Breaking of internal solitary waves generated by an estuarine gravity current

Abstract

Mooring and ship-based data collected in a stratified estuary showed the generation of internal solitary waves by a bottom gravity current. Down-estuary winds drove a counterclockwise lateral circulation over channel-shoal bathymetry. When the lateral flows became supercritical, the pycnocline was sharply raised at the edge of the deep channel, leading to flow convergences and formation of a bottom gravity current. As the lateral circulation weakened during wind relaxation, the gravity current propagated onto the shoal and excited internal disturbances around its head. These disturbances evolved into a train of large-amplitude internal solitary waves that subsequently propagated ahead of the gravity current. The waves moving out of the gravity current broke, generating overturning and turbulence with energy dissipation rate reaching ~1 × 10−4 m2 s−3, 3 orders of magnitude larger than the background value. Our observations suggest that breaking internal waves may be an important source of turbulent mixing in stratified estuaries.