MIXING OF A TRACER IN THE PYCNOCLINE

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 surve yed over a period of 30 months as it dispersed across and along isopyc nal surfaces. The diapycnal eddy diffusivity K estimated for the first 6 months was 0.12 +/- 0.02 cm(2)/s, while for the subsequent 24 month s it was 0.17 +/- 0.02 cm(2)/s. The vertical tracer distribution remai ned very close to Gaussian for the full 30 months, as the root mean sq uare (rms) dispersion grew from 5 to 50 m. Lateral dispersion was meas ured on several scales. The growth of the rms width of the tracer stre aks from less than 100 m to approximately 300 m within 2 weeks implies an isopycnal diffusivity of 0.07 m(2)/s at scales of 0.1 to 1 km, lar ger than expected from the interaction between vertical shear of the i nternal waves and diapycnal mixing. Teasing of the overall patch, init ially about 25 km across, into streaks with an overall length of 1800 km within 6 months supports predictions of exponential growth by the m esoscale strain field at a rate of 3 +/- 0.5 x 10(-7) s(-1). The rms w idth of these streaks, estimated as 3 km and maintained in the face of the streak growth, indicates an isopycnal diffusivity of 2 m(2)/s at scales of 1 to 10 km, much greater than expected from internal wave sh ear dispersion. The patch was painted in, albeit streakily, by 12 mont hs, confirming expectations from analytical and numerical models. Homo genization of the patch continued during the subsequent 18 months, whi le the patch continued to spread with an effective isopycnal eddy diff usivity on the order of 1000 m(2)/s, acting at scales of 30 to 300 km.