Barrier layers in the subtropical gyres of the world's oceans

Barrier layers (BLs) in the subtropical gyres of the world's oceans are detected using all temperature and salinity profiles obtained by Argo floats from Jan. 2000 to Jun. 2005. The synoptic BLs are distributed patchily in the area where climatological BLs are continuously distributed. They are thicker and occur more frequently during Jan.–Apr. (Jul.–Nov.) in the Northern (Southern) Hemisphere with some differences in thickness and occurrence rates among the oceans. We suggest that the same formation mechanism of BLs as that in the North Pacific, i.e., the subduction of high salinity water at sharp salinity fronts on a small scale of approximately 100 km is at work in the other subtropical gyres. Seasonal change of the mixed layer depth contributes to the seasonality of BL thickness and frequency in all subtropical gyres.