Past shifts in the general circulation: causation, structure, and effects on hydroclimate

There is now abundant evidence that an important consequence of recent climate change is the displacement of key elements within the global circulation. Considerable uncertainty exists with regard to the sign and magnitude of these changes, and it is also not fully understood how the roles of individual anthropogenic forcings contribute toward their cause. Because of the potential impacts they may impose upon hydroclimate, these changes in the circulation, and particularly their causation, are of high interest. Our primary goal is to differentiate between the various anthropogenic forcings driving these shifts. We analyze the results from long time-slice simulations performed using a state-of-the-art atmospheric general circulation model, which show that important features in the general circulation have shifted poleward since the preindustrial era. Many of these shifts are characterized by pronounced seasonality and exhibit a tendency to be maximized during the summer months of each respective hemisphere. Furthermore, the magnitude of these shifts tends to exhibit a greater variability both over the southern hemisphere and within the extratropical regions. While reductions in ozone and increases of greenhouse gas concentrations have played a role in causation, the indirect effects from feedback mechanisms have been dominant. In most cases, a linear addition of the changes produced in those experiments prescribed with individual forcings is nearly identical to that for an experiment performed with all forcings included. This result is remarkably consistent, indicating that effects of the mechanisms that lead to changes in the circulation are approximately linearly additive.