Bimodality of the North Atlantic Oscillation in simulations with greenhouse gas forcing

Using high-resolution global "time-slice'' climate change simulations, we show the existence of a bimodal NAO distribution in future climate conditions under the IPCC A2 scenario forcing. The current climate exhibits a unimodal NAO distribution. The second "regime'' in the future climate experiment is located in the positive region of the NAO phase space at about one standard deviation from the maximum, which is located near zero or at slightly negative NAO values. This additional regime grows at the expense of negative NAO occurrences. The change in the NAO distribution is physically related to negative PNA-like change occurring in the Pacific region. The interpretation of the occurrence of the second NAO maximum in the future climate simulation is that large positive NAO events will occur more frequently in future climate conditions, while the rest of the NAO frequency distribution shows only small changes. This implies that European winter climate is expected to show higher variability and increased occurrence of extreme climate conditions associated with the positive NAO phase.