South Atlantic meridional transports from NEMO-based model simulations and reanalyses

The South Atlantic meridional transports are evaluated for four state-of-the-art global Ocean Reanalyses (ORAs) and two Free-Running Models (FRMs) in the period 1997-2010. All products employ the Nucleus for European Modelling of the Oceans model, and the ORAs share very similar configurations. The ORA basin interior transports are consistently modified relative to the FRMs, especially in the Argo period, with an improved representation of the south equatorial currents. The ORAs also exhibit systematically higher meridional transports than the FRMs, in closer agreement with large-scale observational estimates at 35˚S and western boundary measurements at 11˚S. However, the transport impacts by data assimilation still greatly vary between the ORAs, leading to differences up to ~8 Sv and 0.4 PW in the South Atlantic Meridional Overturning Circulation and the Meridional Heat Transports (MHTs), respectively. Large inter-product discrepancies arise in the ORA western boundary currents at both upper and deep levels explaining up to ~85% of the inter-product differences in their total MHTs, and meridional velocity differences, rather than temperatures differences, drive ~83% of this spread. Further analysis shows that only very confined temperature differences right against the western boundary geostrophically explain the large boundary current velocity differences. These findings suggest that the current data assimilation schemes, even with Argo data, can consistently constrain the basin interior circulation in the ORAs, but not the overturning transport component dominated by the narrow western boundary currents as in the South Atlantic