In physical oceanography the admitted limitations of the Bjerknes circulation theorem sometimes are lost sight of and therefore the current trend of inquiring emphasis on these limitations is encouraging. The intelligent interpretation of dynamic calculations and the determination of absolute current directions and velocities therefrom, depend upon a knowledge of the location and shape of a motionless reference surface. In the past the arbitrary selection of isobaric surfaces for reference has been the resort to an approximation found useful in the absence of more definite knowledge regarding the true location and shape of such a motionless surface or surfaces. In recent years various oceanographers have proposed various non-spheroidal reference surfaces such as the oxygen-minimum layer, and a surface approximating an isentrope of constant vertical stability....
Soule, Floyd M.. 1939. "Consideration of the depth of the motionless surface near the Grand Banks of New Foundland." Journal of Marine Research 2, (3). https://elischolar.library.yale.edu/journal_of_marine_research/552