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Abstract A high‐resolution ocean model is used to characterize the circulation and cross‐shelf exchanges in the Brazilian/Uruguayan portion of the southwestern Atlantic shelf. Cross‐shelf exchanges are strongly modulated by the bottom topography. There is ∼1.2 Sv of on‐shelf transport between 21°S and 25.2°S, and ∼1.6 Sv of off‐shelf transport between 35°S and 25.2°S. North of 25.2°S, the cross‐shelf exchanges show a two‐layer structure with an off‐shelf flow in the upper 50m and on‐shelf flow deep below. A Lagrangian diagnostic shows that ∼0.15 Sv of deep waters from the Brazil Current (z > 200 m) are injected into the shelf. Mixing with ambient waters produces a spicier (warmer and saltier) water mass, which is ejected into the open ocean in the southern region. Backward in‐time particle's trajectories analysis reveals that 95% of the southward shelf transport at 32°S originates in the open ocean at 22°S. Our model diagnostics show that there is a very limited connectivity between the shelf regions north and south of Cabo Frio. Correlation analysis shows no significant influence of El Niño Southern Oscillation (ENSO) and Southern Annular Mode (SAM) on the time variability of the cross‐shelf transport. Cross‐shelf transports, however, are significantly correlated with the local wind stress variability.
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Cross-Shelf Exchange in the Southwestern Atlantic Shelf: Climatology and Extreme Events
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.
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- Award ID(s):
- 1645887
- PAR ID:
- 10398935
- Date Published:
- Journal Name:
- Frontiers in Marine Science
- Volume:
- 9
- ISSN:
- 2296-7745
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3389/fmars.2022.855183
