Hydrographic, nutrient and phytoplankton data were observed in the Amazon River Plume (ARP) in the Western Tropical North Atlantic (WTNA) during three cruises in May 2018, June 2019, with RV Endeavor and April/May 2021 with RV Meteor. The fundamental question whether an observed structure is quasi–synoptic or not, depends mainly on the duration of the observations and on the complexity of the flow field. Using daily quasi–geostrophic surface velocity from satellite data, the geographical positions of the stations of observation were transformed onto Lagrangian coordinates to obtain a dynamically coherent and consistent spatial distribution. After the transformation, the observed salinity fields were consistent with the flow fields, the ARP formed a coherent structure and the retroflection of the plume appeared also in Lagrangian coordinates. The use of “synchronous” fields such as that done here by transformation onto Lagrangian coordinates are essential for spatially structured analyses of observations taken over 10s of days in a dynamic region.