Scientists at NOAA’s Atlantic Oceanographic and Meteorological Laboratory (AOML) have shown that the Global Meridional Overturning Circulation (GMOC), commonly known as the global ocean conveyor belt, has changed significantly in the Southern Ocean since the mid-1970s, with a broadening and strengthening of the upper overturning cell and a contraction and weakening of the lower cell. These changes are attributed to human-induced ozone depletion in the Southern Hemisphere stratosphere and increased carbon dioxide in the atmosphere. The study also shows that the changes in the Southern Ocean are slowly advancing into the South Atlantic and Indo-Pacific oceans.
The GMOC is a system of ocean currents that moves heat, freshwater, nutrients, and carbon around the global ocean (Figure 1). As the ocean warms and polar ice sheets melt due to increasing carbon in the atmosphere, near-surface stratification (the separation of a body of water in layers by density) is also increasing. Enhanced near-surface stratification inhibits the sinking of warm surface waters into the deep ocean. As a result, the GMOC is expected to change significantly, posing a risk of disrupting the redistribution of heat, salt, nutrients, and carbon between hemispheres and across ocean basins. These changes can have direct societal impacts on coastal sea levels, marine ecosystems, extreme weather events, and shifts in regional weather and global climate.
Numerous studies based on repeated hydrographic observations have suggested that significant changes in the GMOC have already occurred during recent decades. However, unlike in the Atlantic Ocean where several observing systems are presently in place, it is not currently possible to directly measure the MOC spanning the entire Southern Ocean due to its large size and location. Additionally, the majority of the current climate and ocean models fail to accurately reproduce the mean state or long-term trends of the MOC in the Southern Ocean.
“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,” said Sang-Ki Lee, lead author of the study.
A team of scientists from AOML, the Northern Gulf Institute, National Center for Atmospheric Research, and University of Miami used a diagnostic ocean and sea-ice model to estimate the GMOC and its changes since the mid-1950s that are consistent with historical hydrographic observations.
The study found that the Southern Ocean’s upper and lower overturning cells have changed significantly since the mid-1970s. More specifically, the upper overturning cell has strengthened its flow by 50 ~ 60% and expanded poleward into denser water. These changes are largely due to ozone depletion in the Southern Hemisphere stratosphere and an associated increase in Southern Hemisphere westerly winds and loss of surface buoyancy. The depletion of the ozone layer causes air to cool, resulting in stronger westerly winds in the region.
Conversely, the lower overturning cell has weakened its flow by 10 ~ 20% of the total and contracted during the same period, due to the increased melting of Antarctic ice sheets driven by increasing carbon dioxide in the atmosphere. When ice sheets melt into the ocean, surface waters are fresh and light instead of salty and dense. This freshwater is unable to sink into the deep ocean, which leads to a weakening of the lower cell.
The changes found in the Southern Ocean, as summarized in Figure 2, have an important implication for the ocean’s uptake of anthropogenic carbon. The strengthening and expansion of the upper cell may bring more natural carbon stored in the deep ocean for hundreds to thousands of years to the surface. When the concentration of dissolved carbon at the surface is too high, it is released into the atmosphere through a process known as CO2 outgassing. At the same time, the weakening and contraction of the lower cell may allow for less sinking of carbon dioxide into the deep ocean, leading to reduced anthropogenic carbon uptake. Both of these processes increase anthropogenic carbon concentration in the atmosphere and thus accelerate global warming.
This is the first study to report, based on historical global hydrographic observations, that a significant reshaping of the GMOC has already emerged from the Southern Ocean due to human activity. Additionally, a large-scale readjustment of the GMOC seems to be underway in the South Atlantic and Indo-Pacific oceans during the most recent decade (2005-2017) in response to changes in the Southern Ocean.
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