Direct Pathways From the Drake Passage to the South Atlantic Subtropical Gyre

Abstract

Lagrangian simulations based on 18 years (2002–2019) of high-resolution thermohaline and three-dimensional velocity fields allow revisiting the fate and thermohaline changes of the upper-ocean Antarctic Circumpolar Current (ACC) waters that enter directly the South Atlantic Ocean basin. An advection-diffusion scheme, applied to both climatological annual-mean and daily mean fields, allows estimating the mean pathways and seasonal variability, as well as recirculation volume transports, times, and depths in the South Atlantic subtropical gyre (SASG). The annual-mean diffusive simulation shows that 96.5 Sv of the upper-ocean waters (up to the 28.00 kg m⁻³) crossing the Drake Passage remain in the ACC, while 13.0 Sv join the eastern margin of the SASG. About 8.6 Sv of this eastern input, plus an additional 2.7 Sv that enter the SASG through the interior ocean, reach the North Brazil Current, yielding a total Drake contribution of 11.2 Sv to the upper returning-limb of the Atlantic Meridional Overturning Circulation. The upper-ocean waters that reach the eastern SASG undergo substantial water mass transformations, with a net transfer of 6.7 Sv from intermediate-deep to surface layers and an increase in heat transport by 0.39 PW and salt transport by 8.5 × 10⁶ kg s⁻¹, but remain largely unchanged as they drift westward toward the western boundary at 21°S. Most waters within the SASG (86%) recirculate once, taking a median of 9.1 years, although some complete as many as three loops after reaching 32°S-W. Regarding seasonality, the transit times and transport fraction of the upper-ocean waters flowing into the SASG show higher variability than those remaining in the ACC path.