The Arctic and Antarctic oscillations and their projected changes under global warming

The Arctic Oscillation (AO) and the Antarctic Oscillation (AAO) are the lea ding modes of high-latitude variability in each hemisphere as characterized by the first EOF of mean sea-level pressure. Observations suggest a recent positive trend in the AO and it is speculated that this may be related to global warming. The CCCma coupled general circulation model control simulat ion exhibits a robust and realistic AO and AAO. Climate change simulations for the period 1900-2100, with forcing due to greenhouse gases and aerosols , exhibit positive trends in both the AO and the AAO. The model simulates e ssentially unchanged AO/AAO variations superimposed on a forced climate cha nge pattern. The results do not suggest that a simulated trend in the AO/AA O necessarily depends on stratospheric involvement nor that forced climate change will be expressed as a change in the occurence of one phase of the A O/AAO over another. This pattern of climate change projects exclusively on the AAO pattern in the southern hemisphere but not in the northern hemisphe re where other EOFs are involved. The extent to which this forced climate c hange pattern and the unforced modes of variation are determined by the sam e mechanisms and feedbacks remains an open question.