Datensatz

Zusammenfassung

It has been demonstrated that decadal variations in the North Atlantic Oscillation (NAO) can be predicted by current forecast models. While Atlantic Multidecadal Variability (AMV) in sea surface temperatures (SSTs) has been hypothesised as the source of this skill, the validity of this hypothesis and the pathways involved remain unclear. Here we show, using both reanalysis and data from two forecast models, that the decadal predictability of the NAO can be entirely accounted for by the predictability of decadal variations in the speed of the North Atlantic eddy-driven jet, with no apparent predictability of decadal variations in the latitude. The sub-polar North Atlantic (SPNA) is identified as the only potential source of an SST-based signal common across the models and reanalysis, and the predictability of the jet speed is shown to be consistent with a small but steady forcing from the SPNA taking place entirely within a single season. The pathway is argued to be tropospheric in nature, with the SPNA-induced heat anomaly extending up to the mid-troposphere, which alters the meridional temperature gradient around the climatological jet position, thereby modulating the intensity of the eddies. The transfer of heat from the SSTs to the atmosphere via heatfluxes is shown to be systematically underestimated in the forecast models by approximately a factor of two, with potential implications for the `signal-to-noise paradox'.