Evidence is found for a cutoff gradient Richardson number of Ri~0.25 in the moored measurements of temperature and current obtained during the second Coastal Ocean Dynamics Experiment (CODE 2), conducted off northern California during the upwelling season of March--August 1982. The cutoff at Ri~0.25 was noticed even when the instruments were separated by depth intervals as large at 40 m, or when the time series (having a sampling interval of 7.5 min) were averaged over an hour, During several strong wind events which tended to increase the mean shear, Ri~0.25 occurred in the main thermocline beneath the surface mixed layer over the shelf. On the other hand, several instances were found when a low Ri was not directly related to any wind event, suggesting that superposition of various waves can locally generate unstable conditions. The deeper water over the shelf and slope was generally stable with Ri≫0.25. These results suggest that in coastal areas with strong currents, the intensification of mean vertical shear seems to be very important in bringing about Kelvin-Helmholtz-type instabilities, as compared with the open ocean without strong currents where superposition of internal waves is believed to be primarily responsible for causing the instabilities.

Because the density structure almost everywhere in the ocean is in the form of steps, and because the overall Richardson number across vertical scales as large as 20 m are frequently near critical, it follows that appreciably thick regions in the ocean frequently remain on the brink of Kelvin-Helmholtz instability, even though the actual mixing may take place within thin interfaces between relatively homogeneous layers. A small increase of the overall shear then greatly increases the frequency of billow events and vertical mixing, and a small decrease of the overall shear greatly decreases such activity. It is suggested that in numerical circulation models the Richardson number be constantly checked and vertical mixing be used to maintain the Richardson number above a minimum value of 0.25. ¿American Geophysical Union 1991