J2.4 Annular mode time scales and climate sensitivity in the Intergovernmental Panel on Climate Change Fourth Assessment Report models

Tuesday, 9 June 2009: 11:20 AM
Pinnacle BC (Stoweflake Resort and Confernce Center)
Edwin P. Gerber, Courant Institute of Mathematical Sciences, NYU, New York, NY; and L. M. Polvani and D. Ancukiewicz

A significant component of recent and future climate change consists of a poleward shift of the extratropical jets (and correspondingly, the storm tracks) and this shift projects strongly on the annular mode patterns of variability (e.g. Yin 2005, Miller et al. 2006, and Son et al. 2008). Recent work with idealized models has shown a possible fluctuation-dissipation relationship with the annular modes (Ring and Plumb 2008). Hence their temporal structure may be a gauge of climate sensitivity (Gerber et al. 2008). Here the ability of climate models in the Intergovernmental Panel on Climate Change Fourth Assessment Report (AR4) to capture the temporal structure of the annular modes is evaluated. Models capture the key 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 approximately 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 in the AR4 models.
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