Relationship Between the Barrier Layer Thickness and Upper Ocean Heat Content in the Western Tropical Pacific Ocean

Abstract

Barrier layers form when density-defined mixed layer is shallower than the temperature-defined isothermal layer. It can reduce vertical mixing and entrainment cooling, influencing upper-ocean heat storage. This study examines barrier layer variability near the Mariana Trench using 43 independent underway CTD profiles collected from the R/V Thomas G. Thompson during 27-28 March 2026, together with ERA5 reanalysis surface forcing data. Barrier layer thickness was calculated from vertically binned temperature, salinity, and density profiles. Upper-ocean heat content was integrated over 0-200 m for profiles with sufficient depth coverage, and precipitation, precipitation - evaporation (P - E), wind speed, and wind stress were matched to each profile over 1-, 6-, 12-, and 24-h windows. Barrier layers were common but spatially heterogeneous. The zonal East transect had the thickest and most persistent barrier layers, while the meridional transect showed a sharp transition from thin to thick barrier layers over kilometer-scale distances. Precipitation and P - E were weakly correlated with barrier layer thickness, whereas wind stress showed a stronger negative linear relationship, especially over 24-h window (r = -0.524, p < 0.001). Barrier layer thickness showed a weak positive relationship with upper-ocean heat content (r = 0.223, p = 0.156), but the correlation was not statistically significant. Thus, the hypothesis that thicker barrier layers are associated with greater fixed-depth upper-ocean heat content was partially supported but not statistically confirmed. This suggests that thicker barrier layers may help retain heat, but BLT alone did not explain fixed-depth upper-ocean heat content in this short cruise-based dataset.