Assessing S2S Predictability: A Case Study of Tropospheric Precursors of the January 2009 Sudden Stratospheric Warming (SSW) Event.

Research dedicated to assessing the performance of predicting SSW events shows that skillful predictions of SSWs enhances tropospheric forecasts at subseasonal–to–seasonal (S2S) lead times. This study analyzes the performance of three reforecasts from models in the S2S Prediction Project dataset (BOM, ECMWF, ISAC) initialized 13 days prior to the onset of January 24, 2009 event. At a 13-day lead-time, only one of the three models skillfully forecast the occurrence of an SSW, though it exhibited forecast errors related to the timing and amplitude of the event. The analysis shows that an anomalous ridge over Alaska formed just prior to the SSW event. The ridge was associated with a wave packet and large wave-2 EP flux forcing that led to the “splitting” SSW. Collectively, the ability to skillfully resolve these synoptic-scale processes in the blocking predecessor combined with variability in model resolution were shown to be contributors to the forecast skill. SSW events that were primarily caused by planetary-scale wave-1 EP forcing (e.g., March 2000, December 2001) the BOM, ECMWF, and ISAC forecasts showed more skill in predicting the observed stratospheric winds and temperatures.