Cold versus warm water routes for the upper limb of the South Atlantic MOC

The surface water in the South Atlantic (σ2 < 35.7) is known to originate largely from the Indian Ocean via the Agulhas leakage (e.g., Beal et al., 2011; Gordon, 1986). It is carried northward below the surface mixed layer and brought to the surface via the equatorial Atlantic upwelling. Antarctic Intermediate Water (AAIW) that forms at the surface mixed layer in the southeast Pacific is partly carried to the South Atlantic through the Drake Passage, and serves an important source of the intermediate depth water in the South Atlantic (σ2 = 35.7–36.8; e.g., McCartney, 1977; Talley, 1996). The surface water from the Indian Ocean (warm water route) and the intermediate depth water from the southeast Pacific (cold water route) together feed the upper limb of the Meridional Overturning Circulation (MOC) in the South Atlantic Ocean. According to a data-constrained ocean model study (Lee et al., 2019), the upper limb of the MOC transports 9.1 Sv of the surface water and 7.8 Sv of the intermediate depth water across 30°S. A new study published in Ocean Science used a high-resolution (1/20°) ocean model to better quantify the cold versus warm water routes into the North Brazil Current (NBC), which channels the upper limb of the Atlantic MOC in the tropical South Atlantic. The study used Lagrangian particle trajectories embedded in the ocean model to conclude that out of 11.8 Sv NBC transport across 6°S, about 7.1 Sv (~ 60%) is from the warm route and about 4.7 Sv (~ 40%) is from the cold route. The study also concluded that about 50 ~ 66% of the surface and intermediate depth water reached the surface mixed layer at least once during their transit. This implies that more than half of the water masses supplying the upper limb of the Atlantic MOC are significantly modified through air-sea interactions within the South Atlantic, and thus do not conserve their characteristic properties from the Pacific and Indian Oceans.

Figure 13 from Rühs et al. (2019): Summary of the Atlantic MOC’s upper limb “cold”(DP, blue) and “warm” (mainly AC, red) water routes inferred from O(10⁶) simulated Lagrangian particle trajectories: major (thick arrows) and minor (thin arrows) advective pathways; Lagrangian mean cross section transports in Sverdrups (numbers); respective volumetric contribution of particles entering the mixed layer at least once during the transit (numbers in brackets); and most likely areas of last mixed layer contact.

Rühs, S., Schwarzkopf, F. U., Speich, S., and Biastoch, A. (2019). Cold vs. warm water route – sources for the upper limb of the Atlantic Meridional Overturning Circulation revisited in a high-resolution ocean model, Ocean Sci., 15, 489-512, https://doi.org/10.5194/os-15-489-2019. https://www.ocean-sci.net/15/489/2019/os-15-489-2019.html