South Atlantic meridional fluxes

Abstract

The properties of the meridional overturning circulation (MOC) and associated meridional heat transport (MHT) and salt fluxes are analyzed in the South Atlantic. The oceanographic data used for the study consist of Expendable bathythermograph (XBT) data collected along 27 sections at nominally 35°S for the period of time 2002–2011, and Argo profile data collected in the region. Previous estimates obtained with a shorter record are improved and extended, using new oceanographic sections and wind fields. Different wind products are analyzed to determine the uncertainty in the Ekman component of the MHT derived from their use. Results of the analysis provide a 9-year time series of MHT, and volume transport in the upper layer of the MOC. Salt fluxes at 35°S are estimated using a parameter introduced by numerical studies, the Mov that represents the salt flux and helps determine the basin scale salt feedback associated with the MOC. Volume and heat transport by the western and eastern boundary currents are estimated, and their covariablity is examined. Analysis of the data shows that the South Atlantic is responsible for a northward MHT with a mean value of 0.54±0.14PW. The MHT exhibits no significant trend from 2002 to 2011. The MOC varies from 14.4 to 22.7Sv with a mean value of 18.1±2.3Sv and the maximum overturning transport is found at a mean depth of 1250m. Statistical analysis suggests that an increase of 1Sv in the MOC leads to an increase of the MHT of 0.04±0.02PW. Estimates of the Mov from data collected from three different kinds of observations, contrary to those obtained from models, feature a positive salt advection feedback (Mov<0) suggesting that freshwater perturbations will be amplified and that the MOC is bistable. In other words, the MOC might collapse with a large enough freshwater perturbation. Observations indicate that the mean value of the Brazil Current is −8.6±4.1Sv at 24°S and −19.4±4.3Sv at 35°S, increasing towards the south. East of 3°E, the northward flowing Benguela Current and Agulhas rings have a net northward transport of 22.5±4.7Sv. No significant correlation is observed between the MOC and the Brazil Current transport, and most of the compensation derives from the eastern boundary and interior transports. Products from the Ocean general circulation model For the Earth Simulator (OFES) are used to validate methodology used to extend the XBT record, and to aid in the interpretation of the observed findings.