Role of AMOC in Transient Climate Response to Greenhouse Gas Forcing in Two Coupled Models
Here we use two coupled climate models to study the role of AMOC in transient climate response – the Community Earth System Model version 2 (CESM2) and the Energy Exascale Earth System Model version 1 (E3SM1). While CESM2 and E3SM1 have very similar equilibrium/effective climate sensitivity, our analysis suggests that a weaker AMOC contributes in part to the higher transient climate response to a rising greenhouse gas forcing seen in E3SM1 by permitting faster warming of the upper ocean and a concomitant slower warming of the subsurface ocean. Likewise, the stronger AMOC in CESM2 by permitting slower warming of the upper ocean leads in part to a smaller transient climate response. Thus, while the mean strength of AMOC does not affect the equilibrium/effective climate sensitivity, it is likely to play an important role in determining the transient climate response on the centennial time scale.
This study suggests that although the mean state of AMOC may not have directly affected the equilibrium climate sensitivity of the global climate system, it does affect the transient climate response to elevated greenhouse gas forcing. Although it is not directly demonstrated, our result hinted that the mean state of AMOC may have also inserted influence on the equilibrium climate sensitivity which needs to be studied further.
In this work, we have analyzed simulations from two CMIP6 models, CESM2 and E3SM1, focusing on the AMOC and the transient climate response. Our analysis shows that the preindustrial mean AMOC in E3SM1 is about 40% weaker than that in CESM2, and the mean AMOC strength in CESM2 agrees with observations very well. The weaker AMOC in E3SM1 is likely due to a number of factors, including the absence of deep convection (NADW formation) in the Labrador and Irminger Seas. In response to the rising greenhouse gas forcing, warming of the surface climate is faster in E3SM1 than in CESM2 due to a much stronger ocean stratification. This stronger upper-ocean stratification in E3SM1 can be attributed partially to a weaker AMOC, which generates a weaker sinking of the upper-ocean water and a weaker upwelling of deep water. As the climate warms, the stronger stratification in E3SM1 further increases due to a faster surface warming than the warming in the subsurface ocean. This results in a much faster warming in the upper ocean and less warming in the subsurface ocean in E3SM1 than in CESM2, which further demonstrates that the AMOC mean state and its transient response to greenhouse gas forcing can modulate the ocean stratification, and further affects the model transient climate response.