Growth and Decay of Northwestern Tropical Atlantic Barrier Layers
Creators: name: Saha, Aurpita
name: Serra, Nuno
name: Stammer, Detlef
Publisher: name: FID GEO
Year: 2021

description: The growth and decay mechanisms of barrier layers in the northwestern tropical Atlantic are studied by investigating small‐scale processes embedded in the regional circulation of the tropical Atlantic using output from an eddy‐resolving numerical simulation at 4 km resolution forced by an atmospheric reanalysis. The simulation reproduces well the temporal and spatial patterns of barrier layer thickness (BLT) estimated with Argo and CTD in situ profiles. As seen from an analysis of the salinity and temperature vertical gradient balances, localized large barrier layers form inside North Brazil Current rings during late‐June to July because of a thickening of the isothermal layer in the rings due to horizontal temperature advection, stretching of isotherms and tilting of temperature fronts. These barrier layers decay when the isothermal layer reduces again due to the above mechanisms. Further to the north, along the North Equatorial Current, the seasonal variability of BLT is highly pronounced. Thick winter (January to early March) barrier layers locally grow as the base of the mixed layer shoals mainly due to a tilting of the salinity fronts and partly due to stretching of the isohalines, horizontal salt advection and vertical turbulent mixing. The short‐term barrier layers in this case decay due to a deepening of the mixed layer, whereas they get completely eroded in spring by a shoaling of the isothermal layer due to surface temperature stratification. This work highlights that barrier layers are localized phenomena at times growing solely due to ocean dynamics, without a surface freshwater influx.
description: Plain Language Summary: Oceanic barrier layers exist in regions where salinity is more dominant than temperature in determining upper ocean density. Those layers lie between the bases of a constant‐density layer and a constant‐temperature layer. Barrier layers prevent vertical exchange of energy and mass between the near‐surface and the deep ocean, thus influencing air‐sea interaction. In the western tropical Atlantic, a warmer sea surface due to the presence of barrier layers can fuel hurricanes. Freshwater from the Amazon River and rainfall facilitate the growth of barrier layers, but the dynamics of their evolution are unclear. In this work, we identify/quantify the growth and decay mechanisms of barrier layers using a 4 km resolution simulation. Barrier layers grow/decay inside North Brazil Current eddies in summer because of deepening/shoaling of the constant‐temperature layer inside the eddies due to horizontal heat transport. Further north, barrier layers grow in winter as the constant‐density layer shoals mainly due to northwestward surface freshwater flow and equatorward subsurface salty water flow. Those barrier layers decay when the constant‐density layer deepens, whereas are destroyed when the constant‐temperature layer shoals in spring due to surface heating. These novel results improve the knowledge on barrier layers and help representing them in climate models.
description: Key Points: The North Brazil and North Equatorial Currents are two regions with quasi‐permanent barrier layers in the northwestern tropical Atlantic Large barrier layer thickness (BLT) within the rings occurs during June‐July due to a thickening of the isothermal layer within the eddies Large winter BLT in North Equatorial Current is due to tilting of salinity fronts, stretching of isohalines, advection and turbulent mixing
description: Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659