Biophysical variability at the shelf-break/slope transition: Insights from a high-resolution transect experiment in the Southwestern Tropical Atlantic

Abstract

We investigated how short-term physical variability influences biological distributions at the shelf-break and slope off Northeast Brazil using a process-oriented Multiple Rectangle Transect (~2 × 11 km) repeated over ~26 h, spanning two semidiurnal tides and a full diel cycle. Co-located measurements combined ship-mounted acoustic Doppler current profiling, multifrequency echosounders (38, 70, 120 kHz), continuous thermosalinograph data, and targeted CTD/XBT profiles. Shallow (15–59 m) along- and cross-shelf current series were analyzed with Ensemble Empirical Mode Decomposition to isolate dominant variability scales, and barotropic tidal constituents from a global model were used for comparison. The velocity structure was dominated by the North Brazil Undercurrent, with a jet core (~70–350 m) peaking at 0.6–0.8 m s⁻¹ and a weakened layer around 160–230 m within the core. Near-surface along-shelf flow remained positive (mean ~0.35 m s⁻¹), while cross-shelf flow alternated between shoreward and seaward. Semidiurnal tides explained most shallow variability and organized alternating convergence-divergence patterns across the rectangle; observed amplitudes exceeded tidal model estimates, indicating interaction with the background current. Hydrography showed a warm mixed layer over a sharp thermocline, with a subsurface chlorophyll maximum (~90–110 m) and cooler, oxygen-reduced layers below the undercurrent core. Acoustic data revealed diel vertical migration and multiple sound-scattering layers (SSLs). During daytime, key SSLs preferentially occupied reduced-velocity zones (~100 m above the current maximum and ~205 m within the weakened layer), indicating that fine-scale current structure influenced their vertical organization. The recurrent association of the ~205 m aggregation with the weakened layer suggests that such low-velocity zones may provide favorable conditions for vertically migrating organisms. Deeper SSLs (~310–400 m) coincided with reduced currents and lower oxygen. Overall, the results suggest that flow–topography interactions at the slope create localized low-velocity structures that influence mesopelagic distributions on submesoscales.