The dispersal of the Amazon and Orinoco River water in the tropical Atlantic and Caribbean Sea: Observation from space and S-PALACE floats

Abstract

Abstract The temporal evolution of spatial patterns of the colored water mass associated with the discharges of the Amazon and Orinoco Rivers between 1997 and 2002 was examined using concurrent in situ and satellite observations in the region bounded by 0°N–24°N and 70°W–40°W. Patches of low-salinity (∼32–34) surface waters were frequently observed with Salinity Profiling Autonomous LAgrangian Current Explorer (S-PALACE) floats as far as 2000 km away from the mouths of the Amazon and Orinoco Rivers. Ocean-color data collected with the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) show that these patches originate with these rivers. Chlorophyll (Chl) and colored dissolved organic matter (CDOM) distribution estimated with SeaWiFS data are well correlated with low sea-surface salinity (SSS) throughout the observation period (average correlation coefficients for monthly observations of −0.6 for the SSS–CDOM relationship and −0.5 for the SSS–Chl relationship). The color of the water in the river plumes is dominated by CDOM rather than Chl light absorption, in contrast to adjacent open-ocean and equatorial upwelling regions, where optical properties are primarily controlled by Chl. The mean CDOM absorption coefficient for the area covered by the Amazon's plume showed a peak in July for the 1998–2001 period, which lagged the Amazon's hydrograph by one month. The timing of the CDOM peak along different sections of the Amazon River plume implies an estimated advection speed of ∼35 cm s−1 within the plume. Anti-cyclonic rings (eddies) with diameters of 400–500 km were observed moving along the northeastern coast of South America every year and in all seasons, at a frequency of about one every 30–60 days. These eddies moved to the northwest at a speed of 8–20 cm s−1, with higher speeds observed during the first half of each calendar year. The average depth of the Amazon plume for SSS ranging from 32 to 35 was 20–30 m, and the total fresh-water volume contained in the plume was consistent with discharge rates. Due to uncertainties in the Chl and CDOM absorption coefficient estimates in river plumes, accurate estimation of the rivers’ impact on regional primary production remains a challenge. The paired SeaWiFS and S-PALACE float observations validate the hypothesis that the coherent surface color patches seen annually far offshore in the tropical North Atlantic are caused by dispersal of Amazon and Orinoco River water. In contrast, wind-driven upwelling near the edge of the North Brazil Current retroflection, another mechanism causing color enhancement, is not suggested by these data sets.