Abstract
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). Despite potential ramifications to the global climate, our understanding of the influence of various AMOC and PMOC states on ENSO and global sea surface temperatures (SSTs) remains limited. Five multicentennial, fully coupled model simulations created with the Community Earth System Model were used to explore the influence of AMOC and PMOC on global SSTs and ENSO. We found that the amplitude of annual cycle SSTs across the tropical Pacific decreases and ENSO amplitude increases as a result 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 are also discussed, 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.
Significance Statement
The global-scale ocean circulation named the Atlantic meridional overturning circulation (AMOC) could be slowing due to climate change. Studies suggest that a slowdown of AMOC could trigger the formation of a Pacific counterpart, which would transport upper-ocean water into the North Pacific that is warmer and saltier than present day. Using several century-scale, fully coupled climate model experiments, 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 strength of El Niño–Southern Oscillation, whether a Pacific meridional overturning circulation develops or not, which could amplify climate extremes via tropical–extratropical teleconnections.
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