Two lines of thinking concerning fluid rotation—using either vorticity or circulation—emerged from the nineteenth-century work of Helmholtz and Thomson (Lord Kelvin), respectively. Vilhelm Bjerknes introduced an extension of Kelvin's ideas on circulation into geophysics. In this essay a historical perspective will be given on what has become known as the “Bjerknes circulation theorem.” Bjerknes wrote several papers on this topic, the first being in 1898. As Bjerknes noted, a Polish physicist, Ludwik Silberstein, had previously published an equivalent result concerning vorticity generation in 1896. Silberstein's work had built on an earlier paper by J. R. Schiitz in 1895. In his 1898 paper Bjerknes describes many possible applications of the theorem to meteorology and oceanography including to extratropical cyclones, a subject that made his “Bergen School” famous.
NERC Centres for Atmospheric Science, University of Reading, Reading, United Kingdom
Institut für Physik der Atmosphäre, DLR Oberpfaffenhofen, Oberpfaffenhofen, Germany
Institute of Meteorology and Water Management, Gdynia, Poland