Human-induced changes in the global meridional overturning circulation are emerging from the Southern Ocean

Abstract

In a warming climate, the Global Meridional Overturning Circulation (GMOC) is expected to slow down with a significant risk of disrupting the global redistribution of ocean properties that sustains marine ecosystems, carbon cycle, and others. Here, we analyze the GMOC and its interdecadal changes since the mid-1950s derived from an ocean & sea-ice model with its temperature and salinity restored toward historical hydrographic observations. We find that significant changes in the GMOC have already occurred, most notably in the upper and lower overturning cells in the Southern Ocean. The former has expanded poleward and into denser water and strengthened by 3~4 Sv since the mid-1970s, while the latter has contracted and weakened by a similar rate during the same period. These changes are driven by the increasing Southern Hemisphere (SH) Ferrel cell and associated increases in the westerlies and the surface buoyancy loss over its sinking branch, and the increasing Antarctic meltwater discharge that are resulted from the combined effect of ozone depletion in the SH stratosphere and increasing CO2 in the atmosphere. A large-scale readjustment of the GMOC seems to be underway in the South Atlantic and Indo-Pacific Oceans since the mid-2000s in response to the Southern Ocean changes.