Kara et al. (2000) mixed layer diagnostic

[ali-ramadhan]

Would be useful to pick one diagnostic to compute the mixed layer depth of our simulations (and of SOSE/ECCO data if we want consistency).

@rafferrari suggested the Kara et al. (2000) mixed layer diagnostic which might also be the one used by SOSE(?).

Kara et al. (2000): https://doi.org/10.1029/2000JC900072

cc @sandreza @adelinehillier