Testing concepts for continuous monitoring of the meridional overturning circulation in the South Atlantic

We investigate if and how the monitoring strategy for the meridional overturning circulation (MOC) implemented at 26°N in the Atlantic can also be applied at a latitude in the South Atlantic. The RAPID 26°N strategy to monitor the MOC is based on continuous measurements of zonal density differences across a zonal transect, continuous measurements of the western boundary current, and additional estimates of the zonal wind stress from satellite observations. Here, we simulate a monitoring array akin to the RAPID array at 26°N in the global coupled climate ECHAM5/MPI-OM, forced with the IPCC scenario A1B. We find that the monitoring strategy can provide reliable estimates of the MOC in the South Atlantic, but the latitude needs to be carefully chosen to ensure adequate coverage of the variability arriving from both the north and the south. The limitations in the North Atlantic apply in the South Atlantic, however, we find that direct boundary current observations and bottom velocity measurements are of lesser importance for the time-mean value and the variability than in the North Atlantic. However, western boundary observations and bottom velocity measurements are crucial in capturing the vertical structure of the MOC correctly. We suggest that basin-wide MOC monitoring based on the RAPID strategy at 26°N be conducted only where boundary currents do not hit steep topography, and where bottom velocities are small.