A deliberate tracer experiment in Santa Monica Basin

Abstract

The mixing of heat and chemical constituents across density strata in the ocean plays a major role in ocean circulation and in the response of the climate to thermal and chemical perturbations. We are developing a tracer technique which, in conjunction with fluid dynamical measurements, promises to improve our understanding and our estimates of such diapycnal mixing. In a prototype tracer experiment in Santa Monica Basin, 50km west of Los Angeles, two tracers, sulphur hexafluoride (SF₆) and per-fluorodecalin (PFD), have been injected on an isopycnal surface near the centre of the basin. After 50 days, the tracers had mixed along the isopycnal surface to nearly every part of the basin, although relatively little had penetrated to the basin walls. The diapycnal spreading of the tracer distributions during this first stage of the experiment yielded an apparent eddy diffusivity of 0.33±0.08 cm²s⁻¹ at the ambient density gradient of 4.0±0.5×10⁻⁹g cm⁻⁴.