| This study concerns Northern Hemisphere low-frequency atmospheric regimes as they have been observed in the recent past and simulated for the near future. Non-linear principal component analysis is applied to observed and simulated daily sea level pressure. The objectives are to: (1) characterize the three dimensional spatial, temporal and dynamical signature of the regimes; (2) assess a global climate model reproduction of the regimes and (3) determine the simulated regime response to enhanced levels of greenhouse gases and sulphate aerosols. The main conclusions are that the atmosphere supports three regime states with an average residence time of about seven days. Low- and high-frequency dynamics are both involved in the formation, maintenance and decay of the regimes. The model produces three similar regimes with similar residence times and underlying dynamics. Under enhanced levels of greenhouse gases and sulphate aerosols both the regime residence times and spatial structures are predicted to change. This is in contrast to some earlier studies which suggest that only the residence times would be affected. Finally, it is demonstrated that an overly coarse characterization of regime behavior is obtained when the data is smoothed over time-scales much longer than the intrinsic residence time of the regimes. Importantly, this result helps to reconcile some earlier contradictory results. |
