Predictability of Extreme ENSO Epochs in the CM2.1 Global Coupled GCM

Andrew T. Wittenberg

NOAA Geophysical Fluid Dynamics Laboratory, Princeton, NJ

Session PO45: Ocean's Role in Natural and Anthropogenic Climate Variability
15th Annual Ocean Sciences Meeting, Portland, OR

Presented: February 2010

Abstract: Even without changes in external forcings, a 2000-year control run of the GFDL CM2.1 global coupled GCM exhibits a slow modulation of its ENSO behavior -- with extended epochs of very strong or weak ENSO variability lasting decades to centuries.  To place an upper bound on the long-term predictability of these extreme epochs, we run ensembles of perfect-model reforecasts, initialized from practically-perfect initial conditions at the beginning and middle of each epoch.  Following a rapid nonlocal and global growth of the initially tiny SST perturbations, the reforecast ensembles appear to "forget" most of the ENSO-relevant information in the control initial conditions beyond about 10 years lead.  Thus the control run's extreme ENSO epochs appear to be unpredictable beyond a decade in advance -- consistent with the hypothesis that these unusual epochs arise from little more than random Poisson statistics applied to ENSO's interannual memory.  Possible implications for decadal predictions, model evaluation and intercomparison, historical and paleo reconstructions, and societal vulnerability are discussed.