Rapid change in AMOC intensity in a Holocene transient simulation provides insight into the ocean long term ocean memory

A rapid change in the Atlantic overturning circulation (AMOC) can have a major impact on the hydrological cycle over land and ecosystems. Examples of such events have been widely discussed in palaeoclimate studies investigating the response of the AMOC to rapid freshening of the North Atlantic caused by ice sheet instability and melting during ice ages or in the early Holocene (Wunderling et al. 2024). The rapid decline of the AMOC and the possibility of its collapse in the future could also have a major impact on terrestrial ecosystems. An open question is the identification of precursors to such shifts and the anticipation of impacts and feedbacks on ecosystems. Here we use as a starting point a transient simulation of the last 6 000 years with version IPSLCM6-LR of the IPSL model (Boucher et al. 2020), starting from the PMIP mid-Holocene simulation with this model (Braconnot et al. 2021).  Surprisingly, this simulation shows a rapid shift of 2 Sv in the AMOC, while this type of bifurcation was not seen in the set of CMIP6 simulations run with the same version of the model. (Boucher et al. 2020). Such a shift doesn't occur in a parallel simulation using a slightly different version of the model with fully interactive vegetation. The presentation will discuss the set of simulations used to identify 1) the reason for the shift, 2) the impact of the shift on the vegetation in Africa and Europe. For the latter, we run snapshot coupled simulations using as initial state the ocean state of the Holocene simulation with the AMOC shift, and the corresponding Earth’s orbit and trace gas configuration.  This allows us to estimate the amplification of the changes induced by the vegetation feedback on the regional changes in the hydrological cycle. 

 

Boucher O, Servonnat J, Albright AL, et al (2020) Presentation and Evaluation of the IPSL-CM6A-LR Climate Model. J Adv Model Earth Syst 12:e2019MS002010. https://doi.org/10.1029/2019ms002010

Braconnot P, Albani S, Balkanski Y, et al (2021) Impact of dust in PMIP-CMIP6 mid-Holocene simulations with the IPSL model. Clim Past 17:1091–1117. https://doi.org/10.5194/cp-17-1091-2021

Wunderling N, Von Der Heydt AS, Aksenov Y, et al (2024) Climate tipping point interactions and cascades: a review. Earth Syst Dyn 15:41–74. https://doi.org/10.5194/esd-15-41-2024