No abstract

Atlantic meridional overturning circulation (AMOC) collapses have punctuated Earth’s climate in the past, and future projections suggest a weakening and potential collapse in response to global warming and high-latitude ocean freshening. Among its most important teleconnections, the AMOC has been shown to influence El Niño–Southern Oscillation (ENSO), although there is no clear consensus on the tendency of this influence or the mechanisms at play. In this study, we investigate the effect of an AMOC collapse on ENSO by adding freshwater in the North Atlantic in a global climate model. The tropical Pacific mean-state changes caused by the AMOC collapse are found to alter the governing ENSO feedbacks, damping the growth rate of ENSO. As a result, ENSO variability is found to decrease by ∼30% due to weaker air–sea coupling associated with a cooler tropical Pacific and an intensified Walker circulation. The decreased ENSO variability manifests in ∼95% less frequent extreme El Niño events and a shift toward more prevalent central Pacific El Niño than eastern Pacific El Niño events, marked by a reduced ENSO nonlinearity and asymmetry. These results provide mechanistic insights into the possible behavior of past and future ENSO in a scenario of a much weakened or collapsed AMOC. Significance Statement The Atlantic meridional overturning circulation (AMOC) has collapsed in the past and a future collapse due to greenhouse warming is a plausible scenario. An AMOC shutdown would have major ramifications for global climate, with extensive impacts on climate phenomena such as El Niño–Southern Oscillation (ENSO), which is the strongest source of year-to-year climate variability on the planet. Using numerical simulations, we show that an AMOC shutdown leads to weaker ENSO variability, manifesting in 95% reduction in extreme El Niño events, and a shift of the ENSO pattern toward the central Pacific. This study sheds light on the mechanisms behind these changes, with implications for interpreting past and future ENSO variability.

<p>The Atlantic Meridional Overturning Circulation (AMOC) has a profound impact on both global and regional climate, yet our understanding of the mechanisms controlling remote teleconnections remains limited. In addition, it is unclear how remote processes impact the North Atlantic and alter the strength of the AMOC.  In this presentation I will show how a slowdown in the AMOC can drive an acceleration of the Pacific trade winds and Walker circulation by leaving an excess of heat in the tropical South Atlantic. This tropical Atlantic warming drives anomalous atmospheric convection, resulting in enhanced subsidence over the east Pacific, and a strengthened Walker circulation and trade winds. Further teleconnections include a shift in the ITCZ, enhanced zonal SST gradients across the tropical Pacific, strengthened convection over the West Pacific Warm Pool, and a deepening of the Amundsen Sea Low off Antarctica.  Teleconnections back to the North Atlantic can in turn be triggered by Southern Hemisphere wind anomalies on a relatively rapid time-scale via propagating planetary waves in the ocean.  There is also evidence that tropical Pacific cooling can feedback and influence the strength of the AMOC.  These findings have implications for understanding both intrinsic decadal climate variability as well as longer-term climate change.</p>