Decadal-scale modulation of the NAO/AO by external forcing: Current state of understanding
Analyses of observations show correlations between the mean state of indices representing either the North Atlantic Oscillation (NAO) or the Arctic Oscillation (AO, also called Northern Annular Mode) with various external forcings. These include volcanic eruptions, solar variability, greenhouse gas levels and stratospheric ozone depletion. Climate model simulations have been able to reproduce many aspects of these correlations over a variety of time scales ranging from inter-annual to century scale. This has allowed some insight to be gained into how external forcings modulate these intrinsic variability patterns. I review current understanding derived from comparisons of a range of models with observations to highlight areas of commonality as well as remaining uncertainties. Contrasts between the Northern and Southern Hemispheres suggest that much of the response to external forcing occurs via wave-driven processes. Comparison of the response to forcings at different levels in the atmosphere indicate a sizeable role for both stratospheric and surface-level perturbations. Implications for forcing of changes in Mediterranean climate are presented for pre-industrial times during the last millennium, for the twentieth century, and for the potential future. © Società Italiana di Fisica.
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