The North Atlantic Oscillation as an indicator for greenhouse-gas induced regional climate change

The time-dependent variability of the North Atlantic Oscillation is examine d in an observational data set and several model data sets with greenhouse- gas-induced external forcings, The index of the North Atlantic Oscillation state is derived from the time series of mean latitudinal position and cent ral pressure of the Icelandic Low and the Azores High considering the synch ronous meridional shifting of the two pressure systems. While the North Atl antic Oscillation is characterized by intensive interannual variability, th e low-pass filtered index time series shows a decadal component with a time scale of about 50 y within almost 120 y of observation. Since the late 196 0s we observe a positive trend and a transition to a strong positive phase of the phenomenon indicative of a predominantly zonal circulation over the North Atlantic. This trend occurs equally in the observations and all exami ned model data sets with increasing greenhouse-gas-concentration and atmosp here-ocean coupling. We find statistical evidence that the radiative forcin g by increasing CO2 concentration has a significant influence on the simula ted variability of the North Atlantic Oscillation on time scales of 60 y an d longer, independent of the initial conditions and the model version. The seasonal response is strongest in late summer and winter. The interannual v ariability of the North Atlantic Oscillation states on time scales less tha n 10 y decreases synchronously with the positive trend of its decadal-mean state implying a stabilization of its present and future zonal state.