The Amazon Continental Shelf (ACS) is influenced by two large systems: Amazon and Pará rivers. However, there are no studies on the ACS density field influenced by the variability and stratification of the plumes of these two systems. The aim of this study was to investigate the spatial-temporal variation of the density field in a section along the ACS. For this, data from four oceanographic campaigns obtained under the Costa Norte Project were used: march, July, October and December 2018. Data collection occurred at 8 points, near the isobata of 50 m, at a distance of approximately 200 km from the continent. The vertical TS profiles were obtained using a CTD sensor (Conductive, Temperature, and Depth) Sea Bird model SBE-37 SM, with a sample frequency of 0.2 Hz. The Brunt-Vaisala frequency squared (N²), used to determine fluid stability, was calculated according to the density gradient and the Stratification Parameter (𝑃ɛ) calculated as a function of the vertical gradient of Salinity. In the transitional and dry season (July and October 2018) there was no significant difference in temperature and salinity between surface and bottom (mean of 28 ºC and 36.1 g.kg-1). In the rainy season (March), there was strong vertical stratification between surface and bottom from 28 to 27.5 ºC and 35.4 to 17.2 g.kg-1. In this period, maximum temperature (28.2 ºC) was also observed around 15 meters deep, with an average difference of 0.2 ºC in relation to surface temperature, except at some points where this difference was up to 0.6 ºC. The N² analysis showed greater magnitude during the rainy season and the transitional period between dry and rainy (maximum of Log10(N²) equal to -1.7, always around 7-10 meters deep). These results suggest that despite the presence of continental water on the surface from the continental drainage of the two systems that drain the region, there is a high stability between these bodies of water and, that it appears mainly during the period rainy conditions in a condition of strong vertical stratification (Pɛ equal to 5.9.10-1). It is possible that the thermal inversion observed in the mixing layer is caused by non-instant interaction between these two continental water masses that have their own physical tracers, resulting in two plumes that do not mix.
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