The internal variability of the global meridional overturning circulation (GMOC) in long-term integration of a state-of-art coupled general circulation model is examined. Two distinct modes of the GMOC, which are closely linked to the Southern Hemisphere westerly winds (SWWs) anomalies, are found on multi-decadal and centennial time scales. The dominant mode is characterized by Southern Ocean dynamics: strengthening or poleward shifts of the SWWs yield Ekman-induced northward mass transport, including a zonally asymmetric response in the Southern Ocean sea surface temperature, and a cooling in the tropical Pacific Ocean due to large-scale upwelling. The second mode projects more on the Atlantic meridional overturning circulation (AMOC). It affects the high latitude northern hemisphere sea surface temperature. Our results from a mid-Holocene experiment imply that both modes are independent with the climate background conditions in the Holocene. Finally, we argue that the natural modes of GMOC are important to understand trends in ocean circulation with consequences for heat and carbon budgets for past, present and future climate.