South Atlantic Surface Boundary Current System during the Last Millennium in the CESM-LME: The Medieval Climate Anomaly and Little Ice Age

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.

Projected Atlantic overturning slow-down is to be compensated by a strengthened South Atlantic subtropical gyre

“…The sSEC is considered the zonal extension of the BeC, which defines the northern limb of the SASG and is associated with the origins of both the northward NBUC and southward BC [119][120][121][122] . There is evidence for a multi-banded sSEC 119,123 , and here we define the sSEC as the westward flow between ~10 ∘ -40 ∘ S, partitioned proportionally between southern, central and northern branches (Fig. 2a transects 13, 16 and 17, respectively).…”

Late Holocene Precipitation Fluctuations in South America Triggered by Variability of the North Atlantic Overturning Circulation

“…Sufficiently highly resolved SST records are yet lacking to reconstruct South Atlantic SST variability during the past 5 kyrs. However, recent numerical model experiments using the ensemble mean of the Community Earth System Model Last Millennium Ensemble indicate that reconfigurations of ocean‐surface currents in response to a transient weakening of the AMOC caused a cooling in the extratropical relative to the tropical South Atlantic (Marcello et al., 2019). Observational and reanalysis data (Bombardi et al., 2014) clearly show that the ensuing positive South Atlantic Dipole mode would have led to a strengthened SALLJ bringing more moisture toward Botuverá Cave, while reducing the frequency of cyclones and moisture advection toward central eastern Brazil (cf.…”

Orbital Forcing Strongly Influences the Poleward Shift of the Spring Himalayan Jet During the Past Millennium

“…The CESM‐LME ensembles comprise: “single‐forcing” simulations including individual external forcing factors, namely land use and land cover change (LULC), ozone/aerosols, greenhouse gases, solar irradiance, orbital forcing, and volcanic eruptions; and “fully forced” simulations that include all forcing factors, simultaneously (Otto‐Bliesner et al., 2016). The CESM‐LME has been used to study many aspects of climate and are able to capture the observed character of climate variability (Capotondi et al., 2020; Marcello et al., 2019; Stevenson, Fasullo, et al., 2017; Stevenson, et al., 2018). The CESM‐LME therefore provides an excellent opportunity to diagnose forced trends in STJ position over the Himalayas during the past millennium.…”