SPG convection collapse: an imminent climate tipping point?

In the shorter term, a more imminent potential tipping point could be an SPG convection collapse: Abrupt Climate Change in the Subpolar North Atlantic: Drivers and Impacts

In this study, climate models show that this tipping point is not a matter of if it happens, but when it happens. It may suggest that the tipping point may have already been reached, and the SPG convection is already in terminal decline, or it may be as a result of climate model biases, not including Greenland freshwater runoff, and resolution, which isn't high enough to properly represent eddies, though the model used (CESM2-LENS) is still fairly skillful.

The timing of the collapse is uncertain, but the risk raises significantly in the 2030s, and into mid-century.

The collapse would cause a significant drop in SSTs in the subpolar gyre, so it essentially strengthens the NAWH/cold blob, and is a more minor, abrupt version of an AMOC collapse, that would occur before it. The ensemble mean for the timing of this drop is 13yrs, but there is a significant spread, due to it lasting as little as a few years (so the climate changes faster), or other factors dampen the drop in SST, making it last longer. It's also notable that a drop in MLD corresponds to this collapse, suggesting that the SPG and AMOC is already destabilizing, as a drop in MLD is already being observed, though possibly down to natural variability.

The temperature anomalies that result from these aren't the difference from pre-industrial, but instead the difference between a collapse of the SPG under SSP3, and SSP3 without a collapse, so background warming will still outpace this cooling in areas outside of the NAWH. It finds a cooling of the high latitudes, from a reduction in sea ice retreat in winter. Even without this reduction in summer, western Europe still sees a relative cooling, though this can be attributed to CMIP6 underestimating European heatwaves, due to zonal biases and underestimating atmospheric blocking. Overall, atmospheric circulations shift towards a NAO+. This shift is more intense in winter, than in summer. What I find surprising is that the TPV is displaced towards the Scandinavian/Siberian side of the hemisphere, and that despite the NAO+, there's still a positive height anomaly south of Greenland, possibly due to the positive SST anomaly just southeast of the cold blob, as a weakened Gulf Stream causes heat to build up there. In Britain, this suggests an increase in both warm and cold blocking in winter. The precipitation signal isn't statistically significant for much of the UK, but there is a positive signal for the north, in winter.

During the collapse itself, there's a NAO-, blocking pattern, as high pressure to the south hasn't emerged, and this high pressure south of Greenland is much more expansive and dominant over weather patterns. The first thing that sprung to my mind when seeing this is the transient potential for wintery weather, before the atmosphere manages to reorganize



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