Dynamics and morphology of sinking particles in the equatorial Atlantic during the 2021 Atlantic Nino

The equatorial upwelling system is characterized by a strong seasonal cycle with relatively cold sea surface temperature (SST) and enhanced primary production in the “cold tongue region” of the eastern basin during boreal summer. During the boreal summer of 2021, the equatorial Atlantic witnessed its most intense warm event since the beginning of satellite observations, which is assumed to have a direct impact on the carbon cycle. Here we use data from a BGC Argo float, deployed in the equatorial upwelling region in order to investigate the production peaks of marine particles during two distinct periods: the decay period of the anomalous weak cold tongue and the period of secondary cooling in boreal winter. In situ images of plankton and particles and physical and biogeochemical data provided by the Underwater Vision Profiler 6 (UVP6) and various sensors mounted on the float were analyzed in conjunction with satellite data (sea surface height, SST, ocean color). The float covered the period between 13 July 2021 and 23 March 2022 drifting eastward from 23-7.4°W along the equator and conducting 2000 m profiles every three days. Our data revealed the occurrence of two blooms with high surface chlorophyll concentrations accompanied by the presence of two carbon export events reaching at least 2000 m depth. Both events exhibited high carbon flux at the mixed layer with a flux of 106±5 mgC.m-2d-1 during the first event compared to 122±17 mgC.m-2d-1 during the second while flux between both events remained below 89 mgC.m-2d-1. However, a distinction in the vertical extent of these events was recorded as there was a slightly higher flux at 2000 m for the winter boreal, 30% higher, suggesting a difference between the carbon attenuation flux export associated with the primary upwelling season with the one observed during the secondary cooling period in the boreal winter. The characterization of the morphology of detritus using in situ imaging and clustering method revealed the presence of five different morpho-types with different sinking properties. Two primary classifications—large and small dense aggregates—emerged as the predominant exported detritus to depths while porous aggregates were more concentrated in the surface layer. Our study revealed a dynamic interaction between various layers, involving carbon production in the surface layer, succeeded by its subsequent export to deeper layers. Finally, this study offers new insights into particle dynamics and the morphology of sinking particles within the equatorial region.