No changes in overall AMOC strength in interglacial PMIP4 time slices

Abstract

Abstract. The Atlantic Meridional Overturning Circulation (AMOC) is a key mechanism of poleward heat transport and an important part of the global climate system. How it responded to past changes in forcing, such as those experienced during Quaternary interglacials, is an intriguing and open question. Previous modelling studies suggest an enhanced AMOC in the mid-Holocene compared to the preindustrial period. In earlier simulations from the Palaeoclimate Modelling Intercomparison Project (PMIP), this arose from feedbacks between sea ice and AMOC changes, which were dependent on resolution. Here we present an initial analysis of recently available PMIP4 simulations for three experiments representing different interglacial conditions – one 127 000 years ago within the Last Interglacial (127 ka, called lig127k), one in the middle of the Holocene (midHolocene, 6 ka), and a preindustrial control simulation (piControl, 1850 CE). Both lig127k and midHolocene have altered orbital configurations compared to piControl. The ensemble mean of the PMIP4 models shows the strength of the AMOC does not markedly change between the midHolocene and piControl experiments or between the lig127k and piControl experiments. Therefore, it appears orbital forcing itself does not alter the overall AMOC. We further investigate the coherency of the forced response in AMOC across the two interglacials, along with the strength of the signal, using eight PMIP4 models which performed both interglacial experiments. Only two models show a stronger change with the stronger forcing, but those models disagree on the direction of the change. We propose that the strong signals in these two models are caused by a combination of forcing and the internal variability. After investigating the AMOC changes in the interglacials, we further explored the impact of AMOC on the climate system, especially on the changes in the simulated surface temperature and precipitation. After identifying the AMOC's fingerprint on the surface temperature and rainfall, we demonstrate that only a small percentage of the simulated surface climate changes could be attributed to the AMOC. Proxy records of sedimentary Pa/Th ratio during the two interglacial periods both show a similar AMOC strength compared to the preindustrial, which fits nicely with the simulated results. Although the overall AMOC strength shows minimal changes, future work is required to explore whether this occurs through compensating variations in the different components of AMOC (such as Iceland–Scotland overflow water). This line of evidence cautions against interpreting reconstructions of past interglacial climate as being driven by AMOC, outside of abrupt events.

No Consistent Simulated Trends in the Atlantic Meridional Overturning Circulation for the Past 6,000 Years

“…Brierley et al (2020) found that the AMOC strength in the midHolocene ensemble is not markedly different. These results are supported by associated simulations of the last interglacial (Jiang et al, 2023). There is known to be a resolution dependency (Shi & Lohmann, 2016;Shi et al, 2020), which itself could vary by model (Jackson et al, 2020).…”

“…They also found no consistent changes in AMOC strength (Brierley et al, 2020). Combining those mid-Holocene snapshot simulations alongside last interglacial simulations suggests that variations in precession would not be expected to alter AMOC (Jiang et al, 2023).…”

Quantifying Effects of Earth Orbital Parameters and Greenhouse Gases on Mid-Holocene Climate

“…In addition to the model itself, whether the experimental setting includes the GHG is also an important factor. Our results on the AMOC in the MH are similar to those of (Jiang et al, 2023), but our focus is to find the mutual offsetting effect between external forcing factors. The experiments we conducted are time-slice experiments.…”