Weakening of Antarctic bottom water circulation is noticeable in the North Atlantic

The Antarctic bottom water is part of the Atlantic Meridional Overturning Circulation (AMOC). Like a conveyor belt, the system of ocean currents distributes heat, nutrients and carbon dioxide into the world's oceans. Under the leadership of the University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science, researchers have analyzed two decades of data from several observation programs to determine how the flow of Antarctic bottom water has changed. The results confirm: During the 21st century, the current in this layer has slowed down; it has weakened by around twelve per cent in the last two decades. As a result, less cold water is flowing into higher latitudes, causing the deep sea to warm up.

Dr Torsten Kanzow from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) is also involved in the study. The oceanographer has been studying ocean currents for quite some time: "Perhaps 20 years ago, during my doctoral thesis, I analyzed a data set from an American colleague on deep currents in the tropical North Atlantic. This almost lost data set suddenly became relevant again for this study. That's how I joined the project." For the study, the AWI researcher helped to compare the warming of the bottom water in the North Atlantic with that in the Weddell Sea - one of the origin regions of the Antarctic bottom water. The result: near the source, the warming is twice as great as in the North Atlantic.

The results of this study indicate once again that even the most remote areas of the world's oceans are not unaffected by human activities. This is because they cause environmental changes in the Antarctic that lead to the weakening of the deep-sea circulation of the Antarctic Bottom Water and thus to a rise in sea level in the North Atlantic. “Although these regions are tens of thousands of miles away from each other and abyssal areas are a few miles below the ocean surface, our results reinforce the notion that even the most remote areas of the world's oceans are not untouched by human activity,” said the study’s lead author Tiago Biló, scientist at the Rosenstiel School’s NOAA Cooperative Institute for Marine and Atmospheric Studies. “Our observational analysis matches what the numerical models have predicted—human activity could potentially impose circulation changes on the entire ocean.”

He emphasises that the analysis was only possible thanks to decades of joint planning and efforts by several oceanographic institutions worldwide. In addition to the AWI, the University of Miami, the Rosenstiel School of Marine, Atmospheric, and Earth Science and the Atlantic Oceanographic and Meteorological Laboratory of the National Oceanic and Atmospheric Administration (NOAA) are involved in the study. The project was supported by NOAA, the U.S. National Science Foundation, the EU Horizon 2020 program and the German Research Foundation.

Original publication:

Biló, T.C., Perez, R.C., Dong, S. et al. Weakening of the Atlantic Meridional Overturning Circulation abyssal limb in the North Atlantic. Nat. Geosci. (2024). https://doi.org/10.1038/s41561-024-01422-4