South Atlantic meridional transports from NEMO-based simulations and reanalyses

Abstract

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 ∼ 8 Sv and 0.4 PW 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.

Subtropical‐Tropical Transfer in the South Atlantic Ocean

“…This comparison shows that the reanalysis products are more accurate than the free-running models at representing the AMOC's strength and meridional heat transport at 35°S and the NBUC volume transport at 11°S. Although the heat and volume transports are systematically underestimated across models, GLORYS2V4 is the one that shows the highest agreement with observations (Mignac et al, 2018). Drijfhout et al (2003) have also found that eddy-permitting models are essential to correctly represent the Lagrangian pathways in the thermocline.…”

“…Our region of study is the South Atlantic Ocean between 30°S and 5°S and down to 1,500 m. The position of the northern and southern boundaries ensures that the entire subtropical-tropical boundary is contained in the domain and further warrants that the particles circulate only once through either the subtropical or the tropical gyre before leaving the domain. Mignac et al (2018) have recently performed an exhaustive comparison between current data assimilation products (including GLORYS2V4), free-running models, and observations. This comparison shows that the reanalysis products are more accurate than the free-running models at representing the AMOC's strength and meridional heat transport at 35°S and the NBUC volume transport at 11°S.…”

“…Baringer and Garzoli (2007) and Majumder et al (2016) have shown that the full-column northward heat transport at 30°S is 1.5 PW when excluding the southward western boundary. Further, Mignac et al (2018) used several reanalysis models to show that the full-column integrated net heat transport at 30°S is around 0.5 PW. Both results together imply that there is a southward transport of 1.0 PW through the western boundary.…”

Observational Needs for Improving Ocean and Coupled Reanalysis, S2S Prediction, and Decadal Prediction

“…Ocean reanalyses can be studied to enhance understanding of processes driving observed changes. They are also useful for studying recent changes in the climate for quantities that are difficult to observe continuously, such as transports (Mignac et al, 2018), or those that require consistent spatial data coverage at depth, such as ocean heat content (Balmaseda et al, 2013;Wunsch and Heimbach, 2014). To be able to draw robust conclusions, one must be confident that inhomogeneous time series or abrupt regime changes are caused by physically consistent processes -not artifacts associated with changes in the historical observing network.…”

New Frontiers in Operational Oceanography

“…(h) Shows the total northward flow continuing the integration to the western boundary, and is thus a measure of the overturning component of the circulation. FromMignac et al (2017) Mignac et al (2017). showed results for South Atlantic transports in four reanalyses from…”