The Global Ocean Conveyor Belt is Reshaping from the Southern Ocean

As the surface ocean warms and polar ice sheets melt due to increasing anthropogenic greenhouse gases in the atmosphere, near-surface stratification is increasing almost everywhere, including the major deep water formation regions in the high-latitude North Atlantic and around Antarctica. As a result, the global Meridional Overturning Circulation (MOC), also known as the global ocean conveyor belt, is expected to change significantly with a risk of disrupting the global redistribution of ocean properties that sustains marine ecosystems, carbon cycle, sea-level, climate, and extreme weather. A growing number of observational studies based on repeated hydrographic sections are suggesting that the MOC in the Southern Ocean is undergoing significant changes since the 1980s. However, unlike in the Atlantic Ocean where several observing systems are currently in place, it is not currently possible to directly measure the MOC spanning the entire Southern Ocean. Additionally, the majority of the current state-of-the-art climate and ocean models fail to reproduce even the mean state of the MOC in the Southern Ocean let alone its variability or long-term trend. As such, we are still in the dark while significant shifts in the MOC, likely due to human activity, are currently in progress in the Southern Ocean and neighboring regions where there are no direct measurements and climate models are failing.

Encouraged by recent success in reproducing directly-observed MOC transport values in multiple transects as reported in Lee et al (2019) and Zhang and Thomas (2021), a team of scientists from NOAA’s Atlantic Oceanographic and Meteorological Laboratory, the Northern Gulf Institute, the National Center for Atmospheric Research, and the University of Miami made a novel attempt to utilize a diagnostic ocean & sea-ice model to estimate the global MOC and its interdecadal changes that are consistent with decadally averaged historical global hydrographic observations since the mid-1950s. The derived global MOC and its interdecadal changes show a significant broadening and strengthening of the upper overturning cell (50 ~ 60% increase) and contraction and weakening of the lower overturning cell (10 ~ 20% decrease) in the Southern Ocean since the mid-1970s. These changes are ultimately caused by the combined influence of ozone depletion in the Southern Hemisphere stratosphere and increasing greenhouse gasses in the atmosphere. The study also shows that the externally forced changes in the Southern Ocean are slowly advancing into the other ocean basins.

Figure 6 in Lee et al. (2023). A summary schematic of the global MOC in 2005–2017. Major changes in the global MOC and the associated increases in Southern Hemisphere westerly and Antarctic meltwater discharge and P − E changes in 2005−2017 in reference to 1955–1974 are also indicated.

Lee, SK., Lumpkin, R., Gomez, F., Yeager, S., Lopez, H., Takglis, F. Dong, S., Aguiar, W., Kim, D. & Baringer, M. (2023). Human-induced changes in the global meridional overturning circulation are emerging from the Southern Ocean. Commun Earth Environ 4, 69. https://doi.org/10.1038/s43247-023-00727-3