The ability of climate models in the Intergovernmental Panel on
Climate Change Fourth Assessment Report to capture the temporal
structure of the annular modes is evaluated. The vertical structure
and annual cycle of the variability is quantified by the e-folding
time scale of the annular mode autocorrelation function. Models
vaguely capture the qualitative features of the Northern and Southern
Annular Modes: Northern Hemisphere time scales are shorter than
those of the Southern Hemisphere and peak in boreal winter, while
Southern Hemisphere time scales peak in austral spring and summer.
Models, however, systematically overestimate the time scales,
particularly in the Southern Hemisphere summer, where the multimodel
ensemble average is twice that of reanalyses.
Fluctuation-dissipation theory suggests that long time scales in
models could be associated with increased sensitivity to
anthropogenic forcing. Comparison of model pairs with similar
forcings but different annular mode time scales provides a hint of a
fluctuation-dissipation relationship.

Full Article: http://www.agu.org/journals/pip/gl/2008GL035712-pip.pdf

Last Updated: 2008-10-16