Signature of recent climate change in frequencies of natural atmospheric circulation regimes

A basic paradigm for the effect of increasing atmospheric CO2 concentration is of warming through the lower atmosphere, as a direct response to the radiative forcing due to increased infrared opacity. However, this paradigm is essentially 1-dimensional, and ignores the adjustment of the 3-dimensional dynamics of the climate system to the imposed forcing. Evidence is presented, based on an analysis of mid-tropospheric geopotential data, that changes in northern-hemispheric climate in recent decades can be explained by changes in the frequency of occurrence of preferred nonlinear naturally-occurring atmospheric circulation regimes. This behaviour can be simulated in low-order chaotic models. These results indicate that an over-simplistic interpretation of the "greenhouse effect" paradigm is misleading; recent hemispheric warming (even if anthropogenically forced) may be more directly related to the thermal structure of preferred patterns of natural intraseasonal variability, than to the forcing pattern itself. This scenario can account for the apparent disparity between trends in surface temperature and satellite radiance data, and suggests that climate prediction models must simulate the non-gaussian structure of the climate attractor.