The overturning circulation in the ocean is expected to change in response to anthropogenic forcings, but the lack of direct long-term monitoring makes it challenging to detect human-induced change. A diagnostic modeling approach is used here to demonstrate that Southern Ocean (SO) overturning has undergone significant interdecadal change since the mid-20th century. The low-resolution (1°) CESM1 ocean and sea ice component models are integrated over 1955-2017 with nudging towards observed temperature and salinity fields. The results indicate that the SO upper cell has strengthened by ~3-4 Sv since the 1970s and expanded poleward, while the lower cell has weakened and contracted. The multidecadal shift in the relative strength and position of upper and lower overturning cells are consistent with anthropogenic changes in SO wind and buoyancy forcing. Since the mid-2000s, anthropogenic signals from the SO are found to penetrate the South Atlantic and Indo-Pacific basins.
While models suggest that the Atlantic meridional overturning circulation (AMOC) will decline due to greenhouse gas forcing, mooring arrays deployed in the North Atlantic (RAPID, OSNAP) are still too short to unambiguously detect an anthropogenic influence on AMOC. This research combines models with the much longer record of hydrographic observations to investigate multidecadal change in global ocean overturning circulation (GMOC) and finds that an anthropogenic signal may already be emerging from the Southern Ocean. These results offer novel insight into where and how anthropogenic forcing will alter global ocean circulation.
It is shown that ocean observations from the mid-20th century to present are consistent with a secular change in the Southern Ocean overturning that, in recent decades, has penetrated the deep South Atlantic and Indo-Pacific basins.