Penn State researchers found that the cold blob in the North Atlantic does not respond to a weakening Atlantic Meridional Overturning Circulation in one steady way. The cold blob, also called the North Atlantic warming hole, follows three distinct phases as the current slows.
The region covers a 965,000-square-mile expanse south and southeast of Greenland, and surface waters there cooled by as much as 1.8 degrees Fahrenheit over the past century. The ocean current moves warm surface water northward and returns denser, colder water southward through deeper layers.
Penn State simulations
In the first phase, weakening from the current's highest initial strength produced only a modest decline in surface temperatures. In the second, weakening from a mid-range circulation strength triggered a temperature drop that roughly tripled and spread the cold blob across the subpolar basin.
That pattern matters because direct observational records of the Atlantic Meridional Overturning Circulation go back only about two decades, while the region has long cooled even as broader global ocean waters have warmed. The new simulations show that the surface response depends on where the circulation starts, not just whether it weakens.
North Atlantic warming hole
In the third phase, further deceleration from an already diminished state made the direct cooling effect fade and pushed the colder patch northeast toward Iceland and the Norwegian Sea. The researchers said variable impacts from atmospheric heat radiation and reflections from low clouds helped drive that shift and the weaker surface cooling.
That complicates the long-running link climate observers have drawn between the anomaly and a sustained reduction in Atlantic Meridional Overturning Circulation. A short record can make a changing ocean look simpler than it is, but these simulations show the response can change shape as the current weakens.
Stefan Rahmstorf warns of Possible AMOC shutdown by 2100 in North Atlantic Cold Blob Pattern fits the same concern about a slowing circulation, but the Penn State result adds a sharper point: the cold blob is not moving through one fixed cooling path. For anyone tracking the North Atlantic, the useful takeaway is to read future changes as phase shifts, not a single straight-line decline.
