The impact of internal and barotropic tides on the vertical and horizontal temperature structure off the Amazon River was investigated during two highly contrasted seasons (AMJ: April–May–June; ASO: August–September–October) over a 3-year period from 2013 to 2015. Twin regional simulations, with and without tides, were used to highlight the general effect of tides. The findings reveal that tides have a cooling effect on the ocean from the surface (∼ 0.3 ^∘C) to above the thermocline (∼ 1.2 ^∘C), while warming it up below the thermocline (∼ 1.2 ^∘C). The heat budget analysis indicates that the vertical mixing is the dominant process driving temperature variations within the mixed layer, while it is associated with both horizontal and vertical advection to explain temperature variations below. The increased mixing in the simulations including tides is attributed to breaking of internal tides (ITs) on their generation sites over the shelf break and offshore along their propagation pathways. Over the shelf, mixing is driven by the dissipation of the barotropic tides. In addition, the vertical terms of the heat budget equation exhibit wavelength patterns typical of mode-1 IT. The study highlights the key role of tides and particularly how IT-related vertical mixing shapes the ocean temperature off the Amazon. Furthermore, we found that tides impact the interactions between the upper ocean interface and the overlying atmosphere. They contribute significantly to increasing the net heat flux between the atmosphere and the ocean, with a notable seasonal variation from 33.2 % in AMJ to 7.4 % in ASO seasons. This emphasizes the critical role of tidal dynamics in understanding regional-scale climate.

In this paper, we analyse the results from a numerical model at high resolution. We focus on the formation and maintenance of subsurface equatorial currents in the Gulf of Guinea and we base our analysis on the evolution of potential vorticity (PV). We highlight the link between submesoscale processes (involving mixing, friction and filamentation), mesoscale vortices and the mean currents in the area. In the simulation, eastward currents, the South and North Equatorial Undercurrents (SEUC and NEUC respectively) and the Guinea Undercurrent (GUC), are shown to be linked to the westward currents located equatorward. We show that east of 20° W, both westward and eastward currents are associated with the spreading of PV tongues by mesoscale vortices. The Equatorial Undercurrent (EUC) brings salty waters into the Gulf of Guinea. Mixing diffuses the salty anomaly downward. Meridional advection, mixing and friction are involved in the formation of fluid parcels with PV anomalies in the lower part and below the pycnocline, north and south of the EUC, in the Gulf of Guinea. These parcels gradually merge and vertically align, forming nonlinear anticyclonic vortices that propagate westward, spreading and horizontally mixing their PV content by stirring filamentation and diffusion, up to 20° W. When averaged over time, this creates regions of nearly homogeneous PV within zonal bands between 1.5° and 5° S or N. This mean PV field is associated with westward and eastward zonal jets flanking the EUC with the homogeneous PV tongues corresponding to the westward currents, and the strong PV gradient regions at their edges corresponding to the eastward currents. Mesoscale vortices strongly modulate the mean fields explaining the high spatial and temporal variability of the jets.

Tides and internal tides (IT) in the ocean can significantly affect local to regional ocean temperature and even sea surface temperature (SST), via processes such as vertical mixing, vertical advection and transport of water masses. Offshore of the Amazon River, IT have already been detected and studied; however, their impact on temperature, SST and associated processes are not known in this region. In this work, we use high resolution (1/36°) numerical simulations with and without the tides from an ocean circulation model (NEMO) which explicitly resolves the internal tides (IT), 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 IT are well reproduced by the model, and are in good agreement with observations, for both their generation and their propagation. The simulation with 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. Results show that the observed coastal upwelling during ASO season is well reproduced by the model including tides, whereas the no-tide simulation is too warm by +0.3 °C at sea surface. 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 study further highlights that the IT induce vertical mixing on their generation site along the shelf break and on their propagation pathways towards the open ocean. This process explains the cooler temperature at the ocean surface and 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, mainly via vertical diffusivity along their propagation pathways of approximately 700 km offshore, and tides over the continental shelf, play a key role on the temperature structure off the Amazon River mouth, particularly in the coastal cooling enhanced by IT.

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.

This study investigates the influence of tides on chlorophyll-a (CHL) variability in the North Brazilian Equatorial Margin using daily remotely sensed CHL data from 2005 to 2021. The impact of the tides is deduced by comparing the spring tides with the neap tide signal (fortnightly signal, 14.7 days) in the GlobColour and MODIS-AQUA spring-neap tide composites. Results show that, on the shallow Amazon shelf, significant fortnightly CHL variability is primarily driven by barotropic tide-induced friction on the shelf that produces a significant vertical mixing. On the northwestern shelf, where the Amazon River plume dominates, sediment resuspension (likely driven by stronger tidal mixing during spring tides) suppresses primary production. In contrast, the increased stratification of the river plume during neap tides enhances nutrient availability likely enhancing primary production, leading to negative spring-neap CHL differences (GlobColour: -50 %, MODIS-AQUA: -84 %). Conversely, the northeastern shelf, where low turbid waters are present, exhibits positive CHL differences (GlobColour: +30 %, MODIS-AQUA: +70 %), likely caused by nutrient-rich uplift. Offshore, baroclinic tides, also known as internal tides (ITs), induce CHL-positive spring-neap tide differences in the spring-neap tide composite with a spatial structure of a wave-like pattern along IT pathways. These anomalies are spaced by mode-2 wavelengths (about 68 km), with peak values reaching +3.3 % (GlobColour) and +9.0 % (MODIS-AQUA). The observed wave-like pattern may be attributed to two potential mechanisms. First, tide aliasing caused by the satellites’ orbit characteristics, which consistently capture IT wave crests at similar locations, with concurrent modulation of the deep chlorophyll maximum (DCM) due to IT wave passage along the thermocline. Second, the propagation of internal tides (ITs) as beams into the ocean interior (“Ray theory”), causing upward displacement of isotherms and driving nutrient fluxes that enhance primary production, supported by ray-tracing analysis. Wave patterns in CHL spring-neap tide composites from GlobColour and MODIS-AQUA suggest contributions from mode-1 and mode-2 internal tides. Results indicate also a lag of 1–3 days between spring-neap tides and peak chlorophyll variability, indicative of maximum mixing. The effects of ITs on CHL are more pronounced than on sea surface temperature, likely due to differences in sensor penetration depths and the influence of air-sea interactions.

This study investigates the influence of tides on chlorophyll a (CHL) variability in the Brazilian Equatorial Margin using daily GlobColour and MODIS-Aqua CHL data from 2005 to 2021. The impact of the tides is assessed by comparing the spring with the neap tide signals (fortnightly signal, 14.7 d). Results show that, on the shallow Amazon shelf, significant fortnightly CHL variability is likely primarily driven by barotropic tide-induced friction on the shelf that produces significant vertical mixing. On the northwestern shelf, where the Amazon River plume prevails, CHL levels are higher during neap tides, resulting in a negative spring–neap tide CHL difference (GlobColour: −50 %; MODIS-Aqua: −84 %). Conversely, on the northeastern shelf, characterized by low-turbidity waters, CHL levels are higher during spring tides, leading to a positive spring–neap tide CHL difference (GlobColour: +30 %; MODIS-Aqua: +70 %). Offshore, baroclinic tides, also known as internal tides (ITs), seem to enhance the CHL along their pathways with a spatial structure of a wave-like pattern. The positive CHL peaks are spaced by mode-2 wavelengths (about 68 km), with peak values reaching +3.3 % (GlobColour) and +9.0 % (MODIS-Aqua). Analysis shows that the CHL wave-like pattern suggests contributions from mode-1 and mode-2 internal tides, with mode-2 components having higher spectral coherence with the original signal. A 1–3 d lag between higher CHL variability and tidal potential may indicate delayed nutrient mixing post-spring–neap tides. The effects of ITs on CHL are more pronounced than on sea surface temperature.