Cross-Shelf Exchange in the Southwestern Atlantic Shelf: Climatology and Extreme Events

Abstract

The variability and drivers of the cross-shelf exchanges between the Southwestern Atlantic shelf and the open ocean from 30 to 40°S are analyzed using a high-resolution ocean model reanalysis at daily resolution. The model's performance was first evaluated using altimetry data, and independent mooring and hydrographic data collected in the study area. Model transports are in overall good agreement with all other estimates. The record-mean (1993–2018) cross-shore transport is offshore, 2.09 ± 1.60 Sv. 73% of the shelf-open ocean exchange occurs in the vicinity of Brazil-Malvinas Confluence (~38°S) and 20% near 32°S. This outflow is mostly contributed by northward alongshore transport through 40°S (63%) and the remaining by southward transport through 30°S (37%). The cross-shore flow presents weak seasonal variations, with a maximum in austral summer, and high variability at subannual and weekly time scales. The latter is mainly associated with abrupt wind changes generated by synoptic atmospheric systems. Alongshore wind variations set up sea-level changes in the inner shelf which in turn drive large anomalies in the associated geostrophic alongshore flow. The difference in inner shelf sea-level anomalies at 30 and 40°S is a good indicator of cross-shelf exchange at seasonal and shorter time scales. Episodes of extreme offshore transport that reach up to 9.45 Sv and last about 2 days are driven by convergence of these alongshore flows over the shelf. Large exports of shelf waters lead to freshening of the upper open ocean as revealed by in-situ and satellite observations. In contrast, onshore extreme events drive open ocean water intrusions of up to 6.53 Sv and last <4 days. These inflows, particularly the subtropical waters from the Brazil Current, induce a substantial salinification of the outer shelf.

Wind‐driven currents and water masses shape spring phytoplankton distribution and composition in hydrologically complex, productive shelf waters

“…Daily means of surface current data were downloaded from high‐resolution (1/12°) global Mercator Ocean reanalysis (GLORYS) from Copernicus Marine Environment Monitoring Service (http://marine.copernicus.eu/). The performance of GLORYS was previously evaluated in the study area (30–40°S) on a daily basis using altimetry (1993–2018), mooring and hydrographic data (12 surveys, 2003–2018), concluding that the model is in good agreement with the regional dynamics of the shelf circulation (Berden et al 2022). Likewise for Chl a and wind stress, 8‐d temporal means were computed to analyze the synoptic scale.…”

Western Boundary Current–Subtropical Continental Shelf Interactions

“…However, the spatiotemporal scales of shelf export events may not correspond well to the scales of CO 2 uptake seen in global and regional models. Shelf edge eddies and filaments can facilitate extensive cross-shelf exchanges over limited spatial and temporal scales (e.g., Berden et al, 2022;Suthers et al, 2023). Along the US southeast coast and the southern Brazilian margin, for example, a large fraction of the annual shelf water exchange and shelf primary production is effectively isolated from the atmosphere by a seasonal thermocline.…”

Circulation and Cross‐Shelf Exchanges in the Northern Shelf of the Southwestern Atlantic: Dynamics

“…In the southern region where the Brazil Current collides with the Malvinas Current, offshore eddies are found to play a key role also in the location of the detrainment of the shelfwater to the deep ocean (Manta et al., 2022). Quantitative estimations of the magnitude of those cross‐shelf exchanges have been entirely based on model simulations (Berden et al., 2022; Combes & Matano, 2014; Matano et al., 2010; Palma et al., 2008; Palma & Matano, 2009). Combes et al.…”

“…In the southern region where the Brazil Current collides with the Malvinas Current, offshore eddies are found to play a key role also in the location of the detrainment of the shelfwater to the deep ocean (Manta et al, 2022). Quantitative estimations of the magnitude of those cross-shelf exchanges have been entirely based on model simulations (Berden et al, 2022;Matano et al, 2010;Palma et al, 2008;Palma & Matano, 2009). Combes et al (2021), for example, finds ∼1.2 Sv of onshelf transport mostly restricted to the layer below 50 m north of the SBB and ∼1.6 Sv of offshelf transport with stronger outflow in the upper layer south of the SBB.…”