Transports and Pathways of the Tropical AMOC Return Flow From Argo Data and Shipboard Velocity Measurements

The upper-ocean circulation of the tropical Atlantic is a complex superposition of thermohaline and wind-driven flow components. The resulting zonally and vertically integrated upper-ocean meridional flow is referred to as the upper branch of the Atlantic Meridional Overturning Circulation (AMOC)-a major component and potential tipping element of the global climate system. Here, we investigate the tropical part of the northward AMOC branch, that is, the return flow covering the upper 1,200 m, based on Argo data and repeated shipboard velocity measurements. The western boundary mean circulation at 11 degrees S is realistically reproduced from high-resolution Argo data showing a remarkably good representation of the volume transport of the return flow water mass layers when compared to results from direct velocity measurements along a repeated ship section. The AMOC return flow through the inner tropics (11 degrees S-10 degrees N) is found to be associated with a diapycnal upwelling of lower central water into the thermocline layer of similar to 2 Sv. This is less than half the magnitude of previous estimates, likely due to improved horizontal resolution. The total AMOC return flow at 11 degrees S and 10 degrees N is derived to be similar in strength with 16-17 Sv. At 11 degrees S, northward transport is concentrated at the western boundary, where the AMOC return flow enters the inner tropics at all vertical levels above 1,200 m. At 10 degrees N, northward transport is observed both at the western boundary and in the interior predominantly in the surface and intermediate layer indicating recirculation and transformation of thermocline and lower central water within the inner tropics. Plain Language Summary The Atlantic Meridional Overturning Circulation (AMOC) is one of the major components of the global climate system. In the upper 1,200 m, the northward branch of the AMOC transports large amounts of heat, salt, and biogeochemical tracers across the equator from the South Atlantic through the tropics to the North Atlantic. In this study, we show that a realistic reconstruction of the upperocean circulation at the southern hemisphere western boundary-a bottleneck for the AMOC-is possible based on high-resolution Argo float data, further enabling transport and pathway estimates for the upper and intermediate water mass layers of the inner tropical Atlantic (11 degrees S-10 degrees N). At 11 degrees S, the northward AMOC branch is largely concentrated at the western boundary, whereas, at 10 degrees N, it preferably exits the inner tropics through the western boundary, but also through the interior basin after recirculating in the equatorial current system. When crossing the inner tropics, the water masses forming the AMOC return flow change their characteristics and the associated upwelling of water into the subsurface layer is found here to be less than half as large as previously estimated, likely due to improved horizontal resolution.

Automated summary

The upper-ocean circulation of the tropical Atlantic is a complex combination of thermohaline and wind-driven flow. The AMOC return flow through the inner tropics is associated with a diapycnal upwelling of lower central water into the thermocline layer. The study provides a realistic reconstruction of the upper-ocean circulation and estimates the transport and pathway of water mass layers in the tropical Atlantic.