Northern Hemisphere circulation regimes: observed, simulated and predicted

Nonlinear principal component analysis provides evidence that the Northern Hemisphere extratropical atmosphere supports three distinct circulation regimes with an average residence time of about 7 days. The role of high-and low-frequency dynamics is studied and results indicate that they are both involved in the formation, maintenance and decay of the regimes. A global coupled climate model also supports three distinct circulation regimes with strikingly similar spatial structures, residence times and linked high- and low-frequency dynamics to those observed. The issue related to how long a data record is required to properly resolve the regime structures is addressed by exploiting the model output. Regime residence times and spatial structures are predicted to change over the next century given increasing concentrations of atmospheric greenhouse gases.