Northern Hemispheric atmospheric circulation ... either that or a collapse/severe weakening of the SPG is the next flash point, the latter effect of slowing GMST increases (though with repercussions).
Along the lines of these two articles?
An abrupt weakening of the subpolar gyre as trigger of Little Ice Age-type episodes
E Chamarro et al
https://link.springer.com/article/10.1007%2Fs00382-016-3106-7 free full text
We investigate the mechanism of a decadal-scale weakening shift in the strength of the subpolar gyre (SPG) that is found in one among three last millennium simulations with a state-of-the-art Earth system model.
The SPG shift triggers multicentennial anomalies in the North Atlantic climate driven by long-lasting internal feedbacks relating anomalous oceanic and atmospheric circulation, sea ice extent, and upper-ocean salinity in the Labrador Sea.
Yet changes throughout or after the shift are not associated with a persistent weakening of the Atlantic Meridional Overturning Circulation or shifts in the North Atlantic Oscillation... climate reconstructions describe a transition between a stronger and weaker SPG during the relatively warm medieval climate and the cold Little Ice Age respectively.
The simulated SPG shift is caused by a rapid increase in the freshwater export from the Arctic and associated freshening in the upper Labrador Sea. Such freshwater anomaly relates to prominent thickening of the Arctic sea ice, following the cluster of relatively small-magnitude volcanic eruptions by 1600 CE.
The subpolar gyre (SPG) influences the North Atlantic climate by modulating the transport of heat and salt between the North Atlantic and the Arctic oceans and, particularly, into the Nordic and Labrador seas, where deep water formation takes place.
Variations in the strength and shape of the SPG can lead to major changes in, for example, the intensity of the AMOC or the distribution of sea ice in the Arctic). A recent paleoceanographic reconstruction of the Atlantic mid-depth gyre circulation over the past 1500 years suggested that the SPG weakened between the Medieval Climate Anomaly 950–1250 CE and the Little Ice Age 1450–1850...
Freshening of the Labrador Sea as a trigger forLittle Ice Age development
M Alonso-Garcia ... WS Broecker ... et al
https://www.clim-past.net/13/317/2017/cp-13-317-2017.pdf free full text
The comparison of our Labrador Sea IRD records with other climate proxies from the subpolar North Atlantic allowed us to propose a sequence of processes that led to the cooling that occurred during the LIA, particularly in the Northern Hemisphere.
This study reveals that the warm climate of the MCA may have enhanced iceberg calving along the SE Greenland coast and, as a result, freshened the subpolar gyre (SPG). Consequently, SPG circulation switched to a weaker mode and reduced convection in the Labrador Sea, decreasing its contribution to the North Atlantic deep water formation and, thus, reducing the amount of heat transported to high latitudes.
This situation of weak SPG circulation may have made the North Atlantic climate more unstable, inducing a state in which external forcings (e.g. reduced solar irradiance and volcanic eruptions) could easily drive periods of severe cold conditions in Europe and the North Atlantic like the LIA. This analysis indicates that a freshening of the SPG may play a crucial role in the development of cold events during the Holocene, which may be of key importance for predictions about future climate.