Cyclonic ring formation at the polar front in the Drake Passage

THE first summary of geographical locations of the antarctic convergence (polar front) by Mackintosh1 showed that >50% of the instantaneous locations of the front differed from the mean location by as much as ±50 km. Early hydrographic section work could not resolve the temporal or spatial fluctuations in the horizontal structure of the front. During FDRAKE 75 Gordon2 observed comparable fluctuations in the frontal position in the Scotia Sea but again the spacing between sections was too wide. During FDRAKE 76 (refs 3, 4 and T. M. Joyce et al., unpublished) the small scale structure of the polar front was studied in the Drake Passage. Two ships, the RV Thompson (University of Washington) and the Chilean naval vessel AGS Yelcho, combined efforts in the program which took place during the onset of the austral autumn. We describe here the evolution of the polar front between February 29 and April 5, 1976, a time span of ∼5 weeks.