Changes in Northern Hemisphere stratospheric variability under increased CO2 concentrations

The robustness of stratospheric circulation changes under increased concentrations of carbon dioxide are investigated using the Met Office HadSM3-L64 model. Equilibrium climate change simulations employing forcing of two and four times pre-industrial CO₂ are presented, with particular focus on the temperature response of the Arctic lower stratosphere during Northern Hemisphere winter. High CO₂ loading provides the ability to attain the statistical significance of any response, typically a problem given the large component of interannual variability common to the region. In response to CO₂, the expected global stratospheric cooling is modified by an anomalous dynamical warming of the Arctic winter lower stratosphere. This warming is shown to be associated with an increase in frequency of stratospheric sudden warming (SSW) events. At four times pre-industrial CO₂, the frequency of SSW events per year is doubled with respect to the control simulation. Further, by comparing winters with and without SSW events, it is shown that the warming of the lower stratosphere cannot be achieved without the presence of a frequency modulation of SSW events.