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 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.

Competing effects of wind and buoyancy forcing on recent ocean oxygen trends

“…2a-b,i-j). A steady strengthening of the Southern Hemisphere westerly winds in past decades [31,40] is thought to have strengthened the upper cell of the MOC [41] and to have increased the ventilation of Subantarctic Mode Water and Antarctic Intermediate Water [42][43][44]. In our model results, the upper cell of the Southern Ocean MOC also shows a long-term strengthening (Supplementary Fig.…”

“…In this region, the recent strengthening of westerly winds, caused by increasing greenhouse gas emissions and stratospheric ozone depletion [31], has counteracted the deoxygenation caused by widespread surface warming and mid-depth freshening [64,65]. The poleward shift and implicit movement of the sinking branch of intermediate water masses into denser water may have further accelerated intermediate water mass formation [41], while increasing oxygen solubility in subducted waters.…”

“…By modulating the strength of the Southern Ocean upper circulation cell [41] and setting the formation rates and regions of oxygen-rich intermediate water masses [43,44], the Southern Hemisphere westerly winds are critical for global oxygen supply [33].…”

Deglacial Carbon Escape From the Northern Rim of the Southern Ocean

“…This work highlights the role that shifting fronts played in driving SAMW formation and the past modulation of oceanic CO 2 uptake and release from the Southern Ocean. Modern hydrographic studies are documenting signs that the modern ACC has been both strengthening (Freeman et al., 2016; Shi et al., 2021) and migrating poleward (Lee et al., 2023; Yamazaki et al., 2021) as our climate warms. Much like the deglaciation, the attendant thickening and strengthening of SAMW formation that has been documented in the modern Southern Indian Ocean (Lu et al., 2021) may indicate expansion of the SAZ that could foretell a Southern Ocean shift from a CO 2 sink to a CO 2 source as the globe continues to warm.…”

Wind-driven upwelling sustains weakened Atlantic overturning under climate extremes

“…The SO upper cell at 34.5°S strengthens in 4xCO2 (Extended Data Fig. 5a) due to enhanced SO westerly winds 33,34 that more than compensates the weakening from their poleward shift (Extended Data Fig. 5b).…”