On the spectral characteristics of the Atlantic multidecadal variability in an ensemble of multi-century simulations
The Atlantic multidecadal variability (AMV) is a coherent pattern of variability of the North Atlantic sea surface temperature field affecting several components of the climate system in the Atlantic region and the surrounding areas. The relatively short observational record severely limits our understanding of the physical mechanisms leading to the AMV. The present study shows that the spatial and temporal characteristics of the AMV, as assessed from the historical records, should also be considered as highly uncertain. Using 11 multi-century preindustrial climate simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5) database, we show that the AMV characteristics are not constant along the simulation when assessed from different 200-year-long periods to match the observed period length. An objective method is proposed to test whether the variations of the AMV characteristics are consistent with stochastic internal variability. For 7 out of the 11 models analysed, the results indicate a non-stationary behaviour for the AMV time series. However, the possibility that the non-stationarity arises from sampling errors can be excluded with high confidence only for one of the 7 models. Therefore, longer time series are needed to robustly assess the AMV characteristics. In addition to any changes imposed to the AMV by external forcings, the detected dependence on the time interval identified in most models suggests that the character of the observed AMV may undergo significant changes in the future.