A Basinwide Estimate of Vertical Mixing in the Upper Pycnocline: Spreading of Bomb Tritium in the North Pacific Ocean

Abstract

The vertical diffusivity KV in the upper half-kilometer of the North Pacific subtropical pycnocline is estimated from observations of the spreading rate of anthropogenic tritium. The calculation is based on approximately 300 ocean tritium profiles made since tritium was introduced to the atmosphere via bomb testing in the 1960s. The data coverage does not permit detailed mapping of tritium penetration, especially in the sparsely sampled western Pacific. For this reason, and to minimize advective effects, the spreading rate is averaged within the closed streamlines of the subtropical gyre spanning ?5°N to ?40°N. The result, KV = (1.5 ± 0.7) ? 10?5 m2 s?1, is consistent with inferences from microstructure and purposefully released tracer measurements in the North Atlantic, confirming that the spatially averaged rate of mixing in the upper pycnocline is substantially lower than the canonical Munk estimate for the lower pycnocline.