Global warming is a reality that we feel on every continent and in every ocean – most of the world's oceans have seen an increase in surface temperature over the past century. But there is one fascinating and worrying a place that bucks this trend: subpolar Atlantic, the area south of Greenland. Here, instead of warming, we observe a long-term cooling trend in sea surface temperature (SST), known as North Atlantic “Warmth Hole” (NAWH)Why is this region cooling while the rest of the world is warming? Recent research offers compelling evidence that the key to this mystery lies in slowing down of the Atlantic Meridional Circulation (AMOC).

What is the AMOC and why is it so important?

Atlantic Meridional Circulation (AMOC) is a large ocean current system that acts as a conveyor belt of heat and salt in the Atlantic. It is characterized by the northward flow of warm, salty water in the upper Atlantic a southern reverse flow in the deep AtlanticThis system plays a vital role in global climate change by transporting vast amounts of heat and salt towards the North Atlantic. Although this is a matter of debate, paleo-proxy reconstructions suggest that the AMOC has been weakening since the last century, and projections suggest further slowdowns in the 21st century.

Manifestations of the North Atlantic “Warmth Hole” (NAWH)

NAWH manifests itself as a long-term trend of cooling of sea surface temperatures south of Greenland, which is in in stark contrast to global warmingThe average of six observational datasets showed a distinct cooling trend in annual mean SST of up to −0.3 °C per century.

In addition to cooling, the NAWH region is also observed significant trend of sweetening of surface sea salinity (SSS)Observations from 1955–2005 indicated a trend of sweetening up to −0.25 psu per centuryThis decrease in salinity contrasts with the increase in salinity further south in the Atlantic. Importantly, this Cooling and sweetening are not limited to the surface; they extend deep into the ocean, all the way to the bottom..

AMOC as the main cause of NAWH

The key mechanism by which the AMOC slowdown contributes to NAWH is divergence of ocean heat transport (OHT) in the subpolar North Atlantic. Simply put, a weakened AMOC transports less heat northward, leading to its “dispersion” and cooling in the affected area. Although this cooling is partially offset by increased heat absorption from the atmosphere, the result is still overall cooling. Similarly, a slowing AMOC leads to a divergence of salt transport and an increased influx of freshwater from the surface, which together contribute to sweetening throughout the water column.

Evidence from observations and climate models

Scientists used observations as a reference and climate models as a tool to demonstrate this connection. It was found that only models simulating a weakened historical AMOC were able to generally reproduce the observed cooling and sweetening in the NAWH regionConversely, models that simulated a strengthened AMOC showed warming in the NAWH region, which is at odds with reality.

Study divided 94 CMIP5 and CMIP6 models (two generations of climate models) into two categories: those with a weakened AMOC (AMOC-) and those with a strengthened AMOC (AMOC+). It was shown that the AMOC- models (which recorded a slowdown of -1.35 Sv per century between 1900 and 2005) were able to replicate a pattern similar to that observed in NAWH.

To further confirm this connection, AMOC “fingerprint indices” have been developed based on the dipole pattern of SST and SSS changes between the subpolar region and the region near the Gulf Stream. A correlation was found between AMOC strength trends and these indices. strong and significant correlationBased on this correlation and SST observations, it is estimated that the actual AMOC slowed at a rate of −1.01 to −2.97 Sv per century between 1900 and 2005 (where 1 Sv = 10^6 m^3 s^-1).

Conclusion

The North Atlantic “Temperature Hole” is a striking example of our ocean’s complex responses to climate change. Evidence clearly suggests that the slowdown of the Atlantic meridional circulation is the main driving force this cooling and sweetening trend in an otherwise warming world. Understanding these processes and continuing research and monitoring of the AMOC are essential for more accurate predictions of future climate change and for a better understanding of the broad implications of global warming on our planetary system. Spring

The report is published in the journal Communications Earth & Environment (2025)

Glossary of key terms

• North Atlantic Warming Hole (NAWH): An area in the subpolar Atlantic (south of Greenland) that has shown a cooling trend in sea surface temperatures over the past century, contrasting with the global warming trend.
• Atlantic Meridional Overturning Circulation (AMOC): A large system of ocean currents in the Atlantic, characterized by a northward flow of warm, salty water in the upper Atlantic and a southward reverse flow in the deep Atlantic. It is an important factor in the transport of heat and salt and plays a key role in global climate change.
• Sverdrup (Sv): A unit of volumetric flow, equal to 10^6 cubic meters per second. Used to measure the strength of ocean currents such as the AMOC.
• Sea Surface Temperature (SST): Ocean surface water temperature.
• Sea surface salinity (SSS): The salt concentration in seawater at the ocean surface.
• CMIP5 (Coupled Model Intercomparison Project Phase 5): The previous generation of a suite of climate models used for climate comparisons and projections.
• CMIP6 (Coupled Model Intercomparison Project Phase 6): A newer generation of a suite of climate models that improves on the previous version.
• AMOC models: A category of climate models that simulate a weakening trend of the AMOC between 1900 and 2005.
• AMOC+ models: A category of climate models that simulate a strengthening trend of the AMOC between 1900 and 2005.
• Multi-model average (MMM): An average of results from multiple climate models, used to reduce the impact of internal climate variability.
• AMOC Fingerprint Index (FPI): An index developed based on the dipole pattern of SST or SSS changes between the subpolar gyre and the area near the Gulf Stream, which is used to estimate the strength of the AMOC. There are two types: FPISST (based on SST) and FPISSS (based on SSS).
• Ocean heat transport (OHT) divergence: A situation where heat is removed from a region by the ocean, leading to cooling. A slowdown in the AMOC leads to a reduction in the northern OHT and thus divergence.
• Ocean heat budget: Accounting for heat input and output in the ocean, taking into account heat uptake from the atmosphere, heat transport through the ocean, and heat storage.
• Ocean salt budget: Accounting for the supply and outflow of salt in the ocean, taking into account the virtual flux of salt from the surface and the transport of salt through the ocean.
• Virtual Salt Flow (VSF): A concept that converts the flow of freshwater across the ocean surface into an equivalent flow of salt that would cause the same changes in salinity.
• Solid-state simulations: Climate model experiments where ocean currents are artificially maintained on a fixed seasonal cycle to isolate the impact of circulation changes.
• Free-flow simulations: Common climate model experiments where ocean currents can change dynamically.
• Pre-industrial control run (piControl): A climate model simulation with pre-industrial conditions serving as a reference point.
• Historical simulations: Climate model simulations that attempt to reproduce historical climate conditions.