Distinct Modes of Internal Variability in the Global Meridional Overturning Circulation Associated with the Southern Hemisphere Westerly Winds

The internal variability of the global meridional overturning circulation (GMOC) in long-term integration of the earth system model Community Earth System Models (COSMOS) is examined in this study. Two distinct modes of the GMOC, which are closely linked to the Southern Hemisphere westerly winds (SWW) anomalies, are found to vary on multidecadal and centennial time scales. The dominant mode is characterized by Southern Ocean dynamics: strengthening and poleward shift of the SWW associated with a positive phase of the southern annular mode 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 mainly onto the Atlantic meridional overturning circulation (AMOC). It is driven by a combination of SWW variation and buoyancy forcing. Based on the relationship between the two modes together with the wind perturbation experiments, the authors emphasize that the full AMOC response to the SWW change takes several centuries in their model. The sea surface temperature in Northern Hemisphere high latitudes is significantly affected in this mode, showing a large-scale warming. Their results from a mid-Holocene experiment imply that both modes are independent from the climate background conditions in the Holocene. Finally, the authors 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.