Carina Regina de Macedo

The Amazon shelf–offshore continuum is a dynamic biogeochemical hotspot of the tropical Atlantic, where river discharge, ocean circulation, and strong tides interact to shape nutrient and phytoplankton distributions. The Amazon plume and regional circulation have been widely studied and are known to strongly influence nutrient availability and biological productivity in this region. However, shelf-break tides remain an overlooked pathway linking physical energy to offshore fertilization, and their contribution to seasonal and intraseasonal biogeochemical variability remains unclear. Here, we quantify how tidal dynamics, including internal tides, modulate nitrate supply and chlorophyll distributions from the Amazon shelf to offshore waters. We use a high-resolution coupled physical–biogeochemical model (1/36°), evaluated against climatological, satellite, and in situ observations. The model reproduces the main observed patterns of surface nitrate and chlorophyll, as well as key vertical features such as the nitracline and the deep chlorophyll maximum. We show that tides strongly enhance upward nitrate transport, increasing surface nitrate by more than 50% over the northern shelf, along the shelf break, and within the main internal-tide pathway. This tidally supplied nitrate fuels offshore phytoplankton growth, increasing chlorophyll by about 15–50%, while reducing surface chlorophyll near the Amazon mouth by 30– 40%.Seasonally, surface chlorophyll and nitrate are higher over the Amazon shelf during April–June but lower offshore, revealing a marked cross-shelf contrast. When the tidal contribution is isolated, a similar but weaker spatial structure 30 emerges, with a cone-shaped chlorophyll anomaly extending from the shelf break toward the offshore internal-tide propagation region. Remarkably, tides account for about 63% of the total seasonal variability in surface nitrate, meaning that tidal forcing alone explains more than half of the seasonal nutrient signal. At intraseasonal timescales, tides generate a clear spring–neap rhythm of about 15 days in both nitrate and chlorophyll. This spring–neap tidal pulse propagates from the shelf break toward offshore waters and is especially pronounced near the deep chlorophyll maximum, where oscillations of the upper nitracline periodically modulate nitrate availability and drive a corresponding chlorophyll response., where chlorophyll variability is nearly doubled when tides are included. The concurrent increase in nitrate variability indicates that this spring–neap phytoplankton response is sustained by tidally driven nutrient supply. These findings identify internal tides as a key biogeochemical driver of the Amazon shelf–offshore continuum, linking tidal energy to nutrient injection, offshore fertilization, seasonal redistribution, and rhythmic ecosystem variability in the western tropical Atlantic.

The Amazon shelf–offshore continuum is a dynamic biogeochemical hotspot of the tropical Atlantic, where river discharge, ocean circulation, and strong tides interact to shape nutrient and phytoplankton distributions. The Amazon plume and regional circulation have been widely studied and are known to strongly influence nutrient availability and biological productivity in this region. However, shelf-break tides remain an overlooked pathway linking physical energy to offshore fertilization, and their contribution to seasonal and intraseasonal biogeochemical variability remains unclear. Here, we quantify how tidal dynamics, including internal tides, modulate nitrate supply and chlorophyll distributions from the Amazon shelf to offshore waters. We use a high-resolution coupled physical–biogeochemical model (1/36°), evaluated against climatological, satellite, and in situ observations. The model reproduces the main observed patterns of surface nitrate and chlorophyll, as well as key vertical features such as the nitracline and the deep chlorophyll maximum. We show that tides strongly enhance upward nitrate transport, increasing surface nitrate by more than 50% over the northern shelf, along the shelf break, and within the main internal-tide pathway. This tidally supplied nitrate fuels offshore phytoplankton growth, increasing chlorophyll by about 15–50%, while reducing surface chlorophyll near the Amazon mouth by 30– 40%.Seasonally, surface chlorophyll and nitrate are higher over the Amazon shelf during April–June but lower offshore, revealing a marked cross-shelf contrast. When the tidal contribution is isolated, a similar but weaker spatial structure 30 emerges, with a cone-shaped chlorophyll anomaly extending from the shelf break toward the offshore internal-tide propagation region. Remarkably, tides account for about 63% of the total seasonal variability in surface nitrate, meaning that tidal forcing alone explains more than half of the seasonal nutrient signal. At intraseasonal timescales, tides generate a clear spring–neap rhythm of about 15 days in both nitrate and chlorophyll. This spring–neap tidal pulse propagates from the shelf break toward offshore waters and is especially pronounced near the deep chlorophyll maximum, where oscillations of the upper nitracline periodically modulate nitrate availability and drive a corresponding chlorophyll response., where chlorophyll variability is nearly doubled when tides are included. The concurrent increase in nitrate variability indicates that this spring–neap phytoplankton response is sustained by tidally driven nutrient supply. These findings identify internal tides as a key biogeochemical driver of the Amazon shelf–offshore continuum, linking tidal energy to nutrient injection, offshore fertilization, seasonal redistribution, and rhythmic ecosystem variability in the western tropical Atlantic.

The Amazon shelf–offshore continuum is a dynamic biogeochemical hotspot of the tropical Atlantic, where river discharge, ocean circulation, and strong tides interact to shape nutrient and phytoplankton distributions. The Amazon plume and regional circulation have been widely studied and are known to strongly influence nutrient availability and biological productivity in this region. However, shelf-break tides remain an overlooked pathway linking physical energy to offshore fertilization, and their contribution to seasonal and intraseasonal biogeochemical variability remains unclear. Here, we quantify how tidal dynamics, including internal tides, modulate nitrate supply and chlorophyll distributions from the Amazon shelf to offshore waters. We use a high-resolution coupled physical–biogeochemical model (1/36°), evaluated against climatological, satellite, and in situ observations. The model reproduces the main observed patterns of surface nitrate and chlorophyll, as well as key vertical features such as the nitracline and the deep chlorophyll maximum. We show that tides strongly enhance upward nitrate transport, increasing surface nitrate by more than 50% over the northern shelf, along the shelf break, and within the main internal-tide pathway. This tidally supplied nitrate fuels offshore phytoplankton growth, increasing chlorophyll by about 15–50%, while reducing surface chlorophyll near the Amazon mouth by 30– 40%.Seasonally, surface chlorophyll and nitrate are higher over the Amazon shelf during April–June but lower offshore, revealing a marked cross-shelf contrast. When the tidal contribution is isolated, a similar but weaker spatial structure 30 emerges, with a cone-shaped chlorophyll anomaly extending from the shelf break toward the offshore internal-tide propagation region. Remarkably, tides account for about 63% of the total seasonal variability in surface nitrate, meaning that tidal forcing alone explains more than half of the seasonal nutrient signal. At intraseasonal timescales, tides generate a clear spring–neap rhythm of about 15 days in both nitrate and chlorophyll. This spring–neap tidal pulse propagates from the shelf break toward offshore waters and is especially pronounced near the deep chlorophyll maximum, where oscillations of the upper nitracline periodically modulate nitrate availability and drive a corresponding chlorophyll response., where chlorophyll variability is nearly doubled when tides are included. The concurrent increase in nitrate variability indicates that this spring–neap phytoplankton response is sustained by tidally driven nutrient supply. These findings identify internal tides as a key biogeochemical driver of the Amazon shelf–offshore continuum, linking tidal energy to nutrient injection, offshore fertilization, seasonal redistribution, and rhythmic ecosystem variability in the western tropical Atlantic.

The Amazon shelf–offshore continuum is a dynamic biogeochemical hotspot of the tropical Atlantic, where riverdischarge, ocean circulation, and strong tides interact to shape nutrient and phytoplankton distributions. The Amazon plumeand regional circulation have been widely studied and are known to strongly influence nutrient availability and biologicalproductivity in this region. However, shelf-break tides remain an overlooked pathway linking physical energy to offshore fertilization, and their contribution to seasonal and intraseasonal biogeochemical variability remains unclear. Here, wequantify how tidal dynamics, including internal tides, modulate nitrate supply and chlorophyll distributions from theAmazon shelf to offshore waters. We use a high-resolution coupled physical–biogeochemical model (1/36°), evaluatedagainst climatological, satellite, and in situ observations. The model reproduces the main observed patterns of surface nitrateand chlorophyll, as well as key vertical features such as the nitracline and the deep chlorophyll maximum. We show that tides strongly enhance upward nitrate transport, increasing surface nitrate by more than 50% over the northern shelf, alongthe shelf break, and within the main internal-tide pathway. This tidally supplied nitrate fuels offshore phytoplankton growth,increasing chlorophyll by about 15–50%, while reducing surface chlorophyll near the Amazon mouth by 30–40%.Seasonally, surface chlorophyll and nitrate are higher over the Amazon shelf during April–June but lower offshore,revealing a marked cross-shelf contrast. When the tidal contribution is isolated, a similar but weaker spatial structure30 emerges, with a cone-shaped chlorophyll anomaly extending from the shelf break toward the offshore internal-tidepropagation region. Remarkably, tides account for about 63% of the total seasonal variability in surface nitrate, meaning thattidal forcing alone explains more than half of the seasonal nutrient signal. At intraseasonal timescales, tides generate a clearspring–neap rhythm of about 15 days in both nitrate and chlorophyll. This spring–neap tidal pulse propagates from the shelfbreak toward offshore waters and is especially pronounced near the deep chlorophyll maximum, where oscillations of theupper nitracline periodically modulate nitrate availability and drive a corresponding chlorophyll response., wherechlorophyll variability is nearly doubled when tides are included. The concurrent increase in nitrate variability indicates thatthis spring–neap phytoplankton response is sustained by tidally driven nutrient supply.These findings identify internal tides as a key biogeochemical driver of the Amazon shelf–offshore continuum, linking tidalenergy to nutrient injection, offshore fertilization, seasonal redistribution, and rhythmic ecosystem variability in the westerntropical Atlantic.

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.

Off the Amazon shelf, internal solitary waves (ISWs) generated by internal tides interact with mesoscale eddies, leading to significant modifications in their propagation and structure. For the first time, these interactions are directly observed from repeated high-resolution satellite measurements, notably those provided by the newly launched SWOT (Surface Water and Ocean Topography) mission. The study investigates ISWs detection and the modifications in ISW's characteristics resulting from interactions with eddies across three contrasting scenarios: ISW propagation in the absence of mesoscale eddies, ISW refraction by a cyclonic eddy, and ISW diffraction by an anticyclonic eddy. ISW crests are extracted using a band-pass filtering approach, enabling precise tracking of essential geometric and dynamical features, including propagation direction, crest-to-crest spacing, and wavefront shape. Prior to any interaction with eddies, three mode-1 ISW packets propagate in a steady and coherent manner, exhibiting planar wavefronts and consistent direction, with Absolute Dynamic Topography (ADT) amplitudes ranging from 8 to 14 cm. The results highlight the diversity of ISW responses depending on eddy conditions. In the absence of eddies, interaction with a seamount induces energy transfer from mode-1 to mode-3 ISWswaves (13 crests detected), while propagation direction is maintained and the ADT signature weakens to below 8 cm. In the presence of a cyclonic eddy above the seamount, ISW trajectories are refracted westward by roughly 50 degrees (7 crests detected), with increased wavecrest curvature and intensification of mode-3 generation, while ADT amplitudes decrease below 6 cm. Conversely, near the western boundary of an anticyclonic eddy, ISWs split into two distinct pathways: a western branch is refracted with flatter wavefronts, reduced crest spacing, and ADT amplitudes below 0.2 cm (9 crests detected), while an eastern branch follows the eddy's edge, displaying enhanced curvature, recognizable surface signatures of wave packets, and ADT values exceeding 9 cm (6 crests detected). Together, these findings demonstrate the ability of SWOT-based monitoring to capture the complexity of ISW dynamics and provide new insight into their nonlinear response to interactions with mesoscale and submesoscale oceanic structures.

Off the Amazon shelf, mesoscale eddies interact with internal solitary waves (ISWs), modifying their characteristics. For the first time, such interactions are directly observed through repeated measurements from a set of high-resolution satellite data, including the recently launched SWOT (Surface Water and Ocean Topography) mission. This study investigates ISWs detectable in SWOT Absolute Dynamic Topography (ADT) and characterizes the changes in their properties induced by interactions with mesoscale eddies. The analysis focuses on three scenarios: ISW propagation in the absence of eddies, ISW refraction by a cyclonic eddy, and ISW diffraction by an anticyclonic eddy. ISW crests were identified and extracted using a band-pass filtering technique, allowing accurate tracking of key features such as propagation direction, spacing between individual crests, and wavecrest geometry. Before any interaction with eddies, mode-1 ISWs propagate steadily, with consistent direction and planar wavefronts. A key finding is the variety of ISW responses depending on eddy conditions. In the first case, in the absence of eddy, the interaction of ISWs with a seamount induced energy transfer from mode-1 to mode-3 ISWs, while the propagation direction remains unchanged. In the second case, a cyclonic eddy overlaying the seamount refracted ISW trajectories westward by approximately 50°, while also increasing wavecrest curvature and enhancing the generation of mode-3 waves. In the third case, at the western edge of an anticyclonic eddy near the seamount, the ISWs are split into two distinct paths: one branch refracted westward, exhibiting flatter wave crests and reduced spacing between them; the other branch followed the eastern edge of the eddy, displaying surface signatures of wave packets and enhanced wavecrest curvature. These results demonstrate the effectiveness of the proposed approach in capturing the complex dynamics of ISWs. They offer novel insights into the nonlinear behavior of ISWs and their interactions with mesoscale and submesoscale oceanic features.

Off the Amazon shelf, mesoscale eddies interact with internal solitary waves (ISWs), modifying their characteristics. For the first time, such interactions are directly observed through repeated measurements from a set of high-resolution satellite data, including the recently launched SWOT (Surface Water and Ocean Topography) mission. This study investigates ISWs detectable in SWOT Absolute Dynamic Topography (ADT) and characterizes the changes in their properties induced by interactions with mesoscale eddies. The analysis focuses on three scenarios: ISW propagation in the absence of eddies, ISW refraction by a cyclonic eddy, and ISW diffraction by an anticyclonic eddy. ISW crests were identified and extracted using a band-pass filtering technique, allowing accurate tracking of key features such as propagation direction, spacing between individual crests, and wavecrest geometry. Before any interaction with eddies, mode-1 ISWs propagate steadily, with consistent direction and planar wavefronts. A key finding is the variety of ISW responses depending on eddy conditions. In the first case, in the absence of eddy, the interaction of ISWs with a seamount induced energy transfer from mode-1 to mode-3 ISWs, while the propagation direction remains unchanged. In the second case, a cyclonic eddy overlaying the seamount refracted ISW trajectories westward by approximately 50°, while also increasing wavecrest curvature and enhancing the generation of mode-3 waves. In the third case, at the western edge of an anticyclonic eddy near the seamount, the ISWs are split into two distinct paths: one branch refracted westward, exhibiting flatter wave crests and reduced spacing between them; the other branch followed the eastern edge of the eddy, displaying surface signatures of wave packets and enhanced wavecrest curvature. These results demonstrate the effectiveness of the proposed approach in capturing the complex dynamics of ISWs. They offer novel insights into the nonlinear behavior of ISWs and their interactions with mesoscale and submesoscale oceanic features.

The ocean region off the Amazon shelf including the shelf break presents a hotspot for internal tide (IT) generation, yet its impact on phytoplankton distribution remains poorly understood. While previous studies have extensively examined the physical characteristics and dynamics of ITs, their biological implications – particularly in nutrient-limited environments – remain underexplored. To address this question, we analyzed a 26 d glider mission deployed over September–October 2021 sampling hydrographic and optical properties (chlorophyll a) at high resolution along an IT pathway as well as satellite chlorophyll a and altimetry data to assess mesoscale interactions. Chlorophyll a dynamics were analyzed under varying IT intensities, comparing strong (HT) and weak (LT) internal tide conditions. Results reveal that ITs drive vertical displacements of the deep chlorophyll maximum (DCM) from 15 to 45 m, accompanied by 50 % expansion in its thickness during HT events. This expansion is observed with a dilution of the chlorophyll a maximum concentration within the DCM depth. While direct turbulence measurements were not collected, the observed vertical redistribution of chlorophyll a is indicative of tidally driven cross-isopycnal exchanges, the only physical mechanism explaining the transfer of biomass above and below the DCM. At the surface, turbulent fluxes provide 38 % of the chlorophyll a input, while the remainder is supplied by in situ biological activity. Notably, total chlorophyll a in the water column increases by 14 %–29 % during high internal tide phases, indicating a net enhancement of primary productivity driven by the combined effects of vertical mixing and stimulated surface-layer biological activity. These findings indicate that internal tides can be an important driver of chlorophyll a distribution and short-term biological variability in our study region. By reshaping the vertical chlorophyll a profile through vertical mixing, active internal tides influence primary productivity and may contribute to carbon cycling, particularly in oligotrophic oceanic environments where both a deep chlorophyll maximum and strong internal tides are present.