While otherwise surface temperature of the entire planet has been warming, a region in the subpolar North Atlantic has not only defied this warming trend but has significantly cooled.

This region, according to a new study, is located to the south of Greenland and Iceland and to the west of the British Isles, and it has been dubbed the Atlantic ‘warming hole’ or ‘cold blob’. Analysed sea-surface temperature (SST) variability since 1870 found that the northern Atlantic cooling is part of a pattern anticorrelated with the South Atlantic.

This pattern is linked to Atlantic meridional overturning circulation (AMOC) variations. The AMOC has been weakening since the 1930s. That makes sense as the cold blob region is where the AMOC delivers its heat and passes it to the atmosphere. Much of this heat is drawn from the South Atlantic and transported northward across the equator.

Experts say that is the main reason why the Northern Hemisphere is 1 to 2 degrees warmer than the Southern Hemisphere. There is a strong correlation of AMOC weakening with the cold blob temperature in future global warming simulations.

Earlier studies show that the AMOC has slowed down since preindustrial time, or further indications of a weakening over more recent decades. Weakening of the AMOC could have major repercussions for future climate for millennia. This connection is stronger in the winter half of the year and is expected to involve a time lag of a few years.

A striking example of this was the summer 2023 with record-breaking sea-surface temperatures in the North Atlantic including in the cold blob region, as an exceptionally shallow surface mixed layer – in some areas only 10 m deep-heated up in the summer sun. The cold blob subsequently reappeared after deep winter mixing.

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Warming and Cooling

Experts believe a part of the long-term cooling trend in the subpolar Atlantic could be due to surface forcing. The heat content changes are generally larger and more tightly correlated with ocean heat transport than the surface heat loss variations.

This is physically expected, since any transport changes affect heat content directly, while they affect surface heat loss only indirectly with delay after the sea surface has warmed. It is also clear that phases of heat content increase coincide with phases of anomalously large surface heat loss, so that the surface heat flux does not drive heat content change, but rather responds to surface warming.

Surface heat loss appears to respond as a negative feedback to heat content changes: periods of increasing heat content coincide with periods of large surface heat loss.