In Talley et al.'s Descriptive Physical Oceanography, figure S7.4, it seems that vertical Eddy diffusivity increases with depth between 0 and 30 degrees latitude. Why is this? I thought that diffusivity increases with increasing temperature, but here the opposite seems to be true.
I think you are confusing eddy diffusivity with molecular diffusivity. Whereas the latter increases with temperature, for the former there is no such relationship. Eddy diffusivity describes momentum transport due to eddies. The strength of the eddy diffusivity increases with decreasing buoyancy frequency, which is the case, when moving from the bottom of the mixed layer further downwards in the ocean.