Modeling the Offshore Export of Subantarctic Shelf Waters From the Patagonian Shelf

Abstract

It has been suggested that the Subtropical Shelf Front (STSF) could be a preferential site for the detrainment of Subantarctic Shelf Water (SASW) and related planktonic shelf species onto the open SW Atlantic Ocean. The offshore detrainment of SASW and planktonic shelf species might be an exportation mechanism, affecting the population abundances of fishing resources in Argentina, Uruguay, and Southern Brazil. In this study, we characterize for the first time the 3-D structure of the STSF and the main routes of offshore export of SASW from the Patagonian shelf during austral summer (summer and early fall) and winter (winter and early spring) by using numerical hydrodynamical model results and Lagrangian tracking simulations of neutrally buoyant floats. The transport of SASW toward the open ocean is~1 Sv (1 Sv = 10 6 m 3 /s) during summer and~0.8 Sv during winter. SASW are exported offshore mainly near the Brazil-Malvinas Confluence region during both seasons. The STSF appears to act as an important retention mechanism for the plankton over the inner and middle shelf mainly during late summer and early fall. Our findings could explain the life cycle of distinct fish species that are distributed in the region, as well as the population abundance variability of such species.Plain Language Summary The Subtropical Shelf Front (STSF) has been suggested to be a preferential site for the detrainment of Subantarctic Shelf Water (SASW) from the Patagonian continental shelf toward the open ocean. This offshore detrainment of SASW might be an exportation mechanism to the open ocean of early larval stages of life of shelf fish species, affecting the population abundances of fishing resources in Argentina, Uruguay, and Southern Brazil. In this work we characterize the 3-D structure of the STSF, its dynamics and variability, and the main routes of offshore export of SASW during austral summer and winter for the first time. We discover that the main offshore detrainment of SASW occurs near the Brazil-Malvinas Confluence, while the STSF appears to act as an important retention mechanism for early larval stages of life of shelf fish species mainly during late summer and early fall. Our findings could explain the life cycle of distinct fish species that are distributed in the region, as well as the population variability of such species.

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

“…The other branch of SASW, the northern branch, characterized by 75% SASW concentration, continues northward over the mid-shelf, reaches 35°S (Transect 3) and 50% SASW reaches 33°S (Transect 6), near the STSF location ( Figure 5). This northernmost extent of SASW is consistent with previous observations (Möller et al, 2008;Piola et al, 2000) and numerical simulations reported by Palma et al (2008), and between the extensions found by Franco et al (2018) during summer and winter. Piola et al (2008) suggested that SASW reaches the position of the STSF, where it describes a sharp cyclonic loop and flows to the open ocean.…”

“…The circulation schematic is similar to those presented by Möller et al (2008) and Piola et al (2008) with the addition of the splitting SASW path near 37°S. This is in agreement with the results of Franco et al (2018) based on numerical simulations during summer. Furthermore, the double southward return path of the secondary branch of SASW onshore and offshore of the northern branch is also a new feature of the regional circulation.…”

“…Though a substantial fraction of SASW may be exported offshore close to the northernmost extension of the Malvinas Current (Franco et al., 2018), a fraction of SASW flows northeastward to about 32°S (Palma et al., 2008; Piola et al., 2000), thus establishing the STSF. Interestingly, however, the LADCP data in early October 2013 indicate that most of the middle and outer shelf SASW sampled in Transect 1 flows southward (Figure 8a).…”

“…The flow of high‐chlorophyll shelf waters can have a significant biogeochemical impact on the adjacent deep ocean, as suggested by filaments of high surface chlorophyll concentration detaching from the shelf along the axis of the BMC revealed by satellite ocean color observations. Recent lagrangian studies of high‐resolution simulations suggest that a significant fraction of the northeastward flowing Subantarctic Shelf Water (SASW) is exported to the deep ocean via the BMC (Franco et al., 2018).…”

Subsurface Ocean Warming Hotspots and Potential Impacts on Marine Species: The Southwest South Atlantic Ocean Case Study

“…In the SWAO, the northern shelf is occupied by waters of subtropical origin and the southern shelf by waters of subantarctic origin (Piola et al, 2000). Subantarctic shelf waters are exported offshore mainly near the BMC region (Franco et al, 2018). In the northern SWAO shelf, tropical waters are observed in upper layers (Campos et al, 1995).…”