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

Anthropogenically-induced changes in the global meridional overturning circulation since the 1950s have led the upper overturning cell in the Southern Ocean to expand and strengthen while the lower cell has contracted and weakened, according to diagnostic model simulations. In a warming climate, the Global Meridional Overturning Circulation (GMOC) is expected to change significantly with a risk of disrupting the global redistribution of ocean properties that sustains marine ecosystems, carbon cycle, and others. Here we make a novel attempt to utilize a diagnostic ocean & sea-ice model to estimate the GMOC and its interdecadal changes since the mid-1950s that are consistent with 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, in response to ozone depletion in the SH stratosphere and increasing atmospheric CO2. 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.

Automated summary

According to diagnostic model simulations, anthropogenic activities have caused changes in the global meridional overturning circulation since the 1950s. The upper overturning cell in the Southern Ocean has expanded and strengthened, while the lower cell has contracted and weakened. These changes have significant implications for the redistribution of ocean properties, including marine ecosystems and carbon cycling. A readjustment of the global meridional overturning circulation seems to be occurring in the South Atlantic and Indo-Pacific Oceans since the mid-2000s in response to the changes observed in the Southern Ocean.