Internal waves and whitecaps

Abstract

There are numerous reports of internal waves being 'made visible' on the sea surface by their effect on the surface-wave field and the production of bands of steeper, often breaking, waves separated by zones of relatively calm water^1–5. The effect is sometimes quite dramatic. There are accounts of a 'low roar' as the bands of breaking waves, 'walls of white water', pass a vessel⁶. The bands are sometimes visible from aircraft⁷, on ships, radar^8,9 and are observed from satellites^10–15. In the Bay of Biscay 'boils' have been reported on the sea surface in the calm zones, and appear to be related to pulses of nutrients from the thermocline¹⁶. The position of the roughest water has been commonly reported to be above the part of the internal wave in which the thermocline is most rapidly falling^3,5 (as sketched in Fig. 1) except in very light winds (<3.5 m s⁻¹), when this appears generally to be the calmest zone¹⁶. There are, however, few simultaneous observations of surface and internal waves and none, to our knowledge, which has both quantified the frequency of the breaking waves and established their position relative to the internal waves. Here we have used an upward-pointing side-scan sonar to obtain a sub-surface view of bands of rough water and whitecaps on the sea surface associated with internal waves travelling on a pycnocline. The sonar provides a means to measure the surface currents induced by the internal waves, the position, or phase, of the surface-wave breaking relative to the internal waves, and to quantify the frequency of wave breaking. The effect of wave breaking caused by wave–current interaction is to transfer momentum from the surface waves to the currents, with a phase which tends to diminish the internal waves in the cases studied. The enhanced turbulence caused by wave breaking may be effective in mixing or eroding the pycnocline.