Decadal variability of mixed layer depth is examined with in situ observation of 50 year CalCOFI (California Cooperative Oceanic Fisheries Investigations) dataset and SODA (Simple Ocean Data Assimilation) reanalysis with 0.5oX0.5o horizontal resolution at the southern California coast. MLD fields are calculated by potential temperature profiles or density ones, and they are compared. Both MLD fields show significant decadal changes, which can affect mechanisms of nutrient supply for the primary production in the study area.