How to define the mixing layer depth in CESM 1.3

[fangyu.zhou]

I'd like to ask users of the CESM POP2 ocean data: How is the mixed layer depth defined for each data point? Is the mixed layer depth interpolated? Is the mixed layer depth at each grid point equal to the depth at the exact center of the T cell? In the North Atlantic region I'm interested in, a large area has a zero mixed layer depth. Why is this? Thank you for your patience and guidance. For example, the mixed layer depth in a file name like this: B.E.13.BHISTC5.ne120_t12.sehires38.003.sunway.pop.h.HMXL.185001-0185912.nc