Analysis of climate model simulations for evaluating the impacts of AMOC weakening on the Atlantic tropical rainfall in CMIP6 scenario projections.

Thanks to its large-scale teleconnections, the Atlantic Meridional Overturning Circulation (AMOC) plays an active role in shaping the Earth’s Climate. Many studies and observations indicate that in recent years the AMOC is undergoing a declining phase. However, although this weakening is supposed to have dramatic consequences all over the globe, regional impacts are still largely unknown. With this work I contribute to improve understanding of the climatic responses of tropical Atlantic precipitation and Hadley atmospheric circulation to an AMOC slowdown. To pursue this objective, I analyze and post-process an ensemble of future projections deriving from eighteen global climate models simulations that belong to the Coupled Model Intercomparison Project 6 (CMIP6). The analysis confirms the existence of a declining trend for the AMOC, but the large inter-model spread in the AMOC’s decline rate leads to a high level of uncertainty. Thus, I split the models in two groups, each one composed of six models: one group includes the models that simulate the strongest decline, while the other includes the models that simulate the weakest decline. Through rigorous statistical testing, I attribute the differences between the two groups to the AMOC’s decline impacts. I focus my analysis on the tropical Atlantic, being changes particularly significant in this region. In the group of models with stronger AMOC decline, the annual mean positioning of the ITCZ exhibits a southward shift, whereas the group of models with weaker AMOC decline does not show a southward shift of the ITCZ. To better understand mechanisms of precipitation change, I compute the atmospheric moisture budget. I find that the wind field is the main driver of the equatorial Atlantic moistening occurring in the large AMOC decline group. The present work helps understanding the impacts of the inter-model spread in the simulation of the AMOC decline, elucidating the mechanisms driving differences in the simulation of future precipitation change in the tropical Atlantic.