Circulation and Hydrography in the Western South Atlantic Shelf and Export to the Deep Adjacent Ocean: 30°S to 40°S

Abstract

High sea surface chlorophyll concentration on the Argentine Continental Shelf frequently extends to the deep ocean in the vicinity of the Brazil/Malvinas Confluence (BMC). The offshore transport of shelf waters likely plays a key role in the biogeochemical balance of the western South Atlantic and promotes the offshore transport of planktonic species. We analyze data from an oceanographic survey carried out in the western South Atlantic shelf between 31°S and 38°S in October 2013. We describe the distribution and circulation of the water masses and focus on the exchanges with the open ocean. On-shelf subsurface intrusions of oceanic waters and river discharge supply nutrients to the shelf. A low-salinity tongue of Río de la Plata (RDP) waters extends northward to 32°S. Below these waters Subantarctic and Subtropical Shelf Waters (SASW and STSW) meet to form the Subtropical Shelf Front. The main SASW branch, oversaturated in oxygen and with high-fluorescence mixes with a detachment of Brazil Current waters at 38°S and is exported offshore along the BMC. A second branch of SASW reaches 33°S mixing along its way with RDP and STSW and returns southward after splitting into an onshore and an offshore branch. The offshore branch is exported to the open ocean through the BMC. These export routes are in overall qualitative agreement with those indicated by a high-resolution reanalysis. We estimate a net off-shelf transport (geostrophic [96.2%] plus Ekman [3.8%]) of 3.44 Sv to the open ocean between 32.1°S and 37.7°S. The majority of the offshore flow occurs between 34.7°S and 37.7°S.

Volume Transport Variability on the Northern Argentine Continental Shelf From In Situ and Satellite Altimetry Data

“…Thus, minima in the satellite derived transport (Figure 5b) are likely associated with transport reversals of relatively short duration, as the ones observed during 2015. This is in agreement with recent reports of a transport reversal event in October 2013 detected by hydrographic and lowered ADCP observations obtained near 37°S (Berden et al., 2020). This event was produced by persistent northeasterly winds and is also readily apparent as a minimum (0.5 Sv) in our satellite time series (Figure 5b).…”

“…As pointed out in Auad and Martos (2012), Berden et al. (2020), and Lago et al. (2019), these reversal events might have strong biological consequences, since they can influence the distribution of planktonic species.…”

“…This event was produced by persistent northeasterly winds and is also readily apparent as a minimum (0.5 Sv) in our satellite time series (Figure 5b). As pointed out in Auad and Martos (2012), Berden et al (2020), andLago et al (2019), these reversal events might have strong biological consequences, since they can influence the distribution of planktonic species.…”

New insights of the influence of ocean circulation on the sedimentary distribution in the Southwestern Atlantic margin (23° S to 55° S) based on Nd and Pb isotopic fingerprinting

“…Over the shelf, the extension of the BMC, known as the Subtropical Shelf Front (STSF), separates Subtropical Shelf Waters (STSW, formed by the mixture of the TW and SACW) and Subantarctic Shelf Waters (SASW) (Piola et al, 2000). This narrow and sharp front extends between 32°S at 50 m of water column depth and 36°S over the shelf break, and its distribution appears stable throughout the year (Piola et al, 2000;Berden et al, 2020). At the STSF, the main branch is mixed with waters transported by the BC and exported offshoreward along with the BMC.…”

“…At the STSF, the main branch is mixed with waters transported by the BC and exported offshoreward along with the BMC. A secondary branch is diluted with the PPW and TW and returns along the shelf (Berden et al, 2020).…”