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Ocean Science An interactive open-access journal of the European Geosciences Union
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Volume 14, issue 1 | Copyright
Ocean Sci., 14, 53-68, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 01 Feb 2018

Research article | 01 Feb 2018

South Atlantic meridional transports from NEMO-based simulations and reanalyses

Davi Mignac1,2, David Ferreira2, and Keith Haines2 Davi Mignac et al.
  • 1Postgraduate Program in Atmosphere, Oceans and Climate, University of Reading, Reading, UK
  • 2Department of Meteorology, University of Reading, Reading, UK

Abstract. The meridional heat transport (MHT) of the South Atlantic plays a key role in the global heat budget: it is the only equatorward basin-scale ocean heat transport and it sets the northward direction of the global cross-equatorial transport. Its strength and variability, however, are not well known. The South Atlantic 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 (NEMO) model, and the ORAs share very similar configurations. Very few previous works have looked at ocean circulation patterns in reanalysis products, but here we show that the ORA basin interior transports are consistently improved by the assimilated in situ and satellite observations relative to the FRMs, especially in the Argo period. The ORAs also exhibit systematically higher meridional transports than the FRMs, which is in closer agreement with observational estimates at 35 and 11°S. However, the data assimilation impact on the meridional transports still greatly varies among the ORAs, leading to differences up to  ∼8Sv and 0.4PW in the South Atlantic Meridional Overturning Circulation and the MHTs, respectively. We narrow this down to large inter-product discrepancies in the western boundary currents (WBCs) at both upper and deep levels explaining up to  ∼85% of the inter-product differences in MHT. We show that meridional velocity differences, rather than temperature differences, in the WBCs drive  ∼83% of this MHT spread. These findings show that the present ocean observation network and data assimilation schemes can be used to consistently constrain the South Atlantic interior circulation but not the overturning component, which is dominated by the narrow western boundary currents. This will likely limit the effectiveness of ORA products for climate or decadal prediction studies.

Publications Copernicus
Short summary
Four ocean reanalyses and two free-running models are compared to study the meridional transports in the South Atlantic. We analyse the underlying causes of the product differences in an attempt to understand the potential impact (and limitations) of the data assimilation (DA) in improving the simulated ocean states. The DA schemes can consistently constrain the basin interior transports, but not the overturning circulation dominated by the narrow South Atlantic western boundary currents.
Four ocean reanalyses and two free-running models are compared to study the meridional...