Interactions Between Diurnal Warm Layers and Surface-Layer Fronts

Abstract

Previous studies have highlighted the individual importance of diurnal warm layers (DWLs) and surface-layer fronts within the surface boundary layer (SBL) in regulating energy, momentum, and gas exchange between the atmosphere and the ocean. This study investigates the interactions between DWLs and surface-layer fronts using field observations and numerical turbulence models. Our study provides the real-ocean relevance of the coexistence of DWLs and surface-layer fronts in the SBL in an eddy-rich tropical ocean subjected to intense solar heating and weak winds. We found that the presence of a DWL isolates the deeper layers of the SBL from diabatic and frictional surface forcing, causing these layers to quickly become non-turbulent whereas remaining in a state of marginal stability. This condition suggests that small perturbations from local processes, such as internal tides and waves, can easily trigger instability and turbulence. Additionally, frontal dynamics were observed to deepen the SBL, allowing near-surface diurnal shear associated with DWL dynamics to penetrate to greater depths during nighttime, compared to conditions without a front, thereby facilitating the vertical transport of heat and tracers. Our findings underscore the necessity of accurately representing the interactions between DWLs and surface-layer fronts to enhance the precision of ocean circulation and climate models.