Evidence of the barrier layer in the surface layer of the tropics

TLDR

The distance between the top of the thermocline and the bottom of the mixed layer is called the “barrier layer,” because it impedes vertical heat flux out of the mixed‑layer base. The study aims to identify the distinct mechanisms sustaining the barrier layer across three equatorial regions, including a hypothesis that winter subduction of high‑salinity waters in the northwest Atlantic creates a salinity maximum that forms the barrier layer. The authors compare isothermal and σt‑based mixed‑layer depths across the tropical oceans, and examine region‑specific processes—winter subduction of high‑salinity waters in the northwest Atlantic and monsoonal freshwater fluxes in the eastern Indian Ocean—that generate surface salinity stratification. They find that in all three equatorial regions the mixed layer is shallower than the isothermal layer, indicating a strong halocline, with western Pacific precipitation producing a salt‑stratified layer, and conclude that salinity stratification must be considered when estimating mixed‑layer depth.

Abstract

Comparisons between isothermal depth to the top of the thermocline, and the mixed layer depth based on a σ t criterion were undertaken for the tropical world oceans. In three equatorial regions, a shallower mixed layer than isothermal layer occurs, implying the presence of a strong halocline above the thermocline. This distance separating the top of the thermocline and the bottom of the mixed layer is referred to as the “barrier layer”, in relation to its impediment to vertical heat flux out of the base of the mixed layer. Different mechanisms are responsible for maintaining the barrier layer in each of the three regions. In the western equatorial Pacific Ocean a salinity budget confirmed that heavy local precipitation most likely results in the isothermal but salt‐stratified layer. In the northwest equatorial Atlantic, it is hypothesized that high salinity waters are subducted at the subtropics during winter and advected westward as a salinity maximum in the upper layers of the tropics, resulting in the barrier layer. In the eastern equatorial Indian Ocean, monsoonal related rainfall and river runoff contribute significantly to the freshwater flux, producing salt stratification in the surface. These results suggest the need to include the effects of salinity stratification when determining mixed layer depth.