Internal tide signatures on surface chlorophyll concentration in the Brazilian Equatorial Margin

Abstract. 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.