Climate change in recurrent regimes and modes of Northern Hemisphere atmospheric variability

We identify recurrent regimes of atmospheric states in observed and model-simulated data with and without anomalous greenhouse gas and aerosol forcing by testing against the null hypothesis that the atmospheric states are produced by a Gaussian autoregressive process of the order of 1 in a reduced phase space. In one approach the phase space is described by the leading Northern Hemisphere Empirical Orthogonal Functions (EOFs), while in another, it is described by sectorial EOFs of the northern Atlantic and the Pacific regions. The time series of model output are sufficiently long to show clearly, in the sectorial analysis, that the increased greenhouse gas forcing leads to a change in the frequencies of occurrence of the recurrent regimes. Such evidence is not obtained from the hemispheric analysis of model output. The observational time series is found to be too short to reliably identify changes in recurrent flow regimes with the exception that there appear to have been significant changes in the flow patterns corresponding to the Cold Ocean Warm Land pattern.