Changes in the Atlantic Meridional Overturning Circulation (AMOC) are moderate in most climate model projections under increasing greenhouse gas forcing. This intermodel consensus may be an artifact of common model biases that favor a stable AMOC. Observationally based freshwater budget analyses suggest that the AMOC is in an unstable regime susceptible for large changes in response to perturbations. By correcting the model biases, we show that the AMOC collapses 300 years after the atmospheric CO₂ concentration is abruptly doubled from the 1990 level. Compared to an uncorrected model, the AMOC collapse brings about large, markedly different climate responses: a prominent cooling over the northern North Atlantic and neighboring areas, sea ice increases over the Greenland-Iceland-Norwegian seas and to the south of Greenland, and a significant southward rain-belt migration over the tropical Atlantic. Our results highlight the need to develop dynamical metrics to constrain models and the importance of reducing model biases in long-term climate projection.

The key locations of freshwater input driving Atlantic Meridional Overturning Circulation (AMOC) slowdown and their climate responses remain inconclusive. Using a state-of-the-art global climate model, we conduct freshwater hosing experiments to reexamine AMOC sensitivity and its climate impacts. The Irminger basin emerges as the most effective region for additional freshwater fluxes, causing the greatest AMOC weakening. While global temperature and precipitation responses are relatively homogeneous, subcontinental responses-especially in the northern mid-latitudes-are heterogeneous. At high latitudes, sea ice responses to freshwater fluxes and associated ice-albedo feedbacks determine temperature changes. In tropical and extratropical regions, temperature dynamics are shaped by atmospheric circulation and oceanic heat transport. Precipitation shows seasonal and regional variability due to altered surface turbulent heat flux and the southward movement of the Intertropical Convergence Zone (ITCZ). The widespread heterogeneity in climate extremes underscores the need to monitor freshwater release regions linked to AMOC slowdown. These findings hold vital implications for understanding paleoclimate and future AMOC impacts.

from eight state-of-the-art climate models. That research found that by 2090–2100, the AMOC would weaken by around 18% (with a range of potential weakening The Greenland ice sheet is an ice sheet which forms the second-largest body of ice in the world. It is an average of 1,673 m (5,489 ft) thick and over 3,488 m (11,444 ft) thick at its maximum. It is almost 2,900 kilometres (1,800 mi) long in a north–south direction, with a maximum width of 1,100 kilometres (680 mi) at a latitude of 77°N, near its northern edge. The ice sheet covers 1,710,000 squar Fi…

It is now established that the increase in atmospheric CO2 is likely to cause a weakening, or perhaps a collapse of the Atlantic Meridional Overturning Circulation (AMOC). To investigate the mechanisms of this response in CMIP5 models, Levang and Schmitt (2020) have estimated offline the geostrophic streamfunction in these models, and decomposed the simulated changes into a contribution caused by the variations in temperature and salinity. They concluded that under a warming scenario, and for most models, the weakening of the AMOC is fundamentally driven by temperature anomalies while freshwater flux changes actually act to stabilize it.However, given that both ocean temperature and salinity are expected to respond to a forcing at the ocean surface, it is unclear to what extent the diagnostic is informative about the nature of the forcing. To clarify this question, we used the Earth system Model of Intermediate Complexity (EMIC) cGENIE, which is equipped with the C-GOLDSTEIN friction-geostrophic model (Marsh et al. (2011)). First, we reproduced the experiments simulating the RCP8.5 warming scenario and observed that cGENIE behaves similarly to the majority of the CMIP5 models considered by Levang and Schmitt (2020), with the response dominated by the changes in the thermal structure of the ocean.Next, we considered hysteresis experiments associated with (1) water hosing and (2) CO2 increase and decrease. In all experiments, changes in the ocean streamfunction appear to be primarily caused by the changes in the temperature distribution, with variations in the 3-D distribution of salinity compensating only partly for the temperature contribution. These experiments reveal also limited sensitivity to changes in the ocean's salinity inventory. That the diagnostics behave similarly in CO2 and freshwater forcing scenarios suggests that the output of the diagnostic proposed in Levang and Schmitt (2020) is mainly determined by the internal structure of the ocean circulation, rather than the forcing applied to it.Our results illustrate the difficulty of inferring any information about the applied forcing from the thermal wind diagnostic and raise questions about the feasibility of designing a diagnostic or experiment that could identify which aspect of the forcing (thermal or haline) is driving the weakening of the AMOC.AcknowledgementsThis is a contribution to the WarmAnoxia project funded by the Belgian National Fund of Scientific Research.References:Levang, S. J. and Schmitt, R. W. (2020). What causes the amoc to weaken in cmip5? Journal of Climate, 33(4):1535–1545.Marsh, R., Müller, S., Yool, A., and Edwards, N. (2011). Incorporation of the c-goldstein efficient climate model into the genie framework:" eb_go_gs" configurations of genie. Geoscientific Model Development, 4(4):957–992.