The Barrier layer in the ocean is a layer of water separating the well-mixed surface layer from the thermocline. Barrier layer thickness (BLT) The thickness of the barrier layer is defined as the difference between mixed layer depth (MLD) calculated from temperature minus the mixed layer depth calculated using density. The first reference to this difference as the barrier layer was in a paper describing observations in the western Pacific as part of the Western Equatorial Pacific Ocean Circulation Study. In regions where the barrier layer is present, stratification is stable because of strong buoyancy forcing associated with a fresh lens sitting on top of the water column. In the past, a typical criterion for MLD was the depth at which the surface temperature cools by 0.2 °C (see e.g. DT-02 in the Figure). Prior to the subsurface salinity available from Argo, this was the main methodology for calculating the oceanic MLD. More recently, a density criterion has been used to define the MLD, defined as the depth where the density increases from the surface value due to a prescribed temperature decrease of 0.2 °C from the surface value while maintaining constant surface salinity value. In the Figure, this is defined by Dsigma and corresponds to an isothermal/isohaline layer. The BLT is the difference of the temperature-defined MLD minus the density-defined value (i.e. DT-02 - Dsigma). BLT Regimes Large values of the BLT are typically found in the equatorial regions and can be as high as 50 m. Above the barrier layer, the well mixed layer may be due to local precipitation exceeding evaporation (e.g. in the western Pacific), monsoon related river runoff (e.g. in the northern Indian Ocean), or advection of salty water subducted in the subtropics (found in all subtropical ocean gyres). BLT formation in the subtropics is associated with seasonal change in the mixed layer depth, a sharper gradient in sea surface salinity (SSS) than normal, and subduction across this SSS front. In particular, BLT is formed in winter season in the equatorward flank of subtropical salinity maxima. During early winter, the atmosphere cools the surface and strong wind and negative buoyancy forcing mixes temperature to a deep layer. At this same time, fresh surface salinity is advected from the rainy regions in the tropics. The deep temperature layer along with strong stratification in the salinity gives the conditions for BLT formation. For the western Pacific, the