Abstract: "The northern Atlantic south of Greenland and Iceland is the only part of the world which has cooled significantly since the 19th Century both in the atmosphere and ocean. The oceanic cooling is widely assumed to be a result of reduced ocean heat transport into this region. However, some studies have suggested it could be due to increased net heat loss at the sea surface. Here we use observation-based reanalysis data of ocean heat content and surface flux changes in this region to show that the observed cooling trend cannot be explained by surface heat flux changes, and that multidecadal heat content variations are generally larger and more tightly correlated with ocean heat transport than surface heat flux variability."

From the article: "Our analysis supports the interpretation of the observed ‘cold blob’ as a sign of a weakening AMOC, which is a major component of the lateral heat transport into that subpolar gyre region. A contribution from increasing lateral heat transport out of the subpolar gyre towards the Nordic Seas has also been suggested (Keil 2020), and both may well be dynamically linked (Roewer et al. 2025).

There is substantial evidence for a weakening AMOC independent of the ‘cold blob’. On long time scales this includes paleoclimatic proxy data suggesting the AMOC is at its weakest in a millennium (Caesar et al. 2021, Caesar et al. 2022). Also, salinity in the ‘cold blob’ region is at its lowest in 120 years of data, consistent with reduced AMOC salt transport from the subtropical net-evaporation region (Holliday et al. 2020).

On shorter time scales this includes a robust observed weakening of the Gulf Stream over the past 4 decades (Piecuch and Beal 2023), consistent in magnitude with the 15 % AMOC weakening inferred from the subpolar SST data (Caesar et al. 2018), and an ocean density reduction in the subpolar gyre since 1950 which “is suggestive of a long-term AMOC weakening of 2.2 Sv or 13%” (Chafik et al. 2022)."