John Grue - Large and breaking internal waves at a marine ridge-valley shelf topography in Vesterålen-Lofoten, Norway

The internal waves in consideration are generated locally in combination by:
1) the North Atlantic Current (NAC) that flows along the Norwegian shelf (average speed 0.3 m/s),
2) the local tide (dominant 24 hr mode),
3) the particular shelf geometry,
4) the seasonal pycnocline (at 20 m depth). The typical amplitude is 50 m. The local depth where they exist is 100 m (approx.).
The local internal wave speed of 0.4 m/s is comparable to the current speed.

The wave generation process occurs in a sequence of:
A) locally large depressions of the internal tide occurring at the lee side of two shallow ridges,  
B) the depressions are orthogonal to the current,
C) the depressions move slowly against the current, and
D) eventually fission into nonlinear-dispersive wave groups,
E) they propagate upstream against the current,
F) they are recorded on satellite images and is visible by eye.
 

In a recent RCN-funded project (ECOPULSE) 2020-2023 including research groups at UiO, the Norwegian Meteorological Institute and the Institute of Marine Research in Bergen, field experiments of the mother wave in points A) and C) were undertaken during July-September 2021 and 2022. The measurements show that the waves break and mix in the entire water column by convective breaking in the upper part of the water column, shear instability within the pycnocline, and by a variant of a shear instability near the bottom. The field measurements document the local wave properties and the initial phase of the fission process. The Vesterålen-Lofoten area is of immense political interest because of a) its traditional fisheries and breeding ground for the North Atlantic and Barents Sea Cod and other species, where also a cold water coral reef is located right where these waves are generated.  The waves do also extend across NORDLAND VII, an oil / gas field which may be considered for exploitation. Preliminary results will be presented.