Prediction of Extratropical Storminess
Gilbert P. Compo, NOAA/CIRES/CDC, Boulder, CO; and P. D. Sardeshmukh, M. A. Alexander, and J. D. Scott
This talk is concerned with estimating the predictable variations of extratropical daily weather statistics ("stormtracks") associated with global sea surface temperature (SST) changes on interannual scales, and its magnitude relative to the unpredictable noise. The SST-forced stormtrack signal in each northern winter in 1950-2003 is estimated as the mean stormtrack anomaly in an ensemble of atmospheric general circulation model (AGCM) integrations for that winter with prescribed observed SSTs. Since the stormtrack signals cannot be derived directly from those ensembles with archived monthly AGCM output only, they are diagnosed from the SST-forced winter-mean 200 mb height signals using an empirical linear stormtrack model (STM). For four particular winters, the El Nino of JFM 1987, the El Nino of JFM 1998, the La Nina of JFM 1989, and the La Nina of JFM 1999, the stormtrack 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 stormtrack signal in these four winters, and is thus also suitable for estimating the signal in other winters. We find that a predictable SST-forced stormtrack 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 stormtrack 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. The magnitude of these errors is compared with those computed directly from recent stormtrack forecasts made with coupled ocean-atmosphere GCMs. .
Session 6, Climate Prediction on Seasonal to Interannual Timescales
Wednesday, 1 February 2006, 10:30 AM-12:00 PM, A314
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