Consequences from a slowdown or collapse of the Atlantic meridional overturning circulation (AMOC) could include modulations to El Niño–Southern Oscillation (ENSO) and development of the Pacific meridional overturning circulation (PMOC). Five multi-century, fully coupled model simulations created with CESM were used to expand our understanding of the influence of various AMOC and PMOC states on ENSO and global sea surface temperatures (SSTs).
The global-scale ocean circulation named AMOC could be slowing due to climate change. Studies suggest that a slowdown of AMOC could trigger the formation of a Pacific counterpart. Our study shows that different states of these circulations can dramatically alter Earth’s climate and ocean temperatures, contributing to our understanding of potential future and past geological era climates. Importantly, we show that an AMOC slowdown could increase the amplitude of ENSO, whether a Pacific meridional overturning circulation develops or not, which could amplify climate extremes via tropical–extratropical teleconnections.
We found that the amplitude of annual cycle SSTs across the tropical Pacific decreases and ENSO amplitude increases because of an AMOC shutdown, irrespective of PMOC development. However, active deep overturning circulations in both the Atlantic and Pacific basins reduce ENSO amplitude and variance of monthly SSTs globally. The underlying physical reasons for changes to global SSTs and ENSO were also analyzed, with the atmospheric and oceanic mechanisms that drive changes to ENSO amplitude differing based on PMOC state. These results suggest that if climate simulations projecting AMOC weakening are realized, compounding climate impacts could occur given the far-reaching ENSO teleconnections to extreme weather and climate events. More broadly, these results provide us with insight into past geologic era climate states, when PMOC was active.