The seasonal footprinting mechanism in the Pacific: Implications for ENSO

Dan Vimont
Dept. of Atmospheric Sciences, University of Washington

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Mid-latitude atmospheric variability is identified as an important contributor to the stochastic forcing of ENSO. The effect of the mid-latitude atmosphere is felt in the tropics via the "Seasonal Footprinting Mechanism". During the winter months, when the mid-latitude atmosphere is most energetic, a persistant pattern of mid-latitude atmospheric variability imparts an SST "footprint" onto the ocean via changes in the net surface heat flux. This SST footprint extends into the tropics, where during the spring and summer seasons, it forces atmospheric circulation anomalies that include zonal wind stress anomalies along the equator. These spring and summer zonal wind stress anomalies provide a significant stochastic forcing to the tropical system.

Results are presented for the Commonwealth Scientific Industrial Research Organisation (CSIRO) coupled general circulation models(CGCMs), and for the observed record from 1948-2001. In the CSIRO CGCMs, the seasonal footprinting mechanism accounts for about 25-50% of the model's interannual ENSO variability, and up to 75% of the model's interdecadal variability. In the observed record, the seasonal footprinting mechanism is shown to be strongly associated with ENSO variability. Its existence may have important implications for ENSO prediction, Pacific decadal variability, and our understanding of basic mechanisms responsible for Pacific variability.

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11 Jan, 2002
2 PM/ DSRC 1D 403
(Coffee at 1:50 PM)
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