According to the typical thermal structure of the ocean, the water column can be divided into three layers: the mixing layer, the thermocline and the deep layer. In this study, we provide a new methodology, based on a function adjustment on the temperature profile, to locate the minimum and maximum depths of the thermocline, and therefore its thickness, to separate the water column into layers. We first validated our methodology by comparing the mixed layer depth obtained with the method proposed here with that of two previous studies. Since we found a very good agreement between the three methods we used the function adjustment to compute the monthly climatologies of the mixed layer depth, the maximum depth of the thermocline and the thermocline thickness, throughout the ocean. We also provide an assessment of the regions of the ocean where our adjustment is valid, and consequently the regions where the thermal structure of the ocean follows the three-layer structure. However, there are ocean regions where the water column cannot be separated into three layers due to the dynamic processes that alter it and the major contribution of salinity to stratification. This assessment highlights the limitations of the existing methods to accurately determine the mixed layer depth and the thermocline in oceanic regions that are particularly turbulent as the Southern Ocean and the northern North Atlantic, among others. The method proposed here has shown to be robust and easy to apply, and it can be used in both local and global studies.