Thermohaline gradients and frontal regimes in the northwestern Tropical Atlantic

Abstract

At the edge of the Amazon River plume, stirring by the North Brazil Current (NBC) and its eddies creates sharp surface thermohaline gradients on horizontal scales of O (1-100) km. This study provides a comprehensive picture of these gradients and fronts associated with the region's distinctive dynamics. Saildrone observations show that the plume amplifies density gradient variability at all scales from 1-100 km, with frontal sharpness up to 75× stronger inside the plume than outside, with differences reaching 100× at scales below 3 km. Density gradients are partially reinforced or compensated by temperature-salinity variations, with net frontogenesis observed in both regions. To expand in-situ observations, we use a 1 km resolution CROCO (Coastal and Regional Ocean COmmunity model) simulation to assess the spatial distribution of surface fronts and their spatio-temporal variability. We characterize three distinct frontal regimes: (i) broken-up fronts parallel to shore occupy the plume core over the continental shelf, (ii) thin elongated fronts associated with NBC-plume interactions connect nearshore and offshore regions, and (iii) pools of anisotropic fronts driven by the seasonal mixed-layer cycle are present offshore. Salinity dominates density gradients throughout the year north of 15°N, whereas near-shore fronts exhibit seasonal shifts in temperature-salinity dominance linked to the Amazon dis-charge seasonality and NBC strength. Within the plume, freshwater filaments stirred by NBC rings systematically generate density-compensated fronts on their inner edge and reinforced fronts on their outer rim, a pattern with implications for energy cascades and tracer export.

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BibTeX RIS

BibTeX

@article{napolitano2026thermohaline,
  title = {{Thermohaline gradients and frontal regimes in the northwestern Tropical Atlantic}},
  author = {Dante C Napolitano and Jonathan Gula and Solange Coadou-Chaventon and Sabrina Speich and Cesar B Rocha and James C Mcwilliams and Dongxiao Zhang and Xavier Carton},
  journal = {Ocean Science},
  year = {2026},
  month = aug,
  doi = {10.5194/os-22-2425-2026},
  url = {https://hal.science/hal-05739948v1},
  note = {CC BY 4.0},
}

RIS

TY  - JOUR
TI  - Thermohaline gradients and frontal regimes in the northwestern Tropical Atlantic
AU  - Dante C Napolitano
AU  - Jonathan Gula
AU  - Solange Coadou-Chaventon
AU  - Sabrina Speich
AU  - Cesar B Rocha
AU  - James C Mcwilliams
AU  - Dongxiao Zhang
AU  - Xavier Carton
PY  - 2026
DA  - 2026/08/13
JO  - Ocean Science
DO  - 10.5194/os-22-2425-2026
UR  - https://hal.science/hal-05739948v1
AB  - At the edge of the Amazon River plume, stirring by the North Brazil Current (NBC) and its eddies creates sharp surface thermohaline gradients on horizontal scales of O (1-100) km. This study provides a comprehensive picture of these gradients and fronts associated with the region's distinctive dynamics. Saildrone observations show that the plume amplifies density gradient variability at all scales from 1-100 km, with frontal sharpness up to 75× stronger inside the plume than outside, with differences reaching 100× at scales below 3 km. Density gradients are partially reinforced or compensated by temperature-salinity variations, with net frontogenesis observed in both regions. To expand in-situ observations, we use a 1 km resolution CROCO (Coastal and Regional Ocean COmmunity model) simulation to assess the spatial distribution of surface fronts and their spatio-temporal variability. We characterize three distinct frontal regimes: (i) broken-up fronts parallel to shore occupy the plume core over the continental shelf, (ii) thin elongated fronts associated with NBC-plume interactions connect nearshore and offshore regions, and (iii) pools of anisotropic fronts driven by the seasonal mixed-layer cycle are present offshore. Salinity dominates density gradients throughout the year north of 15°N, whereas near-shore fronts exhibit seasonal shifts in temperature-salinity dominance linked to the Amazon dis-charge seasonality and NBC strength. Within the plume, freshwater filaments stirred by NBC rings systematically generate density-compensated fronts on their inner edge and reinforced fronts on their outer rim, a pattern with implications for energy cascades and tracer export.
SN  - 1812-0784
SN  - 1812-0792
ER  -

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