Bidecadal climate variability in the Northern Hemisphere winter associated to strong tropical volcanic eruptions during the Last Millennium

The simulated decadal and bidecadal response of Northern Hemisphere (NH) winter climates to historical strong tropical volcanic eruptions (SVE) is inspected in the MPI-Earth System Model all-forcing simulation ensemble for the last millennium. Up to 50 tropical SVE producing a peak annual-average top-of-atmosphere radiative perturbation exceeding -1.5 Wm-2 are investigated in ensemble analysis. While post-eruption global and hemispheric winter anomalies of air surface temperature generally revert back to pre-eruption climatological values 4-6 winters after the event, longer-lasting significant repercussions of SVE can be found on NH regional winter climates. In particular, our simulations indicate that surface warming can persist over the Scandinavian/Western Russian region during the first two decades following the eruption, with peaking anomalies occurring around 10-12 years after the event. The warm anomaly is essentially determined by the establishment of a prolonged strengthened phase of the North Atlantic Oscillation (NAO). The proposed mechanism explaining this bidecadal fluctuation involves three essential steps: (1) enhanced stratospheric polar vortex and heat uptake by the tropical ocean trigger an initial strengthened phase of the NAO and a weakened phase of the East Atlantic pattern (EA); (2) persisting cold temperature anomalies in the Arctic region and an active ocean-atmosphere coupling entailing positive feedback between the NAO and the underlying Sea Surface Temperatures (SST) sustains the anomaly. Delayed positive feedback linking the post-eruption anomalous gyre circulation and the meridional overturning circulation (MOC) contributes providing a 5-10 year inertia to the system; (3) negative feedback between the MOC and the NAO-related intergyre gyre circulation drives the second part of the fluctuation, when emerging positive EA anomalies contribute maintaining warmer European temperatures. The whole fluctuation has an average length of 20-25 years: Our results suggest therefore that clusters of tropical SVE during the last millennium can have substantially contributed to the establishment of prolonged anomalously warm regional climate regimes or, similarly, to locally dampen the effects of global-scale cold time periods such as the Little Ice Age. Indeed, our results indicate that the emergence of such bidecadal fluctuations is strongly conditioned by the pre-eruption extent of the Arctic sea ice cover and intensity of the North Atlantic thermohaline circulation, and that it is less likely to occur under a colder global climate.