This study explores potential factors that may influence decadal variability of the South Atlantic meridional overturning circulation (SAMOC) by using observational data as well as surface-forced ocean model runs and a fully coupled climate model run. Here we show that SAMOC is strongly correlated with the leading mode of sea surface height (SSH) variability in the South Atlantic Ocean, which displays a meridional dipole between north and south of 20°S. A significant portion (~45%) of the South Atlantic SSH dipole variability is remotely modulated by the Interdecadal Pacific Oscillation (IPO). Further analysis shows that anomalous tropical Pacific convection associated with the IPO forces robust stationary Rossby wave patterns, modulating the wind stress curl over the South Atlantic Ocean. A positive (negative) phase IPO increases (decreases) the westerlies over the South Atlantic, which increases (decreases) the strength of the subtropical gyre in the South Atlantic and thus the SAMOC.
(a) Regression of SSH PC1 with surface wind stress (vector) and wind stress curl times Coriolis parameter (color) from altimetry data. (b) Same as in Figure 5a but for 20 Century Reanalysis. (c) Schematic diagram of the influence of the IPO on South Atlantic SSH and SAMOC variabilities. Heating anomaly in the atmosphere associated with the positive IPO generates Rossby wave source region extending from the tropical western Pacific toward South America (pink hatching). This generates a stationary wave pattern extending from the source region poleward around the southern tip of South America (labeled H and L for anticyclone and cyclone, respectively). This circulation produces anomalous westerlies in the South Atlantic between 30°S and 40°S, enhancing the northward Ekman transport, which in turn enhances the subtropical gyre circulation and northward SAMOC (dark green arrow).
*Lopez, H., S. Dong, S.-K. Lee, and E. Campos, 2016: Remote influence of interdecadal Pacific Oscillation on the South Atlantic Meridional Overturning Circulation variability. Geophys. Res. Lett ., 43(15):8250 - 8258, doi:10.1002/2016GL069067. [PDF]