Comparison of Subseasonal-to-Seasonal Model Forecasts for Major Stratospheric Sudden Warmings

This study explores forecasts for five distinctive major stratospheric sudden warmings (MSSWs) using subseasonal-to-seasonal prediction data with lead times up to about 2 weeks. Results reveal model-to-model variability of the forecasts, as some models, such as the European Centre for Medium-Range Weather Forecasts and National Centers for Environmental Prediction models, forecast the MSSWs better. Results also demonstrate greater difficulty of forecasting the vortex split MSSWs (three of the five) than the vortex displacement MSSWs (the other two). It is also shown that the variable forecasts among different lead times and models for each MSSW are characterized by how well the models reproduce the weakenings and deformations of the stratospheric polar vortex around the MSSWs and the intensifications of tropospheric planetary waves before them. The variability is well captured by planetary wave activity, or poleward heat flux, in the lower stratosphere. This underlines the importance of enhanced planetary waves in the troposphere and lower stratosphere for successful forecasts of the forecasting MSSWs, although the causality between the MSSWs and enhanced planetary waves will deserve further investigation.