The 2019 New Year Stratospheric Sudden Warming and Its Real-Time Predictions in Multiple S2S Models

Using multiple data sources, favorable conditions for the 2019 SSW event and its predictive skill from 11 subseasonal to seasonal (S2S) forecast models are explored in this study. This mixed-type (displacement to split) SSW event occurred under moderate El Nino, easterly quasi-biennial oscillation (QBO) together with solar minimum, and phases 4-6 of the Madden-Julian oscillation (MJO). A strong positive PNA formed and developed in the troposphere before this SSW event, which is associated with enhanced and wave-1-dominated eddy heat flux. The predictive limit to this SSW onset is beyond 18 days in most S2S models, longer than the average predictive limit in existing literature. This high predictive skill may originate from the favorable initial conditions (QBO, MJO) and boundary conditions (moderate El Nino, solar minimum). Although some difference in the predictability of the 2019 SSW onset is found between models, most models well forecast its onset at a lead time of 18 days. More than 50% of the ensemble members initialized on 13 December forecast the zonal wind reversal, and the anomaly correlation for tropospheric heights during the SSW onset even exceed 0.7 in the multimodel ensemble (MME). In contrast, the predictability of the wave-3 and wave-2 pulses after the SSW onset, responsible for the standing split of the stratospheric polar vortex, is low in the forecast MMEs initialized before the SSW onset. It is a challenge for MMEs initialized before the SSW onset to well forecast the vortex splitting and its persistence 10-20 days after the SSW onset.