Ultra-low-frequency variability in a simple atmospheric circulation model

TO assess the inherent variability of atmospheric flow, we have carried out extended runs of a primitive-equation model of the global troposphere. The longest-period forcing that we explicitly included was the annual cycle, yet the largest-scale structures in the flow were variable on much longer timescales, with maximum variability on periods of 10–40 years. This variability was generated internally by the nonlinear dynamics of the atmospheric flow. Forcings by slow variations of such quantities as sea surface temperature or incoming solar radiation were excluded. The results show that apparent correlations with the sunspot cycle1–3 could be spurious. They also indicate that attempts to monitor systematic climate change require circulation statistics for periods longer than 40 years, and that studies of climate sensitivity based on global circulation models require integrations of at least that length.