23 Mar 2023
 | 23 Mar 2023

Internal tides off the Amazon shelf Part I : importance for the structuring of ocean temperature during two contrasted seasons

Fernand Assene, Ariane Koch-Larrouy, Isabelle Dadou, Michel Tchilibou, Guillaume Morvan, Jérôme Chanut, Vincent Vantrepotte, Damien Allain, and Trung-Kien Tran

Abstract. Tides and internal tides (IT) in the ocean can significantly affect local to regional ocean temperature, including sea surface temperature (SST), via physical processes such as diffusion (vertical mixing) and advection (vertical and horizontal) of water masses. Offshore of the Amazon River, strong IT have been detected by satellite observations and well modelled ; however, their impact on temperature, SST and the identification of the associated processes have not been studied so far. In this work, we use high resolution (1/36°) numerical simulations with and without the tides from an ocean circulation model (NEMO). This model explicitly resolves the internal tides (IT) and is therefore suitable to assess how they can affect ocean temperature in the studied area. We distinguish the analysis for two contrasted seasons, from April to June (AMJ) and from August to October (ASO), since the seasonal stratification off the Amazon River modulates the IT’s response and their influence in temperature.

The generation and the propagation of the IT in the model are in good agreement with observations. The SST reproduced by the simulation including tides is in better agreement with satellite SST data compared to the simulation without tides. During ASO season, stronger meso-scale currents, deeper and weaker pycnocline are observed in contrast to the AMJ season. The observed coastal upwelling during ASO season is better reproduced by the model including tides, whereas the no-tide simulation is too warm by +0.3 °C for the SST. In the subsurface above the thermocline, the tide simulation is cooler by −1.2 °C, and warmer below the thermocline by +1.2 °C compared to the simulation without the tides. The IT induce vertical mixing on their generation site along the shelf break and on their propagation pathways towards the open ocean. This process mainly explains the cooler temperature at the ocean surface and is combined with vertical and horizontal advection to explain the cooling in the subsurface water above the thermocline and a warming in the deeper layers below the thermocline. The surface cooling induced in turn an increase of the net heat flux from the atmosphere to the ocean surface, which could induce significant changes in the local and even for the regional tropical Atlantic atmospheric circulation and precipitation.

We therefore demonstrate that IT, via vertical mixing and advection along their propagation pathways, and tides over the continental shelf, can play a role on the temperature structure off the Amazon River mouth, particularly in the coastal cooling enhanced by the IT. Keywords: internal tides, Amazon continental shelf and slope, temperature, modeling, satellite data, mixing, heat flux.

Fernand Assene et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-418', Clément Vic, 06 Apr 2023
    • AC1: 'Reply on RC1 : Clement Vic', Fernand Bernie Assene Mvongo, 21 Jul 2023
  • RC2: 'Comment on egusphere-2023-418', Nicolas Grisouard, 27 Apr 2023
    • AC2: 'Reply on RC2 : Nicolas Grissouard', Fernand Bernie Assene Mvongo, 21 Jul 2023

Fernand Assene et al.

Fernand Assene et al.


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Short summary
Numerical simulations with and without the tides are used to assess how internal tides (IT) can affect ocean temperature off the Amazon mouth at seasonal scale. We found that In surface layers, IT and tides induce a cooling, which can some when helps to decrease the sea surface temperature (SST) and therefore impact the atmosphere-ocean net heat flux and climate. In turn, deeper below the thermocline, a warming occurs. The main drivers are vertical mixing, and vertical and horizontal advection.