I want to calculate MLD from CTD data, while going through the literature I found different criteria for density (0.1, 0.3, 0.03, 0.25 etc density difference from the surface). What determines the density difference?
If there is a sharp change in density at the base of a well defined mixed layer as shown by density values then almost any criterion will do as all reasonable values will find almost the same depth. When the density change is not abrupt you have to experiment with different criteria to find which one is suitable for your data and the reason(s) for your choice should be specified in your analysis.
Experimenting with different criteria is the way to go as L. J. Hamilton suggested. Plot number of profiles with the UML depth and visually check, how the criteria works.
One might consider also vertical density gradient criteria instead of density difference from surface. I tried and played a lot with this and ended up gradient criteria depending on background stratification: Article Characteristics and variability of the vertical thermohaline...
You can also check the paper discussing the topic:
Article Estimating Mixed Layer Depth from Oceanic Profile Data
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