Mixing of a tracer in the pycnocline


  • James R. Ledwell,

  • Andrew J. Watson,

  • Clifford S. Law


A patch of sulfur hexafluoride was released in May 1992 in the eastern North Atlantic on an isopycnal surface near 300 m depth and was surveyed over a period of 30 months as it dispersed across and along isopycnal surfaces. The diapycnal eddy diffusivity K estimated for the first 6 months was 0.12±0.02 cm2/s, while for the subsequent 24 months it was 0.17±0.02 cm2/s. The vertical tracer distribution remained very close to Gaussian for the full 30 months, as the root mean square (rms) dispersion grew from 5 to 50 m. Lateral dispersion was measured on several scales. The growth of the rms width of the tracer streaks from less than 100 m to approximately 300 m within 2 weeks implies an isopycnal diffusivity of 0.07 m2/s at scales of 0.1 to 1 km, larger than expected from the interaction between vertical shear of the internal waves and diapycnal mixing. Teasing of the overall patch, initially about 25 km across, into streaks with an overall length of 1800 km within 6 months supports predictions of exponential growth by the mesoscale strain field at a rate of 3±0.5 × 10−7 s−1. The rms width of these streaks, estimated as 3 km and maintained in the face of the streak growth, indicates an isopycnal diffusivity of 2 m2/s at scales of 1 to 10 km, much greater than expected from internal wave shear dispersion. The patch was painted in, albeit streakily, by 12 months, confirming expectations from analytical and numerical models. Homogenization of the patch continued during the subsequent 18 months, while the patch continued to spread with an effective isopycnal eddy diffusivity on the order of 1000 m2/s, acting at scales of 30 to 300 km.