Enhanced stability during reduction of stratification in the North Sea

In summer shelf seas are (increasingly with time) stably stratified in density due to solar heating. Previous observations have shown that this stratification may become marginally stable when increased stratification is accompanied by enhanced vertical current differences (‘shear’) that de-stabilize the water column. When the effects of shear become larger than those of stratification, so that the gradient Richardson number Ri<1, mixing may result, most likely by the (occasional) overturning at the layer of strong shear to the point of breaking of short high-frequency internal waves that propagate through the stratification. The shear is dominantly at the low-frequency end of the internal gravity wave band, the local inertial frequency. In autumn the accepted view is that stratification is reduced not only due to internal mixing following enhanced inertial shear, but also due to cooling by the atmosphere and wind mixing. However, in the present paper detailed observations from the northern North Sea clearly demonstrate for the first time that also in autumn periods of increasing stability with time occur. Furthermore, it is shown that enhanced stability may be a prerequisite for generating large amplitude high-frequency internal waves that may eventually break: autumn storms can first reduce inertial shear magnitude, followed by a new equilibrium of marginal stability across a layer of greatly reduced thickness. Such a thin, super-stable (Ri⪢1) layer is then moved vertically by high-frequency (interfacial) internal waves. This ‘creates’ a background of reduced large-scale stratification, associated with the reduced inertial shear.