Session: Last Millennium & Past2K
Author: Fernanda Marcello de Oliveira / email@example.com / University of São Paulo
Co-author: Ilana Wainer, University of São Paulo;
This study investigates changes in the circulation of the South Atlantic Subtropical Gyre (SASG) during the Last Millennium, especially in its northern boundary, represented by the bifurcation of the southern branch of the South Equatorial Current (sSEC) into the North Brazil Undercurrent/Current (NBUC/NBC) to the north and the Brazil Current (BC) to the south. The sSEC Bifurcation Latitude (SBL) marks the transition between waters flowing equatorward within the upper branch of the Atlantic Meridional Overturning Circulation (AMOC) and those flowing poleward and recirculating in the subtropical gyre. Analysis of the large-scale ocean gyre dynamics are performed using simulation results from the Last Millennium Ensemble experiment of the Community Earth System Model version 5 (CESM-CAM5 LME), for the period ranging 850-2005. Results point to an increase in the total anticyclonic circulation and a southward displacement of the subtropical gyre system. More specifically, it is found increased values of Wind Stress Curl, Sea Surface Height and Barotropic Stream Function within the dynamical rims of the subtropical gyre, together with a synchronous poleward migration of the system, which is demonstrated by displaced climatological isopleths of these fields. It is also observed a consistent southward migration of the SBL, associated with a significant increase in the equatorward advection of waters within the sSEC-SBL-NBUC system, which contributes to the AMOC upper branch. Accordingly, time series of the basin-integrated meridional transport which estimates the overturning strength displays a considerable increase in the AMOC cell, suggesting that although the governing dynamics of the subtropical gyre favors a spinned-up circulation, the bulk of the increased sSEC transport is directed to the Northern Hemisphere with the NBUC feeding the AMOC instead of heading southward with the BC and recirculating in the SASG. It is revealed that for the end of the 20th century the observed changes in the SBL and the SASG dynamics have reached levels that had rarely, if ever, been exceeded in the preceding past 1000 years.