Lesson 2: Internal waves

Internal waves (IWs) are of interest to oceanographers and offshore engineers for a variety of reasons. The strong circulatory currents associated with the wave movement may have profound effects on off-shore structures. IWs represent an important mechanism for mixing surface and deeper water, and for the transport of organisms, sediments and pollutants. The mixing and the vertical displacement of the thermocline associated with IWs may also influence ocean productivity in areas of IW activity.

Internal waves propagate along the interface separating waters of different densities, usually where a layer of warmer water lies over a layer of colder water (although differences in salinity may also give rise to the density difference). In thermally stratified waters the thermocline, where the temperature changes rapidly with depth, separates warmer surface waters from the colder deeper water. Internal waves may be observed as undulations in the thermocline. They are similar in many ways to surface waves, except that their scale is generally larger, with wavelengths often of the order of km and amplitudes as much as 100s of metres. They are generated at depths of 10s to 100s of metres when tidal currents flow over areas with variable bathymetry, such as sea mounts or the continental shelf break. The water motion associated with the IWs affects the surface, particularly the distribution of short surface waves and surface films, even though the IWs themselves are generated and propagate many metres below the surface.

The IW characteristics (wavelength, speed, and amplitude) depend on the density differences within the water column. Measurements of the IWs from satellite sensors can thus provide a source of information on the density variations near the thermocline.

Synthetic Aperture Radar (SAR) instruments have become the most important remote sensors for IW detection. The radar detects the surface roughness patterns associated with the IW. Whilst SAR is a long established remote sensing tool for the study of IWs, less is known about their appearance in optical images. However, the surface roughness patterns can also affect optical wavelengths, and ocean colour sensors such as MERIS, which combine increased radiometric sensitivity with greater spatial resolution, make internal wave signatures increasingly evident also in optical data.

This lesson on internal waves falls naturally into two main parts: A general introduction to the use of SAR to study internal waves, and a taster of more recently developed techniques to study internal wave signatures in optical data.

I - Using SAR to study Internal Waves

The first part of the lesson introduces you to IWs, their appearance in SAR images, and some tools for studying their properties. You will look at two case studies:

1. Internal waves off Mozambique
2. Internal waves in South China Sea
3. Internal waves in the Gulf of Cadiz off Spain