Influence of the winter Atlantic Niño on the North Atlantic-European atmospheric circulation

One of the leading modes of sea surface temperature (SST) variability in the Tropical Atlantic is the Atlantic Niño (ATLN). It shows two peaks of variability along the seasonal cycle: the first one occurs in boreal summer, which is the maximum and has been widely studied; the second one takes place in early winter (November-December) and has been much less investigated. Previous studies have shown potential for this early-winter SST variability mode to be a source of predictability for the North Atlantic-European (NAE) region. Coupled simulations and atmosphere-only experiments with the CMIP6 version of the climate model EC-EARTH (T255L91) have been performed and analysed to revisit the ATLN-NAE atmospheric teleconnection and further improve process understanding. The coupled simulation consists in a 250-year long integration, after spin-up, with fixed radiative forcing at present conditions; the atmospheric response is estimated by linear regression onto the winter Atlantic Niño index defined by Okumura&Xie. The atmosphere-only experiments comprise two 150-year long integrations keeping the radiative forcing fixed, a control run with climatological SSTs and a sensitivity run prescribing the observed ATLN with climatology elsewhere; the atmospheric response is evaluated by comparing both experiments. The atmospheric anomaly associated with ATLN shows a local Gill-type response, symmetrically straddling the equator, whose amplitude increases from November-December to January-February. In the extratropics it depicts a barotropic structure, yielding a wave-like pattern in early winter and a dipolar structure at mid-latitudes (different from the North Atlantic Oscillation) in late winter. In both seasons, the atmospheric response to ATLN displays statistically-significant precipitation anomalies over Europe.