Barrier layers, separating the well-mixed surface layer from the thermocline, inhabit turbulent and entrain heat exchange between the atmosphere and the cold subsurface ocean. The formation mechanisms of barrier layers involve intense precipitations, oceanic circulation, wind and runoff.
In this study, salinity and temperature data from 1979 to 2015 derived from the Met Office Hadley Center Observations datasets (EN4.1.1) are used to calculate the BLD following the method of de Boyer Montégut et al(2004, 2007). Precipitation(P)/evaporation (E), surface heat flux and wind stress are analyzed to understand the oceanic structure and generating mechanisms of the barrier layers as well as their variability and trends.
The climatology of BLD shows good correspondence with previous studies. In the tropical Pacific, the warm pool with sufficient precipitation has deep barrier layers. In the Indian Ocean, Bay of Bengal, southeastern Arabian Sea and eastern Indian Ocean are three main regions with deep barrier layers. In the Atlantic warm pool, the formation of salty subtropical underwater may be the origin for barrier layers according to Montégut et al(2007).
Wind, heat flux and P-E affect BLD through their effects on seawater mixing and salinity stratification. The preliminary results show that the interannual-to-decadal variability and trends of BLD from 1979 to 2015 in tropical areas are under the combined action of various factors. In the tropical Pacific, the increasing trend of BLD is mainly affected by both decreasing trend of ocean heat flux P-E. In the Indian Ocean, the increasing trend of BLD in Bay of Bengal is mainly attributable to decreasing trend of ocean heat flux. In the southeastern Arabian Sea, the BLD is slightly increased while in the eastern Indian Ocean, BLD is increased in the Northern Hemisphere and decreased in the Southern Hemisphere corresponding to increasing trend of P-E in the north but deceasing trend in the south. In the Atlantic warm pool, ocean heat flux and P-E also have relevant influence on the interannual-to-decadal variability and trends of BLD from 1979 to 2015.
