Negative viscosity in the Gulf Stream?

This paper discusses whether Webster's and Schmitz & Niiler's measurements verify the proposed existence of negative viscosity for two-dimensional flows put forward by Krause & Rüdiger. It leads to the conclusion, contrary to that of other workers, that only positive viscosity was observed for the Gulf Stream when cross stream directed mean motion, as found in the surveys mentioned above, is considered. This conclusion depends on the validity of our fundamental assumption that the turbulence is caused by an external force field.