Usually,there is a vertically uniform layer of temperature (isothermal layer),salinity (isohaline layer) and density (mixed layer) in the upper ocean.Underneath it there is a layer with strong vertical gradient such as the thermocline (in temperature),the halocline (in salinity) and the pycnocline (in density).In most cases,the temperature determines the density,so the isothermal layer depth (Ht) is identical to the mixed layer depth (Hd).There are areas of the world ocean where the Ht is deeper than the Hd (Lindstrom et al.,1987; Lukas and Lindstrom,1991; Sprintall and Tomczak,1992; You,1995; Kara et al.,2000).The layer difference between Ht and Hd is defined as the barrier layer (BL),which has strong density (salinity) stratification and weak temperature stratification.Less turbulence in the BL than in the mixed layer due to strong density (salinity) stratification isolates the mixed layer from the cold thermocline water.