remote sensing images. The enhanced radiometric resolution of the TerraSAR-X, alongside its wide coverage and detailed spatial resolutions, make it an ideal observational tool for the present study.

SAR Mode Altimetry Observations of Internal Solitary Waves in the Tropical Ocean Part 1: Case Studies

It is well known that internal waves (IWs) of tidal frequency (i.e., internal tides) are successfully detected in sea surface height (SSH) by satellite altimetry. Shorter period internal solitary waves (ISWs), whose periods (and spatial scales) are an order of magnitude smaller than tidal internal waves, have been generally assumed too small to be detected with conventional altimeters. This is because conventional (pulse-limited) radar altimeter footprints are somewhat larger than or of similar size, at best, as the typical wavelengths of the ISWs. Here we demonstrate that the synthetic aperture radar altimeter (SRAL) on board the Sentinel-3A can detect short-period ISWs. A variety of signatures owing to the surface manifestations of the ISWs are apparent in the SRAL Level-2 products over the ocean. These signatures are identified in several geophysical parameters, such as radar backscatter (sigma0), sea level anomaly (SLA), and significant wave height (SWH). Radar backscatter is the primary parameter in which ISWs can be identified owing to the measurable sea surface roughness perturbations in the along-track sharpened SRAL footprint. The SRAL footprint is sufficiently small to capture radar power fluctuations over successive wave crests and troughs, which produce rough and slick surface patterns arrayed in parallel bands with scales of a few kilometers. The ISW signatures are unambiguously identified in the SRAL because of the exact synergy with OLCI (Ocean Land Colour Imager) images, which in cloud-free conditions allow clear identification of the ISWs in the sunglint OLCI images. We show that both sigma0 and SLA yield realistic estimates for routine observation of ISWs with the SRAL, which is a significant improvement from previous observations recently reported for conventional pulse-limited altimeters (Jason-2). Several case studies of ISW signatures are interpreted in light of our knowledge of radar backscatter in the internal wave field. An analysis is presented for the tropical Atlantic Ocean off the Amazon shelf to infer the frequency of the phenomena, being consistent with previous satellite observations in the study region.

Internal Solitary Waves System in the Mozambique Channel

Satellite images of the Mozambique Channel (MC), collected by Synthetic Aperture Radars (SARs) from the Envisat and European Remote Sensing (ERS) satellites, reveal sea surface signatures of oceanic Internal Solitary Waves (ISWs). The MC has been expected to be a major hotspot for the generation of Internal Tides (ITs) in the ocean. New results now show that the Sofala bank (or shelf, located near 20°S, 36°E) is indeed a strong and previously unknown source for the generation of ITs and ISWs. The hotspot region is located between 20 and 21°S, where the barotropic tidal forcing and tidal transports are highest in the area, due to the particular nature of the bathymetry there. Two distinct types of ISWs are observed to travel offshore, away from the shelf-break, and their possible generation mechanisms are discussed. Mode 2 ISWs propagating in MC are also observed in SAR imagery. It is discussed that they may be generated by a tidal beam impinging the thermocline from below, in a similar process to some of the Mode 1 ISWs generated off the Sofala bank. ISW refraction patterns, often observed in SAR imagery in MC are presented, and a particular example is shown where ISWs and eddy-like features are seen to interact. The influence of these rotating features in the refraction patterns is briefly discussed. Finally, it is shown that large-scale atmospheric gravity waves of solitary-like form are also frequently observed in the region, and thus care must be taken when discriminating oceanic and atmospheric ISW signatures in SAR images, if a correct interpretation is to be made.