Dr Simon Michel

Atlantic multidecadal variability is a coherent mode of natural climate variability occurring in the North Atlantic Ocean, with strong impacts on human societies and ecosystems worldwide. However, its periodicity and drivers are widely debated due to the short temporal extent of instrumental observations and competing effects of both internal and external climate factors acting on North Atlantic surface temperature variability. Here, we use a paleoclimate database and an advanced statistical framework to generate, evaluate, and compare 312 reconstructions of the Atlantic multidecadal variability over the past millennium, based on different indices and regression methods. From this process, the best reconstruction is obtained with the random forest method, and its robustness is checked using climate model outputs and independent oceanic paleoclimate data. This reconstruction shows that memory in variations of Atlantic multidecadal variability have strongly increased recently—a potential early warning signal for the approach of a North Atlantic tipping point.

The mid-Holocene (6000 years before present) was a warmer period than today in summer in most of the Northern Hemisphere. In winter, over Europe, pollen-based reconstructions show a dipole of temperature anomalies as compared to present-day, with warmer conditions in the north and colder in the south. It has been proposed that this pattern of temperature anomaly could be explained by a persisting positive phase of the North Atlantic Oscillation during this period, which was, however, not reproduced in general by climate models. Indeed, PMIP3 models show a large spread in their response to the mid-Holocene insolation changes, the physical origins of which are not understood. To improve the understanding of the reconstructed temperature changes and of the PMIP3 model spread, we analyze the dynamical response of these model simulations in the North Atlantic for mid-Holocene conditions as compared to pre-industrial. We focus on the European pattern of temperature in winter and compare the simulations with a pollen-based reconstruction. We find that some of the model simulations yield a similar pattern to the reconstructed one, but with far lower amplitude, although it remains within the reconstruction uncertainty. We attribute the northern warm part of the latitudinal dipole of temperature anomaly in winter to a lower sea-ice cover in the Nordic Seas. The decrease of sea ice in winter indeed reduces the local sea-ice insulation effect, allowing the released ocean heat to reach continental northern Europe. This decrease in winter sea-ice cover is related to an increase in the Atlantic meridional overturning circulation (AMOC) and its associated ocean heat transport, as well as the effect of insolation changes on sea ice in summer, which persists until winter. We only find a slight cooling signal over southern Europe, compared to reconstructions, mainly related to the insolation-induced cooling in winter over Africa. We show that the models that failed to reproduce any AMOC increase under mid-Holocene conditions are also the ones that do not reproduce the temperature pattern over Europe. The change in sea level pressure is not sufficient to explain the spread among the models. The ocean-sea ice mechanisms that we proposed constitute an alternative explanation to the pattern of changes in winter temperatures over Europe in the mid-Holocene, which is in better agreement with available model simulations of this period. Finally, we evaluate if reconstructions of the AMOC for the mid-Holocene can provide interesting emerging constraints on key changes in European climate, and indirectly on AMOC response to on-going and future radiative changes. Although there is a significant link between the response of the mid-Holocene and projections, it remains limited. The proposed mechanism does not appear to be sufficient to explain the large discrepancies between models and reconstruction data for the summertime period.

In the mid-high latitude region, variations of stable isotopic compositions of atmospheric precipitation (δ18Op and δDp) were commonly regarded as reflecting the “temperature effect”. However, some studies have indicated that changes in moisture sources are important controlling factors for δ18Op. To clarify whether there are connections between δ18Op and variations of moisture sources in Southwest France (SW-France), whose implications for speleothem δ18O are of great importance, we have used among the longest isotopic time-series from SW-France (Le Mas and Villars stations) and a 5 days’ reconstruction of air mass history during the 1997–2016 A.D period based on the HYSPLIT tracking model. We found the percentage of initial moisture sources (PIMS) as important factors controlling the oxygen isotope composition of precipitation in SW-France, whether monthly or inter-annual timescale was considered. Additionally, we observed that the δ18Op preserved the signal of local temperature, supporting the “temperature effect”, while no evidence for its “amount effect” has been observed. These quantified links between PIMS/local-temperature and δ18Op appear useful references to understand the link between stable oxygen isotopes and climate parameters. Our long-term monitoring of δ18Op, d-excess, and moisture sources reveals decadal trends, highlighting a tight coupling in hydrologic systems and relatively fast changes on rainfall sources controlled by atmospheric circulations in SW-France.