On the formation of barrier layers and associated vertical temperature inversions: A focus on the northwestern tropical Atlantic

A unique barrier layer (BL) system in terms of persistence, extension, and associated subsurface temperature maximum is present seasonally in the northwestern tropical Atlantic. Based on climatological output of a general circulation ocean model, we show here that its development consists of two phases. In summer, the BL is relatively shallow and thin but subsurface temperature maxima are intense. The latter develop as a result of the specific seasonality of the freshwater discharge in this area, which limits the mixed layer to a very thin depth while the intense radiative heat flux penetrates significantly below, thereby heating the subsurface waters protected from air-sea interactions and inducing a barrier layer between the mixed layer and the ocean interior. In winter, the BL development is due to a surface decrease in salinity associated with the surface freshwater capping, which decouples the pycnocline, and hence the winter mixed layer, from the thermocline. The mechanism is ubiquitous in the sense that it is very similar to that of other areas at the same latitude, as well as at high latitudes in regions of seasonal surface freshening. Results are discussed in the light of a simple linear equation linking the BL development to time evolution of Sea Surface Temperature and Sea Surface Salinity stratification.