15.5
Storm track predictability on seasonal to decadal scales
Gilbert P. Compo, NOAA-CIRES Climate Diagnostics Center, Boulder, CO; and P. D. Sardeshmukh
This talk is concerned with estimating the predictable variations of extratropical daily weather statistics ("storm tracks") associated with global sea surface temperature (SST) changes on interannual to interdecadal scales, and its magnitude relative to the unpredictable noise. The SST-forced storm track signal in each northern winter in 1950-2004 is estimated as the mean storm track anomaly in an ensemble of atmospheric general circulation model (AGCM) integrations for that winter with prescribed observed SSTs. Since the storm track signals cannot be derived directly from the archived monthly AGCM output, they are diagnosed from the SST-forced winter-mean 200 mb height signals using an empirical linear storm track model (STM). For two particular winters, the El Nino of JFM 1987 and the La Nina of JFM 1989, the storm track signals and noise are estimated directly, and more accurately, from additional large ensembles of AGCM integrations. The linear STM is remarkably successful at capturing the AGCM's storm track signal in these two winters, and is thus also suitable for estimating the signal in other winters. We find that a predictable SST-forced storm track signal exists in many winters, but its strength and pattern can change substantially from winter to winter. The pattern correlation of the SST-forced and observed storm track anomalies is high enough in the Pacific-North American sector to be of practical use. In the Euro-Atlantic region, we find much lower correlations, which we argue arise from substantial AGCM error in representing the regional response to tropical SST forcing, rather than intrinsically low predictability. .
Session 15, Climate Predictions on Seasonal to Interannual Time Scales: 2 (parallel with Joint Session 4)
Thursday, 13 January 2005, 1:30 PM-5:15 PM
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