Ocean currents in the North Atlantic, the Gulf Stream, transport warm waters from the tropics to the north ensuring the relatively mild climate in Scandinavian countries. Observations showing a cooling ocean surface trend in the region south of Greenland amidst global warming – known as the warming hole – have alarmed scientists that the Gulf Stream might be slowing down and that it might even be on the way to stop completely. A new study published in Nature Climate Change by scientists from Stockholm University and the Max Planck Institute for Meteorology in Hamburg show that things are not so simple.

n the contrary, the study finds that the best explanation for the warming hole is a strengthening of the circulation at high latitudes near Greenland and Iceland, which transports warm waters further into the Nordic seas and the Arctic. Thereby, the ocean circulation causes the warming hole to form by moving the warm waters faster out of region and further north and replacing it with cold waters in return. To come to this conclusion, the scientists simulated the historical warming from 1850 to present one hundred times in a climate model:

“Our approach was to analyse the entire North Atlantic ocean circulation, not just the Gulf Stream, and also changes in atmospheric conditions. Thereby, we are able to show that many different processes determine the warming hole formation and strength”, says Paul Keil from the Max Planck Institute.

Schematic of warm hole drivers, from Keil et al., 2020
Schematic of warm hole drivers, from Keil et al., 2020.

Interestingly, every simulation would form a warming hole, but the Gulf Stream would either strengthen or weaken mostly due to natural variability. What was common to all historical simulations was that the heat transport to higher latitudes strengthens. The scientists also looked at other factors such as clouds and winds, and although these had some influence on causing the warming hole, they were less important factors than the ocean circulation. The results are in line with another recent observational study showing no decline in ocean heat transport:

"This model-based study helps us better understand our recent oceanographic observational findings from the Nordic Seas region", says Léon Chafik from Stockholm University, who was not taking part in the study.

The model simulations show that the future Gulf Stream heat transport may weaken, and therefore begin to play a role in causing the warming hole to persist despite global warming:

"The warming hole is interesting as it will impact weather systems that form over the North Atlantic and move towards Europe. We still need to understand this better", says Thorsten Mauritsen.


Link to Nature Climate Change study

Link to Max Planck Institute article

The observational study by Thomas Rossby, Léon Chafik and Loic Houpert is published in Geophysical Research Letters. https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2020GL087456.