Decadal Modulations of Interhemispheric Global Atmospheric Circulations and Monsoons by the South Atlantic Meridional Overturning Circulation

This study presents a physical mechanism on how low-frequency variability of the South Atlantic meridional heat transport (SAMHT) may influence decadal variability of atmospheric circulation. A multicentury simulation of a coupled general circulation model is used as basis for the analysis. The highlight of the findings herein is that multidecadal variability of SAMHT plays a key role in modulating global atmospheric circulation via its influence on interhemispheric redistributions of momentum, heat, and moisture. Weaker SAMHT at 30 degrees S produces anomalous ocean heat divergence over the South Atlantic, resulting in negative ocean heat content anomalies about 15-20 years later. This forces a thermally direct anomalous interhemispheric Hadley circulation, transporting anomalous atmospheric heat from the Northern Hemisphere (NH) to the Southern Hemisphere (SH) and moisture from the SH to the NH, thereby modulating global monsoons. Further analysis shows that anomalous atmospheric eddies transport heat northward in both hemispheres, producing eddy heat flux convergence (divergence) in the NH (SH) around 15 degrees-30 degrees, reinforcing the anomalous Hadley circulation. The effect of eddies on the NH (SH) poleward of 30 degrees depicts heat flux divergence (convergence), which must be balanced by sinking (rising) motion, consistent with a poleward (equatorward) displacement of the jet stream. This study illustrates that decadal variations of SAMHT could modulate the strength of global monsoons with 15-20 years of lead time, suggesting that SAMHT is a potential predictor of global monsoon variability. A similar mechanistic link exists between the North Atlantic meridional heat transport (NAMHT) at 30 degrees N and global monsoons.