This curious phrase refers to an oceanic region south of Greenland whose temperature has remained rather constant over the years compared to the warming taking place just about everywhere else. It is not a warming hole; it is a hole in the warming. It has gained prominence among scientists who think this lack of warming might reflect a slowing of the Atlantic meridional overturning circulation (AMOC). This sounds backwards; you would think the cooling was a good sign?! While this region does participate in the AMOC, it produces water of intermediate density, what might be called North Atlantic Intermediate Water (NAIW), not North Atlantic Deep Water (NADW). The distinction is important.
So why the fascination with the warming hole, I wish I knew. Perhaps the fact that apparently it is the only area that isn’t warming in the North Atlantic if not the entire northern hemisphere must mean that it has got to be special. Since part of the AMOC overturns there, maybe something ominous is developing there. So, the search is on for threats to the subpolar gyre AMOC. Some reports find indications in the paleo-record that it might have been stronger in the past. Others examine sea surface temperature in the warming hole and find evidence of increasing nervousness (variability) suggesting it might flip (shut-down). But perhaps that is not the case, there is after all no evidence, whether from direct measurement or inferred from hydrography spanning much of the last century, of a slow-down of the AMOC (more on this below). Perhaps there are other explanations that have gone unconsidered for they haven’t been explored?
One possibility might be that the increased ice-melt from Greenland helps maintain the observed temperature. Ice melts at 0°C, this sets the run-off temperature regardless of warming elsewhere. Since it is thought that run-off contributes to the warming hole, the 0°C melting point might be a reason the warming hole remains cool?
The other possibility that comes to mind is that the water in the warming hole is ‘trapped’ there. It has a cyclonic circulation with cold water from Greenland defining its northern and western edge and the North Atlantic Current as its southern limit. In winter the cold air masses off North America can drive convective overturning to as deep as 2 km and in so doing redistribute or transfer water to greater densities that eventually spread out and leave the area. What exits at depth must be balanced by a convergence at the surface. This water is repeatedly subject to cooling in winter, so the lack of heating reflects the maintenance of the deep (1-2 km) pool of cold water.
Another reason might be that the AMOC is sending more of its heat (as warmer water) to the east and north into the Nordic Seas?!
Some studies suggest that direct measurement of the AMOC started in 2004 with the RAPID program. This is not accurate. The Oleander program has been measuring the strength of the upper ocean transport between New Jersey and Bermuda. This includes the Gulf Stream which carries all water flowing north (both the AMOC and the wind-driven circulation). There is no indication of a slow-down since the start of the program in late 1992. Another study (Rossby et al., 2022) reconstructs 0-1000 m transport (which includes the entire north-flowing AMOC) since the late 1930s. It finds a slight decrease over the last 70-90 years with most of it due to changes in the wind-driven system, not the AMOC (albeit with only modest confidence levels). Further, since there is no evidence for a slow-down of the Nordic Seas MOC and its production of NADW (Rossby et al., 2020), this implies little or no change to the strength of the subpolar component of the AMOC. The point is that the case for a slow-down of the AMOC in recent decades does not square with direct estimates of transport.
Rossby, T., J. Palter, and K. Donohue (2022). What can hydrography between the New England Slope, Bermuda and Africa tell us about the strength of the AMOC over the last 90 years? Geophys. Res. Lett., 49, e2022GL099173. https://doi.org/10.1029/2022GL099173.
Rossby, T., L. Chafik, and L. Hopert (2020). What can hydrography tell us about the strength of the Nordic Seas MOC over the last 70 to 100 years? Geophys. Res. Lett., 47, e2020GL087456. https://doi.org/10.1029/2020GL087456