Abstract
Interocean waters that are carried northward through South Atlantic surface boundary currents get meridionally split between two large-scale systems when meeting the South American coast at the western subtropical portion of the basin. This distribution of the zonal flow along the coast is investigated during the Last Millennium, when natural forcing was key to establish climate variability. Of particular interest are the changes between the contrasting periods of the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA). The investigation is conducted with the simulation results from the Community Earth System Model Last Millennium Ensemble (CESM-LME). It is found that the subtropical South Atlantic circulation pattern differs substantially between these natural climatic extremes, especially at the northern boundary of the subtropical gyre, where the westward-flowing southern branch of the South Equatorial Current (sSEC) bifurcates off the South American coast, originating the equatorward-flowing North Brazil Undercurrent (NBUC) and the poleward Brazil Current (BC). It is shown that during the MCA, a weaker anti-cyclonic subtropical gyre circulation took place (inferred from decreased southern sSEC and BC transports), while the equatorward transport of the Meridional Overturning Circulation return flow was increased (intensified northern sSEC and NBUC). The opposite scenario occurs during the LIA: a more vigorous subtropical gyre circulation with decreased northward transport.
Highlights
The South Atlantic Ocean (SAO) is unique in its establishment of teleconnections between the adjacent ocean basins and as a critical crossroads for the global Meridional Overturning Circulation (MOC) (e.g., [1])
The most remarkable differences were concentrated at the southern limits of the domain, where the maximum Medieval Climate Anomaly (MCA)-warming lied south of the mean-location of the Brazil-Malvinas Confluence (BMC), and the maximum Little Ice Age (LIA)-cooling was located further east, towards the middle of the basin, where there is a region of amplified cooling spreading zonally and northwestwardly
Results show clear opposing MCA/LIA anomalous circulation patterns: the MCA was characterized by overall positive VVEL anomalies off the western boundary (Figure 3a), indicating an intensification of the northward North Brazil Undercurrent (NBUC) and weakening of the southward Brazil Current (BC); while the LIA was subject to overall negative anomalous flows along the coast (Figure 3c) pointing to an NBUC weakening and BC intensification
Summary
The South Atlantic Ocean (SAO) is unique in its establishment of teleconnections between the adjacent ocean basins and as a critical crossroads for the global Meridional Overturning Circulation (MOC) (e.g., [1]). A mixture of Pacific and Indian Ocean contributions is received at the southern opposing corners of the SAO basin, through the Drake Passage (the cold-water route, [2]) and the Agulhas Leakage (the so-called warm-water route, [3]) These are blended together and incorporated within the large-scale South Atlantic Subtropical Gyre (SASG) circulation to continue to the northward flowing upper-limb of the MOC. The sSEC heads westward crossing the basin until meeting the South American (SA) coast At this point, the zonal flow gets divided meridionally between two important large-scale circulation systems. The Last Millennium (LM) spans the most recent past before anthropogenic forcing became significant, representing an ideal opportunity to understand how the climate system varied under natural conditions [17,18] It is marked by two significant climatic events resulting from natural variability: the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA).
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