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.