Seasonal variation and formation mechanism of barrier layers (BLs) in the northern South China Sea (NSCS) were examined based on Argo profiling data and ocean reanalysis data from January 2008 to December 2020. The observed synoptic‐scale BL is thicker and has a patchy distribution relative to the spatially and temporally smoothed picture of climatological BL. Statistics of the occurrence show that autumn and winter are the seasons of the frequently observed thick BLs (>20 m) in the NSCS. The regions with the most frequent observations of thick BLs are to the west of Luzon Island, in the vicinity of Dongsha Island, and to the east of Xisha Island, and the peak of emergence is in September, November, and February, respectively. Furthermore, there is a widespread occurrence of BLs, particularly in spring, which is not seen in the climatology. From July to September, the primary control of BL formation in the NSCS is atmospheric freshwater flux. However, from November to February, the sea surface salinity front that crosses the latitudinal zone of 16–18°N of the basin is found to be an important region for the occurrence of BLs in winter. Analysis from the reanalysis suggests that the contribution of the SSS front to the formation of winter BLs is through the tilting of the front. This hypothesis of winter BL formation is different from the previously proposed vertical stretching mechanism, but the process of the tilting of the front can be seen in the observations from Argo floats.