<p>The Atlantic Meridional Overturning Circulation (AMOC), a key component of the climate system, is projected to weaken in the 21st century. Using the Alfred Wegener Institute Climate Model (AWI-CM 1.1 LR), we conduct a set of numerical experiments to investigate the transient response of AMOC to anthropogenic warming with quadrupled carbon dioxide (4×CO2). The results suggest that circulation adjustment outside the Arctic dominates the AMOC weakening. In warming conditions, northward advection transport from the southern flank of the Atlantic subpolar region will increase, which is expected to enhance the upper ocean stratification over deep convection zones and inhibit deep-water formation, thus weakening the AMOC largely. Stratification enhancement is more pronounced in Nordic Seas than that in the Labrador Sea, implying a more direct role of Labrador Sea in evolution of still-active AMOC. In Nordic Seas, decreased ocean convection is dominated by temperature contributions due to a substantial increase of northward advective heat transport. While in Labrador Sea, both surface heat flux and advective heat transport matter, with comparable thermohaline contributions.</p>

Using the Alfred Wegener Institute Climate Model (AWI-CM 1.1 LR), we conduct sensitivity experiments separating the Arctic and extra-Arctic warming to investigate the transient response of AMOC to quadrupled carbon dioxide (4×CO2) forcings. The results suggest that AMOC weakening is primarily affected by circulation adjustment induced by the outer-Arctic warming, while the effects of Arctic warming are confined to the polar range and contribute less to AMOC changes. When warming forcing is applied outside the Arctic, the increases of northward advective heat transport dominate the weakening of deep convection in the Nordic Seas, while the reduction of heat loss from ocean to atmosphere is prevalent in Labrador Sea. Besides, the weakening of deep convection in Nordic Seas is more pronounced than in Labrador Sea, implying a leading role of Nordic Seas in the weakening of AMOC under global warming.

<p>Global warming since the industrial revolution has led to a series of changes in the atmosphere and ocean. As a key indicator of global ocean circulation, AMOC has shown a weakening in recent decades from both the observed and simulated results. This process which is not only affected by the local variation of the Arctic, but also by the ocean and atmosphere circulation changes in the middle and lower latitudes, might have important implications for future global climate changes. We employ the Alfred Wegener Institute Climate Model (AWI-CM 1.1 LR) and a method of perturbing coupled models to quantify and understand the impact of anthropogenic warming on the slowdown of AMOC. Conducted one control (CTRL) experiment and three sensitivity experiments (60N, 60NS, and GLOB) in which CO2 concentration were abruptly quadrupled either regionally (60N-north of 60°N, 60NS-south of 60°N) or globally (GLOB). The goal of our research is to identify the response of AMOC weakening to the quadrupling of CO2 concentration in different regions and provide future insight into ocean circulation changes in the context of climate warming. Our results show that CO2 forcing outside the Arctic dominates the weakening of AMOC. In a warming climate, the poleward heat transport increased due to the extra-Arctic CO2 forcing, which enhanced the upper ocean average stratification within the mixed-layer depth over Nordic Seas and Labrador Sea and thus weakens the AMOC to a large extent. The warming in upper-layer also lead to the dominant role of temperature contribution to stratification. However, in both the deep convection regions, the mechanism resulting in the strengthening of stratification might be quite different.</p>