Direct measurments of vertical ocean mixing?
I am assuming you know how to get to mixing from velocity field since you are talking about the noise in the ADCP data. The way people get the small fluctuation and vertical mixing profiles that are highly trusted are with "micro-structure" sampling technique. 

Please follow this link for more detail on that technique. 
https://earthobservatory.nasa.gov/blogs/fromthefield/2012/10/04/what-is-ocean-microstructure-and-why-study-it/