On the risk of abrupt changes in the North Atlantic subpolar gyre in CMIP6 models

Abstract

CMIP5 models have been shown to exhibit rapid cooling events in their projections of the North Atlantic subpolar gyre. Here, we analyze the CMIP6 archive, searching for such rapid cooling events in the new generation of models. Four models out of 35 exhibit such instabilities. The climatic impacts of these events are large on decadal timescales, with a substantial effect on surface temperature over Europe, precipitation pattern in the tropics-most notably the Sahel and Amazon regions-and a possible impact on the mean atmospheric circulation. The mechanisms leading to these events are related to the collapse of deep convection in the subpolar gyre, modifying profoundly the oceanic circulation. Analysis of stratification in the subpolar gyre as compared with observations highlights that the biases of the models explain relatively well the spread in their projections of surface temperature trends: models showing the smallest stratification biases over the recent period also show the weakest warming trends. The models exhibiting abrupt cooling rank among the 11 best models for this stratification indicator, leading to a risk of encountering an abrupt cooling event of up to 36.4%, slightly lower than the 45.5% estimated in CMIP5 models.

Exceeding 1.5°C global warming could trigger multiple climate tipping points

“…North Atlantic subpolar gyre / Labrador-Irminger Sea convection (LABC) Convection in the Labrador and Irminger Seas in the North Atlantic-part of the subpolar gyre (SPG)-abruptly collapses in some models as a result of warming-induced stratification, a state which is then maintained by selfreinforcing convection feedbacks (19,65) giving two alternative stable states with or without deep convection. Abrupt future SPG collapse occurs in nine runs across five CMIP5 models at 1.1 to 2.0°C, in one additional model run at 3.8°C (19,65), and in four CMIP6 models in the 2040s (~1 to 2°C) (66). In some models SPG collapse affects AMOC strength but SPG and AMOC have distinct feedback dynamics and patterns of impacts, and SPG collapse can occur much faster than AMOC collapse.…”

“…In some models SPG collapse affects AMOC strength but SPG and AMOC have distinct feedback dynamics and patterns of impacts, and SPG collapse can occur much faster than AMOC collapse. The North Atlantic cooling trend (i.e., the “warming hole”) is centered over the SPG and in models is often closely linked to SPG weakening ( 65 , 66 ), although others have associated it with AMOC slowdown ( 67 ). SPG collapse causes a concentrated North Atlantic regional cooling of ~2 to 3°C, potential global cooling of up to ~0.5°C, a northward-shifted jet stream, weather extremes in Europe, and southward shift of the intertropical convergence zone (ITCZ) ( 65 , 66 ).…”

“…The North Atlantic cooling trend (i.e., the “warming hole”) is centered over the SPG and in models is often closely linked to SPG weakening ( 65 , 66 ), although others have associated it with AMOC slowdown ( 67 ). SPG collapse causes a concentrated North Atlantic regional cooling of ~2 to 3°C, potential global cooling of up to ~0.5°C, a northward-shifted jet stream, weather extremes in Europe, and southward shift of the intertropical convergence zone (ITCZ) ( 65 , 66 ). Given clear tipping dynamics and global impact we class SPG as a global core tipping element (medium confidence), with a best estimate threshold of ~1.8°C (1.1 to 3.8°C) (high confidence), a timescale of 10 years (5 to 50 years) (high confidence), and GMST feedback of ~0.5°C (regional ~−3°C) (low confidence).…”

AMOC Recent and Future Trends: A Crucial Role for Oceanic Resolution and Greenland Melting?

“…Last but not least, the SPG is still suspected to be potentially unstable in the coming century in a number of CMIP6 models, especially among the best in terms of representation of its mean stratification (Swingedouw et al, 2021). This increases the possibility of decadal cooling events of more than 2 • C in the SPG, i.e., at least the double of the 2015 cold blob, whose impact on the occurrence of the summer heat waves over Europe the same year have been further confirmed in new model simulations (Mecking et al, 2019).…”

Early warning signal for a tipping point suggested by a millennial Atlantic Multidecadal Variability reconstruction

“…6a ) can provide robust insights into future AMOC variations. Consequently, the discovered AMV EWS could be a sign of an approaching bifurcation of a more localized feature of the AMOC, such as an abrupt convection weakening in the North Atlantic subpolar gyre region, as observed in some future projections from both the CMIP5 65 and CMIP6 66 AOGCM ensembles.…”