Purposefully released tracers

Abstract

It has been recognized since the late 1970s that it should be possible to use the release of conservative tracers into the deep ocean to measure vertical mixing rates. Diapycnal mixing (i.e. vertical mixing across density strata) is rather crudely described by the present generation of ocean models, usually being taken to be constant irrespective of depth. Model circulations are, however, sensitive to quite minor changes in this assumption. Technology and experimental methods in the open ocean with perfluorinated tracers have now advanced to the state where direct-tracer experiments are feasible and a pilot study in a small basin has given encouraging results. The design of open-ocean experiments is discussed. Consideration of the problem of sampling a streaky distribution leads to the conclusion that the period between release and sampling should be several times the eddy-rotation time. Although any number of tracers could be used for such experiments, sulphur hexafluoride seems to have unique advantages over other candidates: it has been demonstrated to be conservative at least on a timescale of several months, and can be detected with better sensitivity than other possible tracers. Perfluorinated tracers show promise in other areas besides deep-sea oceanography: they are well suited to studies in the coastal ocean and could be used to measure dispersion and air—sea exchange properties at the surface.