Water masses distribution over the continental shelf off the Doce River (19.65°S), Southeastern Brazil: A fuzzy cluster analysis

Abstract

This study investigates the seasonal variability of water masses across three sectors of the Espírito Santo Continental Shelf (ESCS): the North Sector, River Sector, and Environmental Protected Area (EPA) Sector. Utilizing a comprehensive dataset collected between November 2018 and July 2023, we applied fuzzy clustering methods to outline three water masses — Coastal Water (CW), Tropical Water (TW), and South Atlantic Central Water (SACW) — and analyze their distribution across the sectors. Our findings reveal distinct seasonal patterns in these water masses’ distribution and properties, influenced by factors like river discharge, coastal upwelling, and net heat flux. Absolute salinity distribution showed less saline river runoff water constrained to the coastal region around the Doce River mouth and eventually in the coastal North Sector due to increased continental discharge associated with reversal southerly winds. Winter is an exception to the conservative temperature and absolute salinity distribution due to the homogeneity of cold column water and the lower input of freshwater. Particularly noteworthy is the seasonal variability observed in the River Sector, where the influence of freshwater discharge from the Doce River significantly impacts water masses distribution. CW occurrence follows the distribution of absolute salinity and the area affected by the river plume, TW is associated with the Brazil Current, and SACW occurs over the continental shelf throughout the year (despite the weak thermal gradient during the winter). At the surface, TW prevails at all sectors beyond the river influence during the autumn and past the 30 m-isobath in the remaining seasons due to upwelled waters. This study provides valuable insights into the complex interactions shaping the oceanographic characteristics of the ESCS, highlighting the importance of considering seasonal variability in understanding coastal marine ecosystems.